Overvis MC251 to programowalna bramka i kontroler RS-485 do Ethernet / GSM / LTE z priorytetem chmury, zaprojektowany do zastosowań w automatyce przemysłowej i zdalnym monitoringu.
Mostkuje protokoły Modbus RTU/ASCII do platformy chmurowej, umożliwiając bezproblemową integrację starszego sprzętu przemysłowego z chmurą.
Może również mostkować do nowoczesnych sieci TCP/IP, systemów SCADA i infrastruktury IoT.
This guide walks you through setting up your Overvis MC251 gateway from wiring to successful connection in about 15 minutes.
The Overvis MC251 is a programmable gateway that bridges RS-485 Modbus devices to Cloud and modern networks, letting you monitor and control industrial equipment remotely via Ethernet or cellular connections.
Mount your MC251 on a standard 35mm DIN rail in a well-ventilated location. The device operates from -35°C to +55°C, but avoid areas with excessive vibration, humidity, or corrosive atmospheres.
Install memory card: Insert the provided 2GB microSD card into the SD slot. The memory card is required for firmware updates, data logging, and automation logic programming features.
Install SIM card (if using GSM/LTE): If you plan to use cellular connectivity, insert your SIM card (with data service enabled) into the SIM slot now. Attach GSM antenna to the ANT connector.
Network connection: Connect the Ethernet cable from MC251’s LAN port to your router or directly to your computer.
Before making any electrical connections, ensure all devices are powered off.
Tighten terminal screws to 0.4 N·m — enough for solid contact without damage.
RS-485 wiring: Connect twisted-pair cable to MC251’s RS-485 terminals: terminal A for non-inverted signal, terminal B for inverted signal (your devices might label these as A/D+/+ and B/D-/- respectively). Run the cable to your Modbus devices and connect accordingly. Use shielded twisted-pair cable (Category 1 or better) for reliable communication over distance.
Power: Connect 12V DC power to the 9÷30V terminals. For safety, install a 2.5A fuse in the power circuit. Use stranded wire with ferrules, stripped 5mm. Wire cross-section should be 0.5-3 mm².
When you apply power, all LEDs light up briefly during the 2-second initialization, then the device spends 10-15 seconds establishing network connections.
The PWR LED stays on to confirm power. The LAN LED turns on when Ethernet connects (blinking during data transfers). The GSM LED blinks slowly (every 1.5s) when registered to cellular network, or rapidly (3 times/second) after TCP/IP link start.
The display shows IP addresses (prefixed with (E) for Ethernet or (G) for GSM), plus data rates and signal strength.
Step 3: Access Web Interface and Configure Internet Connection
Note the IP address from the display (press R to wake it). If connected via Ethernet with DHCP, you’ll see the assigned address.
Connecting directly to a PC?
Without a DHCP server, the device uses the default 192.168.0.111, showing it on the display in a minute after power-up.
Configure your computer’s network adapter with IP 192.168.0.10 (or any address from 192.168.0.1-254 except 111), subnet mask 255.255.255.0, leave gateway blank.
See the Connections Guide for OS-specific instructions.
Open a web browser and enter the IP address shown on the display (e.g., http://192.168.0.111).
Login to quick setup: You’ll see a login page. Press the R button on MC251 for temporary password-free access.
Configure network: Enable/disable DHCP, set static IP if needed, adjust subnet mask and gateway. For GSM, enter your mobile operator’s APN (Access Point Name) and SIM card PIN (if present).
For detailed information about all web interface options, see the Web Interface Guide.
Overvis Cloud provides remote monitoring and control through a web dashboard.
The device label includes a QR code and PIN for quick setup.
It is necessary to connect the device to Overvis, which allows to further configure, operate and monitor MC251.
Access Overvis server: Scan the QR code on the device label or manually enter the link from the label (format: https://c.overvis.com/ABCD1234 where the last part is your PIN). The link redirects to Overvis server’s login page.
Login or create account: Enter your credentials if you have an account. New users should create an account first.
Create Endpoint: After login, Overvis displays the Workspace page. If you came from the QR code/link, the Create endpoint form will be displayed with the PIN pre-filled automatically. Otherwise, select Manual setup, Endpoints and press the Create endpoint button. Enter the PIN from the device label. Give your endpoint a descriptive name (a “endpoint” represents your MC251 plus all connected Modbus devices). Click Create to verify MC251 is online.
Authenticate access: After checking MC251 is online, Overvis displays the Confirmation form. Prove you have physical access to the device by entering the temporary code displayed by MC251, or shortly pressing the button on its front panel.
Configure Network & Verify Operation Ask AI copilot by pressing the yellow star AI button on how to add the devices connected to MC251, configure them and display the data in convenient visual form.
Display shows (E) 0.0.0.0: → Normal during first 20-60 seconds while DHCP negotiates. If it persists, check cable connection and router DHCP. Try direct PC connection with static addressing settings.
Display shows (E) 192.168.0.111: → DHCP failed or is unavailable. MC251 has switched to its factory default static IP address. Either configure your client device to the same subnet (192.168.0.x) or enable DHCP on your router.
Can’t access web interface: → Verify IP address entered in browser matches what’s on the display. Ensure Ethernet address is used (the one marked with (E) in the second line). Check PC and MC251 are on the same subnet. Temporarily disable firewall and/or proxies. Press R for password-free access. Clear browser cache.
GSM won’t connect: → Verify SIM card is fully inserted (until you hear a click), check the antenna is attached, check the GSM settings. Check that data service is active. Check signal strength on display (should be >0%). Try relocating antenna to better position. Try to disable PIN. Try manually configured APN settings.
No data response from MC251 itself → Check Modbus ID in the cloud device configuration for “MC251” device
MC251 responds but RS-485 devices don’t: → Most common issue is mismatched settings. Select MC251 device (in newly created network) and configure baud rate and parity to match all the devices. Check A/B wiring polarity (swapping these is very common - this doesn’t cause equipment damage, but prevents communication). Try increasing delays in MC251 settings for RS-485. Ensure protocol (Modbus RTU/ASCII) and parity are the same for all the devices. Start with 9600 baud, RTU, AUTO-STOP parity.
Wrong data returned as a response → Check register address matches your device’s documentation. Check the parameter type in the configuration of the cloud device parameter (with this address)
This Operating Manual explains the design, safety requirements, operating rules, and maintenance procedures for the Overvis MC251 Cloud-first Modbus Gateway with 4G/Ethernet/RS-485.
The Overvis MC251 is a Cloud-first programmable RS-485 to Ethernet / GSM / LTE gateway and controller. It:
collects data from connected Modbus devices;
transfers data to a cloud server;
provides access to data via Modbus TCP or SMS text messages;
tracks events and executes actions (for example, sending SMS notifications, writing values to Modbus devices, or logging values to a memory card).
The Overvis MC251 provides the following capabilities:
Flexible connection options: Ethernet (wired) or wireless GPRS / FDD-LTE, automatic selection of the connection method to the cloud server, automatic or manual selection of GSM options and communication parameters, automatic or manual Ethernet settings
Access security: no default password, encrypted Cloud VPN connection, passwords for setup mode and connections to the Modbus network, passwords for SMS-based device control
Flexible RS-485 communication: Modbus RTU or ASCII, even/odd/no parity, wide range of baud rates, adjustable delays
Programmable logic for data collection, event tracking, and actions in response to events (see Operations logic programming)
Service functions: real-time clock, astronomical timer, diagnostic logs, automatic or manual firmware update
LAN LED is on when connected to Ethernet network, and is blinking during the data exchange on the network;
The display shows the state of the device, active connections, device addresses, and the load of communication interfaces;
GSM LED flashes every 1.5 s when GSM (2G) communication is available; flashes 3 times per second when LTE (4G) communication is available, or when GPRS or FDD-LTE data is being exchanged;
AL LED warns about the error as a result of analysis of the received data;
SD is on when there is a memory card in its slot, and is blinking during the data exchange with the memory card;
SRV is on when the connection is present with the Overvis Cloud VPN server, and is blinking during the data exchange via this connection;
485 is on when waiting for data from a device on RS-485 bus, and is blinking during the data exchange on this bus;
R service button (accessible through a hole in the front panel, press with a thin non-conducting object) grants quick access to MC251 or resets the controller.
MC251 provides monitoring and control over Modbus RTU/ASCII devices in an RS-485 network for the cloud server. It also supports reading Modbus data.
The secure VPN connection to a cloud server is established:
via Ethernet network using an integrated Ethernet interface chip;
via GPRS / FDD-LTE using a built-in modem (used when Ethernet connection is not available).
MC251 also provides access to RS-485 devices via Ethernet, GSM / LTE, or SMS.
In addition, MC251 can connect via the Modbus TCP protocol to exchange data with Modbus TCP devices or with another MC251 controller.
MC251 receives and processes SMS messages that contain a password and a read/write command for Modbus devices.
When you insert a memory card with prepared operation logic program files, MC251 may load this program into internal memory. This program defines data collection and event tracking. It runs in the background.
You can write collected data to the memory card in tabular or binary format. For registered events, you can configure actions such as sending SMS messages or writing corrected Modbus values.
MC251 stores network settings, security parameters, and operation logic in its built-in memory.
If the temperature of MC251 after transportation or storage differs from the ambient operating temperature, keep MC251 under operating conditions for at least two hours before connecting it to the power supply. This prevents condensation on internal components.
To ensure reliable electrical connections, use flexible (stranded) wires. Strip the insulation from the wire ends by 5±0.5 mm and crimp with suitable ferrules. It is recommended to use wire with a cross-section of at least 1 mm².
When connecting to the RS-485 bus, use twisted-pair cable of category 1 or higher. A shielded cable is recommended; in that case, ground the shield according to “ANSI/TIA/EIA-485-A-1998”.
When connecting to Ethernet, use the supplied cable or a category 5e twisted-pair cable with an 8P8C (RJ-45) plug.
Route and fasten wires so as to avoid mechanical damage, twisting, or abrasion of the insulation.
For a reliable contact, tighten the terminal screws with the force indicated in Technical specifications.
To improve safety and reliability, it is recommended to install the fuse F1 (or its equivalent) in the MC251 supply circuit, rated for a current of no more than 2.5 A.
Connect the RS-485 bus cable to the RS-485 terminals (A and B) and to the RS-485 bus (or directly to the device with an RS-485 interface).
If the MC251 should connect to the Internet via wired communication, to a local network, or directly to a computer, connect the Ethernet cable to the LAN connector and to the Ethernet network (or directly to a computer). Connection details, depending on the network type, are provided in Connections.
Connect the appropriate DC power supply to the 9÷30V power connector.
Insert the memory card (microSD) into the MicroSD slot if you plan to use data logging or operation logic programming features.
If the MC251 is to connect to the Internet via wireless communication (or needs SMS exchange), insert the SIM card of the GSM operator into the SIM slot and connect the GSM antenna to the ANT connector (SMA F connector).
After power is turned on, all indicators except LAN and GSM light up, and MC251 performs initialization.
After approximately 2 seconds, the indicators (except for the power indicator) go out, and the device proceeds to start the communication interfaces with networks.
The display shows general information about the device (Fig. 4). Startup can take up to 15 seconds.
Fig.4 – Displaying of general information about the device on the display
After this, if enabled in settings, MC251 establishes a connection to the cloud server (or VPN) and starts executing the operation logic (if a program is loaded from the memory card). By default, cloud connections are disabled.
If configured, MC251 establishes TCP connections (via Ethernet and GSM/LTE networks) and waits for incoming TCP connections.
If a SIM card is installed, the GSM indicator shows the cellular connection status: blinking once every 1.5 seconds indicates successful network registration (but no data transfer yet); blinking 3 times per second indicates active TCP/IP data transfer via GPRS or FDD-LTE.
The display shows the interfaces load, the GSM signal strength and the IP addresses used, as shown in Figure 5.
┌─────────────────────┐
│ E: 5.2к S: 15% │
│ (E) 10.0.0.1 │
│ │
└─────────────────────┘
┌─────────────────────┐
│ 4G: 7-0.3к S: 15% │
│ (G) 87.1.1.1 │
│ │
└─────────────────────┘
Fig.5 – Displaying the status of connections on the display
E: 5.2k – the transmission speed via Ethernet is 5.2 kB/s;
4G: 7-0.3k – the LTE signal level is 70 % and the FDD-LTE transmission rate is 0.3 kB/s;
S: 15% – the load of RS-485 is 15 %.
(E): 10.0.0.1 – connection to the local network with the address 10.0.0.1;
(G): 87.1.1.1 – wireless Internet access with the address 87.1.1.1.
The Overvis MC251 is a protocol converter that bridges RS-485 Modbus RTU/ASCII networks and TCP/IP Modbus TCP devices to be available at the cloud server.
It operates in multiple communication modes simultaneously, providing flexible connectivity through Ethernet, GSM/LTE networks, and RS-485 interfaces.
Each mode serves a distinct purpose and can be used independently or in combination to meet your application requirements.
MC251 can establish and maintain a secure VPN outbound connection to a cloud server (using Wireguard protocol).
MC251 initiates the outbound connection to the server, bypassing firewall issues
MC251 authenticates at server and receives VPN parameters
The VPN connection is started
After the connection is established, MC251 operates as a Modbus TCP server in slave mode, waiting for connections and processing server requests
The server connects to MC251 and sends Modbus and HTTP requests through the VPN tunnel
MC251 forwards these requests to RS-485 devices or responds with its own data
Returns responses back to the server
If the SRV LED is on, the connection to the server VPN has been successfully established
If the SRV LED blinks, data is being exchanged via this connection
Use case: Centralized monitoring and control of distributed equipment through Overvis Cloud platform, enabling secure remote access from anywhere without configuring firewall port forwarding or static IP addresses.
In master mode, MC251 forwards Modbus requests from TCP clients to devices on the RS-485 bus and returns their responses.
MC251 receives Modbus requests from incoming TCP connections
Translates Modbus TCP requests to Modbus RTU/ASCII format
Forwards requests to target devices on the RS-485 network
Returns responses back to the TCP client
Supports both Modbus RTU and Modbus ASCII protocols
When an operation logic program is loaded, MC251 can also autonomously poll devices at configured intervals for data logging or event tracking purposes.
In tunnel mode, MC251 accepts data “as is” (without protocol verification) and forwards it to all other directions that are configured for this mode.
This allows transmission of data in formats different from the Modbus protocol. For example, arbitrary data received via RS-485 can be redirected to a remote TCP server, and vice versa.
Tunnel mode can be configured individually for:
Each connection to a remote TCP server
The RS-485 interface
Incoming connections to the Ethernet TCP port
Incoming connections to the GSM/LTE TCP port
First, a data packet from one direction is fully received (for Ethernet or GSM/LTE, this is one TCP packet; for RS-485, the packet length is determined by the Modbus RTU maximum-pause rules). Then it is sequentially forwarded to the other tunnel directions (if there are more than two directions).
The maximum data packet length in tunnel mode is 254 bytes.
Use case: Connecting equipment with non-Modbus compatible protocols, or extending serial communication over IP networks.
Pair two MC251 units to extend RS-485 networks beyond physical distance limitations by converting to/from TCP.
One MC251 operates in master mode on its RS-485 interface, while the other is in slave mode
First MC251 receives RS-485 signals and converts them to TCP packets
Data is transmitted over any distance via 4G/LTE or Ethernet networks
Second MC251 receives TCP packets and converts them back to RS-485 signals
Creates a transparent bridge between two RS-485 networks or segments
Works in both Modbus protocol mode (with address translation) and tunnel mode (fully transparent)
Each MC251 can operate in master or slave mode on its RS-485 interface as needed
Use case: Connecting RS-485 networks in separate buildings without running long cable runs, extending RS-485 beyond the 1200m distance limit, or accessing remote sites via cellular networks.
When an operation logic program is loaded into internal memory, MC251 reads specified parameters at a configured rate. These parameters may include registers of connected Modbus devices, MC251’s own registers, or MC251 memory.
The device then performs configured calculations and evaluates the received data. As a result, the following actions can be executed:
writing the read values to a log on the memory card;
sending SMS notifications on events;
writing new values to parameters.
The program is loaded into internal memory from the memory card. The procedure for preparing and loading the program into the device is described in Operations logic programming.
Use case: Autonomous data logging to memory card for later analysis, or automated responses like sending alarm notifications or regulation commands when temperature exceeds thresholds.
MC251 configuration can be performed in three ways:
quick setup of the Internet connection via the web interface using a browser (see Web interfaces);
quick setup via the Modbus protocol using any Modbus client software that works with MC251’s own registers (see Modbus interface).
advanced configuration at the Overvis cloud server device page.
Use the R button to grant quick access to MC251, restart the controller, or reset factory settings.
Access the button through a hole on the front panel and press with a thin non-conductive object.
To reset the device to factory settings and clear the internal operation logic memory:
Press and hold the service button R for at least 8 seconds. After 2 seconds, the AL indicator will light up. After 8 seconds, the settings are reset, internal memory is cleared, and the device restarts; the indicators blink once.
Release the R button.
To safely remove the memory card and restart the device (while preserving user settings):
Press and hold the service button R for 2 to 8 seconds.
When the AL indicator lights up, release the R button. If needed, remove the memory card after the SD indicator turns off.
To wake up the display, show connection information, and grant quick access to the device:
Briefly press and release the service button R.
The display lights up and shows connection information. For several minutes, configuration access (without a password) is granted for incoming connections.
You may transport and store MC251 in the original package at temperatures from minus 45 to +60 °C and relative humidity of no more than 80%. When transporting MC251, protect it against mechanical damage.
The Overvis MC251 uses standard TCP/IPv4 addressing for Ethernet communication. DHCP is enabled by default, allowing the device to automatically obtain network settings from your router. Without DHCP the factory default IP address is 192.168.0.111.
IP Addressing Fundamentals
When devices communicate over an Ethernet network using the TCP/IPv4 protocol, each device uses IP addressing settings to identify the sender and receiver of data. MC251 stores the following in its memory:
IP Address: A unique address within the subnet (four bytes written as integers 0-255, separated by dots)
Subnet Mask: Identical for all devices within the subnet (written similar to IP address)
Gateway Address: Used to communicate with other networks
Requirements for Proper Communication:
Devices within a subnet communicate properly when these conditions are met:
Common Subnet Mask: All devices in the subnet must use the same mask. Most small LANs use 255.255.255.0.
Valid Mask Format: The mask starts with consecutive 1 bits, followed by consecutive 0 bits.
Subnet Address: All IP address bits corresponding to the mask’s 1 bits must be identical across all devices. Common LANs use 192.168 as the address prefix with mask 255.255.255.0. The third byte can serve as a subnet number in complex LANs; in smaller LANs, it’s typically 0.
Unique Host Address: The IP address bits corresponding to the mask’s 0 bits must be unique to each device within the subnet.
Gateway Configuration: A router or gateway device that communicates with other networks typically uses an address like 192.168.0.1, 192.168.0.100, or 192.168.0.101. Other devices can use this address as their gateway. The gateway address is not required for intra-subnet communication—only for connecting to external networks (such as the Internet).
In some cases you may need to connect MC251 directly to your computer to access its web interface and change settings. Connect the device with an Ethernet cable to the LAN port on your PC, then configure your network adapter as described below.
The following example shows how to configure a Windows PC (Windows 7/8/10/11) to communicate directly with MC251 using factory settings. For other operating systems, configure your client device’s addressing according to its documentation.
How to access network adapter settings on Windows 7 / 8 / 10
Log in as administrator
Select Start → Control Panel
If items are categorized, select Network and Internet
Go to Network and Sharing Center
In the left sidebar, select Change adapter settings
How to access network adapter settings on Windows 11
In the Connections window, select the desired network adapter. If multiple connections appear, choose the correct one by adapter name or consult your system administrator.
Right-click the connection icon and select Properties. The Properties window opens (Fig. 1).
Fig. 1 – Connection properties window in Windows
Fig. 2 – TCP/IPv4 properties window in Windows
Select Internet Protocol Version 4 (TCP/IPv4) from the component list. Ensure it’s enabled (checkbox is checked). Click Properties. The TCP/IPv4 properties window opens (Fig. 2).
Select Use the following IP address
In the IP address field, enter an address in the range 192.168.0.1 to 192.168.0.254 (excluding 192.168.0.111, which MC251 uses)
In the Subnet mask field, enter 255.255.255.0
Leave Default gateway, Preferred DNS server, and Alternate DNS server fields blank or unchanged
Connect MC251 to a local network router with DHCP addressing enabled
Ensure the router is connected to your Internet service provider (ISP)
Do not connect MC251 directly to the ISP’s cable
This setup enables outgoing connections (e.g., VPN connection to Overvis cloud server, connections to other servers using hostnames or static IP addresses).
graph LR
A[MC251] -->|Ethernet| B[Router]
B -->|WAN| C[Internet/ISP]
B -->|LAN| D[Local Network]
C --> E[Overvis Cloud Server / Remote Modbus TCP Devices]
D --> F[Modbus TCP Devices]
Fig. 3 – MC251 Internet connection topology via Ethernet with DHCP
To access the device from the Internet via incoming connections (direct TCP connection or web interface):
Acquire a Static IP: Get a dedicated line with a static IP address from your ISP
Connect the ISP cable to the router’s uplink port (usually color-coded, see router documentation)
Use a straight-through Ethernet cable to connect MC251 to the router
Configure Router: Following your ISP’s recommendations, configure the router for Internet access using the router documentation.
Set Up Port Forwarding: Configure the router to redirect queries from your static public IP address to MC251’s local IP address (factory default: 192.168.0.111)
For web interface access: redirect to port 80 on MC251’s local address
For Modbus TCP or tunnel access: redirect to the incoming Ethernet connections port (factory default: 502).
graph LR
A[Internet Client] -->|Public IP:Port| B[Router]
B -->|Port Forward| C[MC251]
C -->|RS-485| D[Modbus Devices]
Fig. 4 – MC251 incoming connections via port forwarding
Configure DHCP Reservation: Either:
Configure the router to always assign the same IP address to MC251 via DHCP, OR
Disable DHCP in MC251 settings and use a static IP
This ensures port forwarding works correctly.
Verify Security: Confirm that MC251’s Internet connection is protected by standard security measures (see Connection Security)
Access the Device: When accessing MC251 from the Internet, use the static IP address provided by your ISP and the port specified in your port forwarding configuration
Obtain SIM Card: Get a SIM card from your GSM operator
Install Hardware: Insert the SIM card into MC251 and connect the appropriate antenna to ensure proper signal strength at the MC251 location
Verify SIM Recognition: After starting MC251, confirm the SIM card is correctly identified
After communication initialization, the GSM indicator should blink continuously
If the indicator remains off for more than 4 seconds, check the SIM card, antenna, and GSM signal level on the MC251 display
Verify Internet Connection (if using GPRS/LTE): Ensure the operator settings are correct
When connected to the Internet, the GSM indicator flashes 3 times per second
If flashing less frequently, check the APN settings
Configure APN: If necessary, manually configure the APN according to your operator’s recommendations (see APN Configuration Examples below)
This setup enables outgoing connections (connecting to Overvis cloud server, or to other servers with static IP addresses on the Internet).
graph LR
A[MC251] -->|GSM/LTE| B[Cellular Network]
B --> C[Internet]
C --> D[Overvis Cloud Server / Remote Modbus TCP Devices]
A -->|RS-485| E[Modbus Devices]
Fig. 5 – MC251 Internet connection topology via GSM/LTE
MC251 provides several security features to protect against unauthorized access.
Access for writing and/or reading via Modbus or SMS can be deactivated in settings.
Device settings can be changed remotely by entering a password (minimum 5 characters).
Access passwords can be set for restriction of writing and/or reading via Modbus or SMS.
When entering the password, all settings are only available to the specific client using the specific protocol.
In case of no requests from the client over a long period, the access returns to locked mode.
To link the MC251 to your Overvis account, use the registration information from your MC251 label:
Method 1: Use PIN/QR code from the device label
Method 2: Manually enter and follow the link from the label
sequenceDiagram
participant MC251
participant Overvis
participant User
MC251->>Overvis: Connect & send handshake (unique key)
Overvis->>MC251: Send VPN configuration
MC251->>Overvis: Connect to VPN & send handshake
Overvis->>MC251: Confirm handshake
Note over MC251,Overvis: Connection established, waiting for binding
User->>Overvis: Scan QR code / Enter PIN URL
User->>Overvis: Login to account
User->>Overvis: Select "Create endpoint" option
User->>Overvis: Submit PIN in "Create Endpoint" form
Overvis->>Overvis: Find matching connection IP by device key
Overvis->>MC251: Send temporary authorization code to MC251
MC251->>User: Temporarily show the authorization code
Overvis->>User: Ask to prove physical access (enter the authorization code displayed by the device or press the button on the device)
User->>Overvis: Enter authorization code
Overvis->>Overvis: Link PIN to device key address
Overvis->>User: Create network under user's account
loop Data Communication
User->>Overvis: Request device data
Overvis->>MC251: Read registers (via established VPN connection)
MC251->>Overvis: Return data
Overvis->>User: Display data
end
Connection Flow: MC251 initiates outbound connection to Overvis VPN and maintains it. User registers by linking the PIN to this existing connection. No port forwarding required.
Prerequisites:
MC251 connected to Internet (via Ethernet or GSM)
QR code label on device (or PIN code from label)
Overvis Cloud account (or create during setup)
Connection Steps:
Verify Internet Connection
Ensure MC251 is connected to the Internet. Check the device display for a valid IP address — the prefix (E) indicates Ethernet connection, (G) indicates GSM. Address should not be 0.0.0.0.
Verify Overvis VPN Connection
Make sure MC251 is connected to the cloud VPN. Check the device display for a valid IP address under the Wireguard word — the prefix (E) indicates Ethernet connection to the VPN, (G) indicates GSM.
Access Overvis Server
Scan the QR code on the device label with your phone or tablet, OR manually enter the URL from the label (format example: https://c.overvis.com/ABCD1234). The link automatically redirects to Overvis server login page with the PIN embedded in the URL.
Login or Create Account
If you have an account, enter your credentials. New users should register for a free account first.
Create Endpoint
After login, Overvis displays the Workspace page. If you came from the QR code/link, the Create endpoint form will be displayed with the PIN pre-filled automatically. Otherwise, select Manual setup, Endpoints and press the Create endpoint button. Enter the PIN from the device label. Give your endpoint a descriptive name (a “endpoint” represents your MC251 plus all connected Modbus devices). Click Create to verify MC251 is online.
Authenticate access
After checking MC251 is online, Overvis displays the Confirmation form. Prove you have physical access to the device by entering the temporary code displayed by MC251, or shortly pressing the button on its front panel.
Configure Network & Verify Operation
Ask AI copilot by pressing the yellow star AI button on how to add the devices connected to MC251, configure them and display the data in convenient visual form.
The only method of connection MC251 to other cloud servers and SCADA systems is the direct connection method via Modbus TCP.
Configure MC251 in the monitoring system as a remote Modbus TCP device.
This requires setting up port forwarding on your router to redirect external connections to MC251’s local IP address and Modbus TCP port (default: 502).
Your server will initiate connections to MC251 through your router, similar to Method 3 described above.
You’ll need a static public IP address from your ISP, or alternatively, you can use a secure WireGuard VPN tunnel to bypass the need for public IP exposure.
Since MC251 uses the standard Modbus TCP protocol for direct connections, no special server software is required — any Modbus TCP client can communicate with the device.
See the Modbus Interface documentation for complete register mappings and communication protocols.
The DHCP server is not responding. Wait up to 60 seconds for MC251 to switch to its static IP 192.168.0.111. If the issue persists, verify the network cable is properly connected and check your router’s DHCP settings.
Problem: Cannot access MC251 web interface
Verify your client device is on the same subnet as MC251. Check firewall settings on your client device and confirm the MC251 IP address shown on the display. You can press the R button briefly to grant temporary password-free access.
Problem: LAN LED is off
Check the Ethernet cable connection and verify it’s not damaged. Try using a different cable to rule out cable faults.
Problem: GSM LED stays off
The SIM card may not be inserted correctly—remove and reinsert it. Check that the SIM card PIN is disabled (disable it using a phone before inserting). Verify the antenna is connected properly. Poor signal strength may require relocating MC251 or using an external antenna.
Problem: GSM LED blinks slowly (every 1.5s) but no Internet
This indicates incorrect APN settings. Verify the APN configuration with your operator. Check that your SIM card has an active data plan and review the APN configuration in MC251 settings.
Problem: Cannot receive SMS commands
Verify the SIM card phone number is correct. Check the SMS format according to the Modbus Interface documentation. Ensure SMS service is enabled on the SIM card and verify any SMS passwords are configured correctly.
Problem: SRV LED stays off
MC251 is not connected to the Internet — check your Ethernet or GSM connection. Check that the server address and port are correct in settings. Your network firewall may be blocking outgoing connections.
Problem: “Device already registered” error
The device is bound to another Overvis account. Use the Restart Activation button in the Cloud settings page to unbind it, or contact the previous owner to remove the device from their account.
Problem: Intermittent connection drops
Check that the power supply voltage is within the required range (9-30V DC) and can provide sufficient current (up to 500mA). Look for electrical noise or interference sources near the device. Update MC251 firmware to the latest version.
Problem: RS-485 devices not responding
Verify the RS-485 wiring is correct (A and B terminals). Check that the RS-485 bus termination is properly configured. Verify Modbus RTU/ASCII settings match the connected devices. See the Modbus Interface documentation for detailed communication settings.
Community: Check the user forums for solutions from other MC251 users
For warranty service or hardware issues, please contact your authorized distributor or the manufacturer directly.
z
MC251 Documentation
User Web Interface
The Overvis MC251 has a built-in web interface for quick-start configuration of general options.
You can access this interface using any standard web browser connected to the same network as the device.
Further configuration and management using the full web interface are available at the cloud server.
After logging in, the Quick setup provides a real-time overview of the device status, allows to configure clock and Internet, and provides firmware update option (if update is available).
Fig. 2 - The quick setup page
The quick setup displays:
TCP/IP interfaces status
Current system time
Cloud connection and activation
The quick setup allows the management and configuration for:
System Time: Configure the internal clock.
Ethernet: Configure IP addresses, subnet, gateway, and DHCP.
GSM: Configure APN settings, roaming options and PIN code.
Cloud VPN Connection: Select the cloud VPN connection method and server address.
The Search Devices button allows to list the slave devices connected via serial RS-485 interface.
Fig. 6 - The Search Devices tool options
The search covers all the Modbus unit addresses.
The search can also cover RTU or ASCII framing and commonly used configurations (baudrate, parity and stopbits), or some of the parameters can be set fixed (to filter and speedup the search).
The search can be optionally simplified to stop matching the configuration once the first device answers on the RS-485 bus.
If there is a set of devices with the same serial settings (or a single slave device on the bus), the Fast search option can be flagged.
If the number and configuration of the devices are unknown, it is recommended to run a thorough search once (can take from half an hour to an hour).
Fig. 7 - The devices search progress
After the search has been started, its progress and the current action are displayed, along with the list of the found devices.
For each device, there are shown:
Modbus unit address (UID);
the MC251 configuration which has matched;
the data or the exception code returned by the device.
The exception code during the search does not mean a certain device problem (in most cases the device just didn’t have the Modbus resources requested). The exception still means the device has received the MC251 request and has answered.
The search can be stopped by pressing the stop button or by closing the search tool.
The MC251 configuration is left unchanged by the search. If the needed device configuration differs, either it has to be configured to match the MC251, or the MC251 serial settings have to be manually matched with this device.
The Send Request button allows to check the MC251 configuration or to check the available Modbus devices (both at RS-485 and at remote TCP servers).
Fig. 8 - The Send Request tool options
The MODBUS request (with device address byte, but without any other headers or checksums) can then be entered as a hexadecimal string of bytes.
The request can also be built by specifying a device Modbus address (UID), function to call and function parameters. These include address (e.g. holding register address) and count of items to read (or a single value to write).
The Build request button would convert the specified parts into a hexadecimal string below. This should be used before sending the request.
Fig. 9 - The Modbus request results
After the request has been sent, the answer wait message and then the results are displayed.
These could be one of the following:
The factory default configuration includes no monitoring task.
If the memory card is present, the monitored parameters can be added to the list on the right.
Each parameter has an address on a Modbus device and the value type (the way it would be converted from the device registers).
The optional comment can be added.
The Save list button allows to generate the automation task /TASKS/PARAMLOG.TXT and then reload the tasks.
The saved parameters log files would be displayed on the left of the page.
A folder would be created for each month. Selecting the folder allows to expand it and show the files.
The needed file can then be downloaded by pressing the corresponding button to the right of it.
The Delete list button allows to remove the automation task and then restart the tasks for the changes to take effect. This does not affect the data logged.
The Reread list button allows to cancel any unsaved changes and read the list from the automation task /TASKS/PARAMLOG.TXT.
The factory default configuration includes no logic tasks.
If the memory card with taskfiles in the TASKS folder is present, the tasks would be read into the internal memory (see Logic Programming).
This page indicates the result of reading the folder TASKS, including the number of discovered files.
If during reading and verifying the program errors were detected, then it indicates the type of error, file and line number of the file error. If the program consisted of several files in the folder TASKS, then the internal memory will read all files except those in which errors are detected. Therefore, during error correction you should reread tasks, to clear again the internal memory.
The successfully read files are displayed below in the logic tasks table.
The original taskfile name, the task programmed period of reruns, the task last run time, and the last run error, if any, are displayed for each of the tasks.
If the task was recently run, the last run time is displayed in green.
If the task hangs or otherwise misses its scheduled runs, the last run time is displayed in red.
The Reload Tasks button allows to clear the internal tasks memory and then reload the tasks (provided the memory card is present).
Folders can be opened and items can be selected in the tree view of the card contents.
There are options for a folder to re-read its contents, create a new subfolder, upload a file from the local device, remove a folder (provided it is empty).
Files have options for download or removal.
The Eject Memory Card button allows to safely remove the card.
If the card is not removed within 30 seconds, it would be automatically re-mounted.
The Overvis MC251 operates as a Modbus gateway, listening for Modbus TCP connections on port 502 (configurable). It supports connections from standard Modbus TCP client applications. Windows client software for basic testing is available for download here.
Upon receiving a connection request, the MC251 checks its list of active clients. If the maximum list size (as per technical specifications) has not been reached, the new client is accepted.
Once connected, the MC251 processes Modbus requests from the client. In RS-485 Slave Mode, it also accepts requests from a Modbus Master on the RS-485 bus.
The device analyzes each request based on the requested function and the client’s access rights (determined by passwords entered).
Blocked Requests: If a request is blocked due to insufficient rights, the MC251 sends back a Modbus exception (default code 1).
Internal Requests: If the request addresses the MC251 itself, it is processed internally, and a reply is sent back.
Redirection:
RS-485 Master Mode: Requests for other devices are converted (Modbus TCP/RTU/ASCII) and redirected to the RS-485 bus. The RS-485 indicator lights up while waiting for a response.
Remote Server: If configured, requests can be redirected to one or several remote Modbus TCP servers via Ethernet or GSM/LTE.
If a valid response is received, the MC251 forwards it back to the client.
Exception Illegal Function (Code 1) is returned for a request: Sent either by MC251 (if insufficient rights for the request) or by the target device (if the request unsupported). Check the MC251 settings, Administration tab for protection settings. Send the correct password to the MC251 before using this request. Try allowing the requests.
Exception Gateway Path Unavailable (Code 10 or 0x0A) is returned for a request: Sent if the request cannot be redirected. Check the connection to the remote is configured and established (if the target should be requested using Modbus TCP). Check the Modbus Master mode is configured (if the target should be requested using Modbus RTU/ASCII). Check the Modbus address ranges on MC251 Modbus settings tab.
Exception Target Device Failed to Respond (Code 11 or 0x0B) is returned: Sent if no response is received from the target device. Check the device settings match the MC251 Modbus tab configuration. Check the target device is powered on.
Some exception (not 1, 10 or 11) is returned: Sent by the target device if it could not perform the request or produce the response data. Check the device manual. Try some other request.
The data has been acquired, but the values seem wrong: Check the device manual. Ensure the Modbus device address and Modbus register address in the request are correct. Check the data formatting in the Modbus client software (which is sending this request). Try dividing the acquired value by 10, 100 or 1000.
Integer number (16 bit) in standard range of Modbus register values
1
Number
-32768 – +32767
Integer number (16 bit) in complementary arithmetic
1
Number
0 – 4294967295 in two registers, MSB part – first
Integer number, which value can exceed the limit for Modbus register (65535)
2
Number
-2147483648 – +2147483647 in two registers, the MSB part is the first
Integer number (32 bit) in complementary arithmetic
2
Character string
In every register – number of 0 to 255 - ASCII character code or 0 (the end of string)
A set of values, each of which is equal to the code of one character in the ASCII encoding. If the string is shorter than the maximum length, the code 0 is placed after the last character.
Max. length of string for this parameter
IP-address (IP-mask)
In every register – one byte (0 – 255)
Set of four byte of address IPv4, from left to right
4
MAC-address
In every register – one byte (0 – 255)
Set of six byte of address MAC-48, from left to right
6
Table 3 - Parameter Groups
Group
Description
Access
Address
Device description
Device and firmware identification
Any mode, read only
0 – 3
Current mode
Access and general commands controls
Password entry is available in any mode, command entry - only in Setup Mode (after password entry)
100 – 120
Current status
Interfaces and software modules operation, current time and statistics
Any mode, read only
121 – 295, 900 – 944, 2000–2023
Changeable settings
Interfaces and features selection and configuration
Only in Setup Mode, reading or writing
300 – 899,
Active settings
The configuration being used by the device at the moment
In any mode, read only
2300 – 2899,
Saved settings
This set is saved regardless the power of the device and is used at restart
Only in mode of setting, read only
3300 – 3899,
Clock setting
See Table 5.8.
Only in clock setting mode, for reading or writing
If the dynamic addressing is switched off or not available, IP-address of the device in Ethernet network is equal to this value
300 – 303
Subnetwork mask
IP-mask
255.255.255.0
It is used only with static IP-address
304 – 307
Gateway
IP-address
192.168.0.1
It is used only together with static IP-address for communication with other networks, or as an address of DNS/DHCP servers
308 – 311
Switch on the dynamic addressing with a help of DHCP
0 – 1
1
0 – for addressing in Ethernet, the specified values of the IP address, mask and gateway are used;
312
1 – If DHCP server is available in the network, then IP address, mask and gateway are received from the server
DHCP behavior options
0 or 6
6
0 – standard DHCP address keeping;
313
6 – renew DHCP leased address more often, and keep the old address until the new one has been leased
Switch on the use of server gateway DNS
0 – 1
1
It is used if DHCP is not available (switched off):
314
0 – DNS of gateway is not used;
1 – The gateway DNS is used to determine the IP addresses of other servers, if they are specified by host names
IP-address of DNS server
IP-address
8.8.8.8
It is used if DHCP is not available (switched off);
315 – 318
When the server of DNS gateway is used, it sets IP-address of additional DNS server
IP-address of additional DNS server
IP-address
0.0.0.0
It is used if DHCP is not available (switched off);
319 – 322
0.0.0.0 – it is not used
Unused parameter
0
0
Should be 0 for compatibility
323 - 329
GSM network
PIN-code of SIM-card
0 – 65535
65535
0 – 9999: this code is used for the SIM card if it demands the PIN code;
330
Other values: the code isn’t used; SIM card and GSM are unavailable if the card demand a code
Enable automatic detection of GSM operator’s APN
0 – 1
1
0 – connection is established by manually specified APN parameters;
331
1 – APN is automatically determined for the operator by SIM card ICCID code
Enable roaming exchange
0 – 1
1
0: GPRS/LTE is blocked in roaming;
332
1: GPRS/LTE may be used in roaming
Enable SMS sending while in roaming
0 – 1
0
0 – SMS can be only received in roaming;
333
1 – SMS can be received and sent in roaming
Connection port via GSM
0 – 65535
0
It is used for external connection to the device via GSM with static IP, for communication using the Modbus TCP protocol or in tunnel mode.
334
0 – it is disabled
Enable GSM module
0 – 1
1
0: GSM module is off;
335
1: GSM module, SMS and GPRS/LTE link are kept on
Parameter is not used
0
0
It is not used; it should be equal to 0 for compatibility
336 – 351
APN log-in of GPRS/LTE service
Character string
Provided by the GSM service provider; up to 40 characters
352 – 391
APN password of GPRS/LTE service
Character string
Provided by the GSM service provider; up to 24 characters
392 – 415
APN address of host
Character string
Provided by the GSM service provider; up to 34 characters; there cannot be spaces in a string
416 – 449
TCP Server
Connection port via Ethernet
1 – 65535
502
It is used for external connection to MC251 via Ethernet for exchange via Modbus TCP protocol or in tunnel mode
450
Disconnect inactive clients
0 – 1
1
0 – incoming TCP connection is kept regardless of the time between requests from the client;
451
1 – disconnect clients that did not send requests for longer than a specified time
Max. request waiting time, s
0 – 600 000
90
Used if disconnection of inactive clients is selected
452 – 453
Unused parameter
0
0
Should be 0 for compatibility
454 - 456
Own Modbus-identifier of MC251
0 – 247
111
0 – all queries are sent via Modbus to the Modbus network, the device registers are unavailable by Modbus;
457
1 - 247 – the device responds Modbus queries with this Modbus identifier without forwarding them on
RS-485 network
Bit rate via RS-485, bit/sec
75 – 230 400
9 600
It is used in case of data exchange between the devices via RS-485, the same value for the devices on the same RS-485 bus cable
458 – 459
Quotient for Modbus RTU silence time between frames
0 – 5
1
It is used for transmissions via RS-485 in Modbus RTU mode. During response reception, if the pause between the bytes is longer than the silence time, the frame is considered complete.
460
0 - standard silence time (depends on the transmission speed and is equal to the transmission time of 3.5 bytes, or 1.75 ms for speeds above 19200 bps)
1 - 5: N quotient for a prolonged silence time multiplied by 2^N
Byte format when transmitting via RS-485
0 – 5
5
It is used in case of data exchange between the devices via RS-485. See Table 7.1 for formats.
461
Waiting time for starting the Modbus RTU response, ms
0 – 60 000
200
It is used for transmissions via RS-485 in Modbus RTU mode. After transmission of query, if the first byte of the response was not received within this time interval, the waiting for the response is terminated. The response is always waited for at least the silence time between frames
462
Enable ASCII exchange mode in Modbus network
0 – 1
0
Exchange mode via RS-485, the same value for all units on the same RS-485 bus cable.
463
0 – RTU exchange mode (format: 1 start bit, 8 data bits, 2 stop-bits, parity bit, and stop bit or only 1 stop bit – total from 10 to11 bits);
1 – ASCII exchange mode (format: 1 start bit, 7 data bits, 2 stop-bits or parity bit and stop bit - total is 10 bits). The non-standard byte formats (register 461, values 4 and 5) are not available in this case, format 3 (2 stop bits) is used instead
Response time for subsequent Modbus ASCII character, ms
0 – 60 000
1 000
It is used in case of data transfer via RS-485 in Modbus ASCII mode. If you receive a response, if the next byte of the response was not received within this time interval, then the response waiting is stopped. Waiting is always not less than the transmission time of one character (depends on the transmission speed)
It is used to access the configuration mode. The string of 5 to 10 characters in length can be indicated as password. This string should not have any spaces
510 – 519
Parameter is not used
0
0
It is not used; it should be equal to 0 for compatibility
520 – 529
Password for writing permission using incoming SMS
Character string
specified on the device label
It is used to verify the authenticity of incoming SMS with request for record or with acknowledgment of the fault. The string of 3 to 10 characters in length can be specified as password. This string should not have any spaces
530 – 539
Password for reading permission using incoming SMS
Character string
specified on the device label
It is used to verify the authenticity of incoming SMS with request for reading or with acknowledgment of the fault. The string of 3 to 10 characters in length can be specified as password. This string should not have any spaces
540 – 549
Password for writing permission via Modbus to the other devices
Character string
It is used to access devices connected to the MC251, to request write or control functions that can change the status of these devices. The string up to 10 characters in length can be specified as password. This string should not have any spaces
550 – 559
Password for reading permission via Modbus
Character string
It is used to access devices connected to the MC251, to request read functions, or to access the MC251 registers, except for registers of version, password, mode and tabs. The string up to 10 characters in length can be indicated as password. This string should not have any spaces
560 – 569
Enable the protection mode against writing via SMS
0 – 1
0
0 – Protection against recording is regulated with help of other parameters (password);
570
1 – Blocking of queries via SMS for function of writing
Enable the protection mode against reading via SMS
0 – 1
0
0 – Protection against reading is regulated with help of other parameters (password);
571
1 – Blocking of queries via SMS for function of reading
Enable the protection mode against writing via Modbus
0 – 1
0
0 – Protection against recording is regulated with help of other parameters (password) or deactivated;
572
1 - Blocking of any queries for functions, excepting functions of Modbus 1, 2, 3, 4, 7, 17, 20
Enable the protection mode against reading via Modbus
0 – 1
0
0 – Protection against reading is regulated with help of other parameters (password) or deactivated;
573
1 – Blocking of queries for functions of Modbus 1, 2, 3, 4, 7, 17, 20, excepting reading using function 3 of registers of version, mode and tabs
Parameter is not used
0
0
It is not used; it should be equal to 0 for compatibility
574
Miscellaneous
Parameter is not used
0
0
It is not used; it should be equal to 0 for compatibility
575 – 630
Restart time, min
5 – 7 200
120
Used when automatic restart is enabled.
631
Automatic restart mode
0 – 2
0
It is used when automatic restart is enabled:
632
0 – the automatic restart is disabled;
1 – the device is restarted after a specified period of time since the start;
2 – the device is restarted after a specified period of time since the last transmission via Ethernet or GSM networks.
Modbus exception code generated when access is denied
0 – 255
1
0 – if the access to Modbus registers is denied, the response to the client is not returned;
633
1 – 255 – if you deny access to the client who sent the request, this exception code is returned
Modbus exception code generated when there is no response
0 – 255
11
0 – if there is no response from the addressee (Gateway Timeout), the response to the client is not returned;
634
1 - 255 – if there is no response from the request recipient, this exception code is returned to the client
Parameter is not used
0
0
It is not used; it should be equal to 0 for compatibility
635
Modbus exception code generated if there is no connection to query addressee
0 – 255
10
0 – If there is no connection to the query addressee (Gateway Path Unavailable), response is not returned to the client;
636
1 – 255 – if there is no connection to the query addressee, this exception code is returned to the client
RS-485 transmission mode
0 – 2
0
0 – Master mode (Modbus Master): RS-485 is used to send queries;
637
1 – Slave mode (Modbus Slave): RS-485 is used to receive queries from additional client;
2 – tunnel mode, used to transmit data “as is”, without protocol verification
First Modbus-identifier of RS-485
1 – 255
1
Two parameters define a range of Modbus identifiers used for RS-485.
638
In the master mode the queries with addresses in this range (and also the broadcast ones with address 0) are sent via RS-485.
In the slave mode the queries with addresses in this range
(and also the broadcast ones and the queries to MC251 address) are received via RS-485
Last Modbus-identifier of RS-485
1 – 255
255
same as above
639
Connection to the first remote TCP server
IP-address of the remote server
IP-address
192.168.0.112
It is used with enabled redirection of queries to TCP remote server. IP-address of the remote server wherewith connection is maintained
640 – 643
Port of the remote server connection
0 – 65535
502
Port of the remote server to which the TCP connection will be established
644
Time to wait for response from remote server, ms
0 – 60 000
1 000
It is used during redirection of queries to the remote server. After the query transfer, if the correct response failed to be received within this time interval, response waiting is stopped
645
Standby time to repeated connection to the remote server, s
0 – 240
20
It is used during redirection of queries to the remote server. After connection with the server is lost, the repeated connection will be performed after preset standby time
Preset month for transition to day-light saving time
1 – 12
3
It is used if you selected the automatic transition to daylight saving time on the specified days. The month when the clock will be set one hour ahead
701
Preset week of the month for transition to daylight saving time
1 – 10
10
It is used if you selected the automatic transition to daylight saving time on the specified days. Week of the month when the clock will be set one hour ahead.
702
1 – 5 – week of the month, including the part weeks;
other values – the last week of the month
Preset day of the week for transition to daylight saving time
1 – 7
7
It is used if you selected the automatic transition to daylight saving time on the specified days. The day of the week when the clock will be set one hour ahead
703
Preset hour for transition to day-light saving time
0 – 22
2
It is used if you selected the automatic transition to daylight saving time on the specified days. The hour of the day at which the clock will be set one hour ahead
704
Preset month to revert to standard time
1 – 12
10
It is used if you selected the automatic transition to daylight saving time on the specified days. The month when the clock will be set one hour back
705
Preset week of the month to revert to standard time
1 – 10
10
It is used if you selected the automatic transition to daylight saving time on the specified days. Week of the month when the clock will be set one hour back.
706
1 – 5 – week of the month, including the part weeks;
other values – the last week of the month
Preset day of the week to revert to standard time
1 – 7
7
It is used if you selected the automatic transition to daylight saving time on the specified days. The day of the week when the clock will be set one hour back
707
Preset hour to revert to standard time
1 – 23
3
It is used if you selected the automatic transition to daylight saving time on the specified days. The hour of the day at which the clock will be set one hour back
708
Sunrises and sunsets calculation
Sun day
0 – 3
1
0 – official;
709
1 – civil;
2 - nautical;
3 - astronomical;
Latitude, degree
0 – 89
46
The absolute value of the latitude
710
Latitude, minute
0 – 59
29
711
Latitude, second
0 – 59
10
712
Longitude, degree
0 – 179
30
The absolute value of the longitude
713
Longitude, minute
0 – 59
43
714
Longitude, second
0 – 59
40
715
Quadrant
0 – 3
0
0 – N latitude, E longitude;
716
1 – N latitude, W longitude;
2 - S latitude, E longitude;
3 - S latitude, W longitude
Connection to the service servers
NTP server connection mode
0 – 4
0
0 – clock synchronization with the server is not used;
717
1 – to connect to the servers using Ethernet or GSM, preferably via Ethernet;
2 – to connect to the servers using Ethernet or GSM, preferably via GSM;
3 – to connect to the servers only via Ethernet;
4 – to connect to the servers only via GSM
Time period of connection to NTP servers, h
1 – 240
24
It is used if you have enabled synchronization of clocks with the server clock. The time interval over which the server time is received
718
Minimum shift of clock for synchronization, s
1 – 180
2
It is used if you have enabled synchronization of clocks with the server clock. The synchronization is performed after receiving the server time, if the difference between the clocks is no less than this value
1 – to connect to the servers using Ethernet or GSM, preferably via Ethernet;
2 – to connect to the servers using Ethernet or GSM, preferably via GSM;
3 – to connect to the servers only via Ethernet;
4 – to connect to the servers only via GSM
Parameter is not used
0
0
Not used, must be equal 0 for compatibility
721 – 723
Parameter logging
Minimum supply voltage for safe removal of memory card, mV
0 – 24 000
9 000
If the supply voltage is below the specified value, the memory card will be removed safely. The card can be used again after the supply voltage exceeds the minimum plus 0.5 V.
724
0 – do not remove the memory card, including in a case if the supply voltage is unknown
Format for logging parameters in task files
0 – 7
2
Used if there is a memory card and logging actions in task files.
725
0 – not used
1 – compact file of data bytes;
2 – CSV table with text separator ”;“;
3 – similar 2 with a separator ”,“;
4 – similar to 2 with delimiter - tab character
5 – CSV table without headers, with text separator ”;“;
6 – similar 5 with a separator ”,“;
7 – similar to 5 with delimiter - tab character
Maximum limited size of log files, kB
0 – 65535
1023
Used if there is a memory card present, logging actions in task files, and logging is enabled.
726
The size of the generated files is limited to the specified size plus 1 kB
Minimum stored period in recorder mode, days
0 – 184
184
Used if there is a memory card present, logging actions in task files, and logging is enabled.
727
0-183 – the oldest files (older than the specified number of days ago) can be deleted to write new data;
Other values – old files are saved, new data recording is suspended when the memory card is full
Parameter is not used
0
0
Not used, must be equal to 0 for compatibility
728 – 739
Abonents
Main abonent phone number
Character string
Can be used to send SMS. Up to 20 characters. There can be no spaces in the line
740 – 759
Connection to the second remote TCP server
Remote server IP address
IP address
192.168.0.113
Similar to 640–643
760 – 763
Remote server connection port
0 – 65535
502
Similar to 644
764
Waiting time for a response from a remote server, ms
0 – 60 000
1 000
Similar to 645
765
Waiting time before reconnect-ting to a remote server, ms
0 – 240
20
Similar to 646
766
Remote server connection mode
0 – 8
0
Similar to 647
767
First Modbus identifier of the remote server
1 – 255
1
Similar to 648
768
Last Modbus ID of the remote server
1 – 255
255
Similar to 649
769
Connection to the third remote TCP server
Remote server IP address
IP address
192.168.0.113
Similar to 640–643
770 – 773
Remote server connection port
0 – 65535
502
Similar to 644
774
Waiting time for a response from a remote server, ms
0 – 60 000
1 000
Similar to 645
775
Waiting time before reconnect-ting to a remote server, ms
0 – 240
20
Similar to 646
776
Remote server connection mode
0 – 8
0
Similar to 647
777
First Modbus identifier of the remote server
1 – 255
1
Similar to 648
778
Last Modbus ID of the remote server
1 – 255
255
Similar to 649
779
Parameter not used
0
0
Not used, must be 0 for compatibility
780 – 799
Preset password for access to the first remote Modbus TCP server
Character string
Used only if connection to a remote server is selected and its ID is set (reg. 730). If a password is set, it will be entered immediately after connecting to the server. Obtained access rights depend on the settings of the remote server. A string up to 10 characters long can be specified as a password. No spaces can be in the string
800 – 809
Preset password for access to the second remote Modbus TCP server
Character string
Similar to 800 – 809
810 – 819
Preset password for access to the third remote Modbus TCP server
Character string
Similar to 800 – 809
820 – 829
Parameter not used
0
0
Not used, must be 0 for compatibility
830 – 849
Protocol for incoming Ethernet connections
0 – 1
0
0 – Modbus TCP protocol;
850
1 – tunnel mode, used for data transmission “as is”, without protocol verification
Protocol for incoming GSM connections
0 – 1
0
0 – Modbus TCP protocol;
851
1 – tunnel mode, used for data transmission “as is”, without protocol verification
Current gain. It is set during manual transition to the daylight saving time, when selecting the automatic mode it will be adjusted within 5 minutes. The value must be a multiple of 15
34817
Time zone offset, min
-1440 – +1440
120
It is used during synchronization of the clock with the server clock. The value must be a multiple of 15
34818
Second
0 – 59
The time is to be set at the clock
34819
Minute
0 – 59
34820
Hour
0 – 23
34821
Day
1 – 31
34822
Month
1 – 12
34823
Year
0 – 65534
34824
Set the clock
0 – 65535
0
It is used to set the clock. When recording to this register with any value, the new clock settings in registers 34817 - 34824 will be set
34825
Table 7.1 - RS-485 Byte Formats (Register 461)
Value
Format
Description
0
EVEN
1 parity bit, 1 stop bit
1
ODD
1 parity bit, 1 stop bit
2
0 (SPACE)
1 zero bit, 1 stop bit
3
1 (MARK)
1 unit bit, 1 stop bit (similar to 2 stop bits)
4
ABSENT
No parity bit, 1 stop bit
5
AUTO-STOP
No parity bit, 2 stop bits (tx), 1 stop bit (rx)
Table 7.2 - Cloud Server Connection Modes (Register 465)
Value
Description
0
Connection to server is not used
1
Connect via any interface (prefer Ethernet)
2
Connect via any interface (prefer GSM)
3
Connect only via Ethernet
4
Connect only via GSM
5
Connect to VPN server via any interface (prefer Ethernet)
6
Connect to VPN server via any interface (prefer GSM)
7
Connect to VPN server only via Ethernet
8
Connect to VPN server only via GSM
Table 7.3 - Remote Server Connection Modes (Register 647)
Value
Description
0
TCP remote server is not used
1
Modbus TCP connection via Ethernet or GSM (prefer Ethernet)
2
Modbus TCP connection via Ethernet or GSM (prefer GSM)
Q: What is the default Modbus TCP port and device address?
A: MC251 listens on TCP port 502 (configurable in register 450) and uses Modbus address 111 by default (configurable in register 457). You can view the current IP address by pressing the R button on the device.
Q: How do I enter Setup Mode to change configuration?
A: Write the password (found on the device label) to registers 100-119. If correct, register 121 will read 1 (Setup Mode). You can then modify settings in the Changeable Settings registers (300-899).
Q: I have saved the new settings, how do I restart MC251 remotely?
A: Write command 1 to register 120 to restart. MC251 will reboot and usually be available again in half a minute. You may need to reconnect then.
Q: How do I save my configuration changes?
A: Write command 2 to register 120 to save (then restart MC251 for the changes to take effect), or 4 to save and apply immediately (only for Modbus settings). Write 9 to cancel unsaved changes.
Q: How do I configure RS-485 communication parameters?
Q: What is the difference between Master and Slave RS-485 modes?
A: In Modbus Master mode (default), MC251 sends queries to RS-485 devices. In Modbus Slave mode, MC251 receives queries from an external Modbus Master on RS-485. Use Slave mode when MC251 should act as a slave device on an existing RS-485 network.
**Q: I have configured RS-485 and set MC251 modbus device ID parameter to match my RS-485 device address. But I still get errors like Illegal address.
A: This can happen because MC251 (configured this way) reads its own virtual Modbus device registers instead of redirecting the requests to your RS-485 device. Do not set virtual Modbus ID in the Modbus server parameters to any of your devices addresses. However, reading registers of any device with ID other than this virtual ID (111 by default) should work.
Q: How can I connect the RS-485 device if its protocol differs from Modbus?
A: Select Tunnel mode for RS-485. You may also need to select the tunnel for either the MC251 server or one of the remote servers connections.
Q: How do I send Modbus commands via SMS?
A: Format: [Password] [R/W][DeviceID][H/I/D/C][Address] [Value]. Example: abc r1h100 reads holding register 100 from device 1 (password “abc”). The response SMS contains the request command without the password, and result data or error code.
Q: How do I set the device clock via Modbus?
A: First, write command 35381 to register 120 to enter Clock Setting Mode. Then write time values to registers 34819-34824 (second, minute, hour, day, month, year). Finally, write any value to register 34825 to apply. Write command 35431 to register 120 to cancel.
Q: How can I connect MC251 to multiple remote Modbus TCP servers?
A: MC251 supports up to 3 remote TCP servers. Configure each in registers 640-649 (first), 760-769 (second), and 770-779 (third). The main parameters are the connection mode and the server IP address. Also, set Modbus ID range for each server to avoid extra traffic and delays.
Q: What does the “virtual identifiers” option do for remote servers?
A: When enabled (register 647 for the first remote server, values 5-8), MC251 remaps Modbus addresses before forwarding. The range is renumbered starting from 1. For example, if range is 15-17, address 16 becomes 2 on the remote server. This helps to avoid address conflicts, when multiple gateways share address space, or when multiple Modbus TCP devices of the same model are connected.
Q: How do I reset the device to factory defaults?
A: Enter Setup Mode by writing the password, then write command 444 to register 120. All settings will be reset to factory defaults, including passwords. Restart MC251 for the changes to take effect.
Q: Why can’t I read certain status registers?
A: Some registers (like GSM IP address, register 900-903) are only available after the authentication. Enter the password first. Also, the clock setting registers (34817-34825) require Clock Setting Mode.
Q: How do I check the current firmware version?
A: Read register 1 for the firmware version number.
The IP address of the device in the GSM network is commonly a dynamic one. To access the device via GSM using its IP address, please contact the GSM operator. ↩↩2
The content of these registers is available only in Setting Mode. ↩↩2↩3↩4↩5
Registers for clock setting are available only in Clock Setting Mode (see registers 120, 122). ↩
In virtual identifier mode, before forwarding the request to the remote server, the virtual identifier of the addressee in the request is replaced by the real one so that numbering in the server range starts from 1. For example, for the range 15-17, virtual identifier 16 will be replaced with real 2. Broadcast identifier 0 is processed without changes. ↩↩2↩3↩4
MC251 Documentation
Memory Card Data Saving
The Overvis MC251 supports microSD compatible memory cards formatted as FAT/FAT32.
Only the first volume of the card is used (the maximum usable capacity on the card is 32 GB).
The card can be inserted before powering the device or while the device is in operation.
MC251 uses a memory card for the following actions:
taskfiles operations;
logging of collected data;
diagnostic logging;
export and import of settings;
firmware updates;
navigation on the memory card, downloading and uploading files through the WEB-interface.
When the device is started or when a memory card is inserted, its parameters are checked (it may take up to 3 seconds).
After that, the card can be used for other actions.
When the device is restarted, or when the supply voltage drops below the value specified in the settings (see Modbus Register Map, register 724), MC251 safely ejects the memory card.
Before unmounting, temporary data is saved and files are closed.
MC251 saves the collected data to the log in the LOGS\TASKS folder on the memory card. The order of data collection and conditions for logging are specified in the taskfiles (see Logic Programming).
FolderLOGS/
FolderTASKS/
FolderYYYY_MM/
YYMMDDII.EEE(Log files)
The log files are organized by registration time (UTC+00:00). For each month, a subfolder is created with a name in the format YYYY_ММ, where:
YYYY – year;
ММ – month.
In this subfolder, for each day of the month, a file is created with the name in the format YYMMDDII.EEE, where:
YY – the last two digits of the year;
ММ – month;
DD – day of the month;
II – index;
EEE – file extension CSV or DAT (depending on the settings, see Modbus Register Map, register 725).
The data is appended to the end of the current file. A new file with the next index is created when one or more of the following conditions are met:
the file is filled up to the maximum size (specified in the settings, from 1 KB to 64 MB, see Modbus Register Map, register 726);
an error occurs while writing to the log file,
the card is unmounted or removed, or the device is restarted.
Before creating the file, the presence of files in the subfolder with names for the given day of the month is checked, and the maximum occupied index is found.
Files are numbered starting from 01.
Numeric indices: 01 … 99
Alphanumeric indices: A0 … A9, AA … AZ, B0 … ZZ
In total, up to 1035 indexes can be used for one day of the month. After that, recording is suspended until the date changes.
When the binary log format is selected, the MC251 saves the collected data in a compact form into files with the DAT extension.
Records of a fixed size of 32 bytes are appended to the files.
Each record can contain the value of one parameter or a service message.
Table 1 - Format of the service record in the log data bytes file
Bytes
Field
Range of values
Description
0 – 3
Timestamp
0 – 4294967295
Time in Epoch format: number of seconds since midnight 1.01.1970 UTC+00:00
4 – 7
Service record code
4294967295
Indicator for distinguishing from other types of records
8 – 15
ID
18446744073709551615
All bits 1 for unspecified ID (as the losses are summed for any ID)
16 – 23
Message type
0
0 – losses due to repetitive errors
24 – 31
Message
1 – 4294967295
For loss reporting – the number of bytes
Table 2 - Format of parameter recording in the log data bytes file
Bytes
Field
Range of values
Bits
Description
0 – 3
Timestamp
0 – 4294967295
Time in Epoch format: number of seconds since midnight 1.01.1970 UTC+00:00
4
Parameter type
0 - 49, 128 – 177
0 – 6
Parameter type index: 6 – parameter is the bit (Modbus coil or digital input) other values – parameter is in the registers (Modbus holding or input) (see Parameter Types)
7
0 – parameter is from read/write table (Modbus coil or holding register) 1 – parameter is from the read-only table (Modbus discrete input or input register)
5
Source type
0 or 1
0 - MC251 own virtual Modbus device 1 - Modbus device with ID (address) in range 1 - 255
6 – 7
Parameter address
0 – 65535
starting address of the parameter on the Modbus device
8 - 15
Source ID
1 – 255
address of the Modbus device (unused for the MC251 own virtual Modbus device)
16 – 23
Parameter value
-9223372036854775808 – +9223372036854775807
The value converted to a signed 64-bit integer
24 – 31
0 - 18446744073709551615
Data read from the device before being converted to a parameter value
When the text table log format is selected, MC251 saves the collected data in text form into files with the CSV extension.
Records are text strings with variable length fields.
Fields are separated by the character selected in settings (see Modbus Register Map, register 725).
Lines are separated by CR + LF.
Maximum record length: 100 bytes (without comment), up to 200 bytes (with comment).
Table 3 – Format of records in the file of text tables of the log
Column
Field
Description
1
Timestamp
Date and time in ISO8601 format when the parameter value was received
2
Value
The value converted to a signed 32-bit integer
3
Type
Parameter type name similar to that used in the taskfile (see Parameter Types)
4
Item address
Starting address of the parameter on the Modbus device
5
Item kind
One symbol representing the source data table: H – holding registers; I – input registers; D – discrete inputs; C – flags
6
Source ID
Address of the device (1..255 for Modbus)
7
Source kind
String representing the kind of source: MBSELF – MC251 own virtual Modbus device; MB – Modbus device with ID (address) in range 1 - 255
8
Losses
The number of bytes that could not be written (and were removed from the write queue before creating this record) 0 – no losses
9
Comment
An empty field or comment line if the entry was created by a command of logging with a comment. (it is not recommended to include the table field separator character in the comment)
The time it takes for an empty memory card to be filled, can be calculated using the formula:
Tfull ≈ (Vfree * Tupd) / (Npar * Lsiz)
, where:
Tfull – the time till the card is full;
Vfree – free space on the memory card;
Tupd – task run (parameters update) period;
Npar – the number of parameters to be logged (if the log record is made with each task run);
Lsiz – the size of the log entry (depends on its format).
Examples:
Binary data format: Writing 7 parameters every 40 seconds. A 2 GB card will be filled in ~383,479,222 s (or more than 12 years).
Text table format: Writing 3 parameters 10 times per second (without comments). A 32 GB card can be filled in ~16,361,780 s (or more than 6 months).
MC251 can export the saved settings from the internal memory to the SETTINGS\MC251SET.DAT file, or import the settings from this file and save them to the internal memory.
A: MC251 supports microSD compatible memory cards formatted as FAT or FAT32. Only the first volume is used, with a maximum usable capacity of 32 GB.
Q: Can I insert or remove the memory card while the device is running?
A: You can insert the card while the device is running. However, removing the card during operation may result in data loss. To safely remove the card, wait until after a device reset while the device title is displayed on screen.
Q: What happens if the power supply drops while the card is in use?
A: MC251 monitors supply voltage and safely unmounts the memory card when voltage drops below a configurable threshold (register 724). Before unmounting, temporary data is saved and files are closed to prevent corruption. A high-capacitive power supply is recommended to give enough time for this feature.
Q: Which log format should I use — binary (.DAT) or text (.CSV)?
A:
Binary (.DAT): More compact (32 bytes per record), better for long-term logging with limited card space, but requires parsing tools to read.
Text (.CSV): Human-readable, can be opened directly in spreadsheet software, but takes more space (up to 200 bytes per record with comments).
Q: How long will it take to fill my memory card with log data?
A: Use the formula: Tfull ≈ (Vfree * Tupd) / (Npar * Lsiz). For example, logging 7 parameters every 40 seconds in binary format on a 2 GB card takes over 12 years to fill. Logging 3 parameters 10 times per second in CSV format on a 32 GB card takes about 6 months.
Q: What happens when the memory card is full?
A: It depends on the recorder mode setting (register 727). When enabled, the oldest files are automatically deleted to make room for new data. When disabled, logging pauses until you manually free up space.
Q: How much data can be logged per day?
A: The file size limit is configurable (up to 64Mb). And up to 1035 files per day can be created in the day folder. After all file indices are used, recording is suspended until the date changes.
Q: What happens if a write error occurs during logging?
A: Data remains in a temporary write queue and the device retries for up to 10 minutes. If writing still fails, the data is removed from the queue and the number of lost bytes is recorded for reporting in subsequent log entries.
Q: Can I use the same memory card for multiple MC251 devices?
A: Yes, but be careful with taskfiles — each device will try to load and execute taskfiles from the TASKS folder. Settings export/import files are also shared. Consider using separate cards or carefully managing folder contents if moving cards between devices.
Q: Where are taskfile compilation results stored?
A: In the same TASKS folder alongside your source files:
.MAP files contain token lists from compilation
.OBJ files contain compiled bytecode
.CNF files store taskfile runtime parameters
Q: How do I back up my MC251 settings to the memory card?
A: Export settings to the SETTINGS\MC251SET.DAT file using Modbus commands. This file (up to 16 KB) can be imported later to restore settings or transfer them to another device.
Q: Why does the SD card take up to 3 seconds to become available after insertion?
A: MC251 performs parameter checks on the card after insertion or device startup. This verification ensures the card is properly formatted and usable.
Q: Does SYS.LOG have a size limit and/or get cleaned up automatically?
A: SYS.LOG, ATM.LOG and RESTARTS.LOG are appended to by the device and are not cleaned or removed automatically. They have a file system limit of 2GB.
This HTTP API provides an interface for interacting with the Overvis MC251 device.
All requests are made to paths starting with /api (except for memory card file downloads, which start from /card).
The API supports GET and POST methods, some of which require authorization.
Press the button on the front panel of the MC251 device once.
After this, a user with Guest rights temporarily receives access level 🟨 1 (Power User) for 10 minutes.
Saves cloud registration data to the default settings.
This request is intended for one-time use during device production. Newly programmed devices do not have cloud server connection links filled in. After the registration links and other registration data are saved, the device no longer accepts this request.
POST/api/register/
Request Parameters:
Field
Type
Description
manufacturerName
string / null
Manufacturer name used for registration.
labelName
string / null
Registered device model name.
sku
string / null
Registration model code.
labelLink
string / null
Cloud server device link.
modbusServer
string / null
Cloud Modbus TCP callback server address and port.
discoveryServerUrl
string / null
WireGuard configuration server URL and authorization request base URL.
vpnStaticConfig
object / null
Static WireGuard connection configuration, if used.
endpoint
string
WireGuard server address and port.
endpointPublicKey
string
Public key of the WireGuard server.
deviceIpAddress
string
Device address in the VPN.
allowedIps
array
VPN allowed IP ranges.
persistentKeepalive
integer
Maximum idle period between WireGuard packets.
pinCode
string / null
Unique cloud product code, up to 18 characters, or null if unsaved.
After running, Overvis MC251 starts execution of the operations logic program, if it was placed earlier in the internal memory.
If there is no program in the internal memory, and a memory card is present (formatted as FAT or FAT32), MC251 may load the taskfiles automatically.
Such a reading runs once after startup or after installing a new memory card, but only if the internal memory does not contain a program.
MC251 searches for and checks the taskfiles placed in the folder TASKS on the memory card.
Files can have arbitrary names and extensions (except for .MAP, .OBJ and .CNF) and placed in subfolders of the folder TASKS.
The found correct files are read into the internal memory and they form the operations logic program.
The internal logic memory can be cleared along with the settings reset to factory defaults.
To preserve the settings and clear the internal logic memory only, you should use either Modbus or Web-interface command.
Using Web-interface
access the device WEB interface using the cloud (see Web Interface);
open Tasks tab;
Press the button Reload Tasks and confirm clearing the memory.
Via MODBUS
connect to the device via MODBUS and enter the setup mode (see Modbus Register Map);
to clear the internal logic memory, write the value 40959 into the control command register 120.
Taskfile describes set of actions repeated at a specified time interval.
The actions can include gathering data, processing and comparing the values and special actions after fulfillment of the preset conditions (according to the processing results).
File consists of lines. Each line can be one of the following:
empty;
option;
definition;
action.
Lines can include comments preceded by ; or #. The text after these symbols is ignored.
Lines can start from labels, followed by :. Labels are used for action references (e.g. CALL function1 … function1: RETURN).
Line parts are separated by or TAB symbols.
This option is a set interval between two consequtive runs of the task.
Actual interval depends on the controller load and overall taskfiles complexity (some actions such as parameter read/write or SMS sends can prolong the run execution).
If the next run time comes before the other run has been finished, the new one is delayed and will be executed as soon as possible.
Option value is seconds integer in range 0..2000000.
This options is a modifier for the set interval between two consequtive runs of the task.
If set to non-zero value, set interval is managed with subsecond precision and equals ‘UPDATE’ / ‘UPDATEDIVISOR’.
This option defines forced wait delays between device parameter read/write operations (this does not affect memory parameters, see Parameter definition below).
This allows more time for the other clients requests processing.
The delay is proportional to the time taken by the previous parameter operation.
Option value is percents integer in range 0..100.
100 - no forced delays.
50 - delay time equals the previous operation time.
25 - delay time is 3 times greater than the operation time.
Wait limit for device parameter read/write operations (this does not affect memory parameters, see Parameter definition below).
After timeout expires, operation results in error.
Option value is milliseconds integer in range 0..5000.
String definition is marked by " pair, with the string inside.
String can include special references, enclosed in * (the escape character * should be doubled to be included into the string itself).
Special references include:
VAR(variable) - for a numeric value out of a variable. lasterror variable may be also used here;
ERR(variable) - for a literal representation of an error code name out of a variable. lasterror variable may be also used here;
PHONE(index) - for a number out of a list of GSM abonents in the device settings. Index is a number in range 0 to 4.
There is one pre-defined string: emptystring. This string has zero length.
Modbus device can be used as a parameter source or storage.
Modbus device is defined by one of the following types:
MBWRDENIED - only reading functions may be used;
MBWRSINGLE - single register/coil write functions can be used;
MBWRMULTI - multiple register/coil write functions can be used;
MBWRANY - both single/multiple register/coil write functions can be used.
MBSELF - same as MBWRANY * 100 100, it stands for own MC251 virtual Modbus device.
For MBSELF, no arguments are needed.
The other Modbus device types are followed by 2 or 3 arguments:
Modbus unit address - one of the following: fixed ID in range 1..255, own Controller virtual device ID as *, or indirect ID as a variable plus integer value (e.g. some_variable+10);
read registers/coils per one Modbus request limit, in range 1..125;
the argument is present only for MBWRMULTI, MBWRANY types - write registers/coils per one Modbus request limit, in range 1..125.
Parameters are used to access external data: get data to or from variables.
Parameter is defined by one of the data types.
Table 1 - Parameter types available for any variable type size
No.
Type
Description
0
UINT16
unsigned (non-negative) 16-bit (2 bytes) integer, serialized as Big Endian (most significant byte first, e.g. 258 is stored as 0x01, 0x02);
1
INT16
signed 16-bit (2 bytes) integer, serialized as Big Endian;
2
INT16BLE
signed 16-bit (2 bytes) integer, serialized as Little Endian (least significant byte first, e.g. 258 is stored as 0x02, 0x01);
3
INT32
signed 32-bit (4 bytes) integer, serialized as Big Endian (most significant byte first, e.g. 66051 is stored as 0x00, 0x01, 0x02, 0x03);
4
INT32BLE
signed 32-bit (4 bytes) integer, with bytes serialized as Little Endian (least significant byte first, e.g. 66051 is stored as 0x03, 0x02, 0x01, 0x00);
5
INT32WLE
signed 32-bit (4 bytes) integer, with words serialized as Little Endian (least significant word first, e.g. 66051 is stored as 0x02, 0x03, 0x00, 0x01);
6
BIT
1-bit integer (used for example to access Modbus coils and discrete inputs);
7
INT32BE
same as INT32
8
F32EP0R
IEEE 754 single precision (4 bytes) floating-point, serialized as Big Endian;
9
F32BLEEP0R
IEEE 754 single precision (4 bytes) floating-point, with bytes serialized as Little Endian;
10
F32WLEEP0R
IEEE 754 single precision (4 bytes) floating-point, with words serialized as Little Endian;
11
F32EP1R
single precision (4 bytes) floating-point, serialized as divided by 10 Big Endian;
12
F32BLEEP1R
same as F32BLEEP0R, but divided by 10 before serializing;
13
F32WLEEP1R
same as F32WLEEP0R, but divided by 10 before serializing;
14
F32EP2R
single precision (4 bytes) floating-point, serialized as divided by 100 Big Endian;
15
F32BLEEP2R
same as F32BLEEP0R, but divided by 100 before serializing;
16
F32WLEEP2R
same as F32WLEEP0R, but divided by 100 before serializing;
17
F32EP3R
single precision (4 bytes) floating-point, serialized as divided by 1000 Big Endian;
18
F32BLEEP3R
same as F32BLEEP0R, but divided by 1000 before serializing;
19
F32WLEEP3R
same as F32WLEEP0R, but divided by 1000 before serializing;
20
UINT16BLE
unsigned (non-negative) 16-bit (2 bytes) integer, serialized as Little Endian;
21
UINT8
unsigned (non-negative) 8-bit (1 byte) integer;
22
INT8
signed 8-bit (1 byte) integer;
Table 2 - Parameter types available for 64-bit variable type
No.
Type
Description
23
UINT32
unsigned (non-negative) 32-bit (4 bytes) integer, serialized as Big Endian (most significant byte first, e.g. 66051 is stored as 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x02, 0x03);
24
UINT32BLE
unsigned (non-negative) 32-bit (4 bytes) integer, with bytes serialized as Little Endian (least significant byte first, e.g. 66051 is stored as 0x03, 0x02, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00);
25
UINT32WLE
unsigned (non-negative) 32-bit (4 bytes) integer, with words serialized as Little Endian (least significant word first, e.g. 66051 is stored as 0x02, 0x03, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00);
26
INT64
signed 64-bit (8 bytes) integer, serialized as Big Endian (most significant byte first, e.g. 66051 is stored as 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x02, 0x03);
27
INT64BLE
signed 64-bit (8 bytes) integer, with bytes serialized as Little Endian (least significant byte first, e.g. 66051 is stored as 0x03, 0x02, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00);
28
INT64WLE
signed 64-bit (8 bytes) integer, with words serialized as Little Endian (least significant word first, e.g. 66051 is stored as 0x02, 0x03, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00);
29
F64EP0R
IEEE 754 double precision (8 bytes) floating-point, serialized as Big Endian;
30
F64BLEEP0R
IEEE 754 double precision (8 bytes) floating-point, with bytes serialized as Little Endian;
31
F64WLEEP0R
IEEE 754 double precision (8 bytes) floating-point, with words serialized as Little Endian;
32
F64EP1R
double precision (8 bytes) floating-point, serialized as divided by 10 Big Endian;
33
F64BLEEP1R
same as F64BLEEP0R, but divided by 10 before serializing;
34
F64WLEEP1R
same as F64WLEEP0R, but divided by 10 before serializing;
35
F64EP2R
double precision (8 bytes) floating-point, serialized as divided by 100 Big Endian;
36
F64BLEEP2R
same as F64BLEEP0R, but divided by 100 before serializing;
37
F64WLEEP2R
same as F64WLEEP0R, but divided by 100 before serializing;
38
F64EP3R
double precision (8 bytes) floating-point, serialized as divided by 1000 Big Endian;
39
F64BLEEP3R
same as F64BLEEP0R, but divided by 1000 before serializing;
40
F64WLEEP3R
same as F64WLEEP0R, but divided by 1000 before serializing;
41
F64EP4R
double precision (8 bytes) floating-point, serialized as divided by 10000 Big Endian;
42
F64BLEEP4R
same as F64BLEEP0R, but divided by 10000 before serializing;
43
F64WLEEP4R
same as F64WLEEP0R, but divided by 10000 before serializing;
44
F64EP5R
double precision (8 bytes) floating-point, serialized as divided by 100000 Big Endian;
45
F64BLEEP5R
same as F64BLEEP0R, but divided by 100000 before serializing;
46
F64WLEEP5R
same as F64WLEEP0R, but divided by 100000 before serializing;
47
F64EP6R
double precision (8 bytes) floating-point, serialized as divided by 1000000 Big Endian;
48
F64BLEEP6R
same as F64BLEEP0R, but divided by 1000000 before serializing;
49
F64WLEEP6R
same as F64WLEEP0R, but divided by 1000000 before serializing;
The type is followed by 1 or 3 arguments:
source - one of the following: MEMTEMP, MEMBAT, MEMFILE, MEMFLASH (for memory areas) or a Modbus device defined earlier.
For Modbus device source it is the Modbus data table: one of the following C, D, H, I for coils, discrete inputs, holding registers or input registers respectively. For memory sources it is optional - if specified, it maps the memory parameter to own controller virtual device addresses.
For Modbus device source it is the starting address (either fixed in range 0..65535, or indirect as a variable plus integer value, e.g. some_variable+0). For memory sources it is optional - if specified, it maps the memory parameter to own controller virtual device addresses. Parameter may occupy several addresses depending on its type and size.
Memory sources for the parameters allow to read presets or store settings and temporary values to be processed at consequtive runs.
MEMTEMP - RAM space, data is lost at powerdown or controller reset, small area size but excellent write speed;
MEMBAT - memory area powered by internal clock battery, very limited size but good write speed;
MEMFLASH - controller settings area, data retains at reset, small size and usually read-only;
MEMFILE - extendable file area, requires memory card with the taskfiles to be present, average speed but excellent area size.
The holding registers of the own controller virtual device in range 5000..5999 and 50000..59999, and the input registers in range 50000..59999 are reserved for mapping additional memory sourced parameters.
Variables are used to operate integer values.
All variables have the same type, which is set in taskfile options.
Variables can be mostly defined automatically when first mentioned.
Some operations though (like ‘ISKNOWN’, copying, or array items assignment) require the variable to be defined earlier.
Variables are defined with VAR type.
Variable arrays are defined with VARS type followed by array size in range 1..65535 (array of a single variable is effectively the same as a simple variable).
There is one pre-defined variable: lasterror.
Error occurrences write the error code to it.
It cannot be written like the other variables, but RAISE command effectively writes to it.
This variable can be used in error handlers for error-specific actions.
Conditions can be mostly defined automatically when first mentioned.
Some operations though (like ‘ISKNOWN’, copying or array items assignment) require the condition to be defined earlier.
Conditions are defined with COND type.
Condition arrays are defined with CONDS type followed by array size in range 1..65535 (array of a single condition is effectively the same as a simple condtion).
Actions form iterative logic algorythm. Actions can be one of the following:
assertion;
check;
command.
Actions are executed consequtively with these exceptions:
check actions would skip a single action, if their condition is not TRUE;
command actions like GO, CALL, RETURN would jump to a label or return from the function call;
command action RAISE or any error occurrence would either jump to the error handler or stop this execution run due to an unhandled error;
command action EXIT stops this execution run.
There are 2 labels used as the main taskfile function definition:
run - each time the taskfile run is executed, the action labelled run is performed first (if there is no such label, the first action in the taskfile is the first to be performed);
onerror - if an error occurs with no other handlers specified, a handler labelled onerror is used (if there is no such label, the error would be unhandled, see Error handling below).
The action may in some cases reference the other items which were not defined earlier in the taskfile:
variable arrays and condition arrays should be defined before they are referenced;
evaluations which can take arguments of different types (like direct copying), require arguments to be defined earlier;
other variables and conditions can be referenced without pre-definition;
labels can be referenced by CALL, TRYCALL or GO before they are defined;
strings can be referenced by SENDSMS, SMSRCVD, NOSMSRCVD, SYSLOG or PARAMLOG before they are defined;
in all the other cases the referenced item should be defined earlier in the taskfile.
Assertions are actions producing some result data, which they store in one of the holding items (variable or condition).
Assertions start with a PUT followed by result target item name.
Then comes the function with the proper result type for this target (see Variable evaluation, Condition evaluation below).
If no function is specified, the argument is directly copied into target (this can also be used to get the items from the array indexed by the variable, or put them back).
Checks are actions used for branching.
They skip the next action if their function (with condition result, that is boolean) produces FALSE or an unknown value.
Checks start with IF followed by the function with the condition result type (see Condition evaluation below).
If no function specified, the argument should be a condition name.
Commands are actions producing no result data or discarding it.
Commands are one of the following:
EXIT - stops taskfile execution. Has 1 argument: error name, variable with the error code or lasterror, or OK (if no error).
RAISE - forces an error occurrence. Has 1 argument: error name, variable with the error code or lasterror (for raising the same error higher).
GO - next action will not be performed, execution will continue from the specified label. Has 1 argument: label name.
CALL - starts some user function execution. Next action execution is postponed, execution will now continue from the specified label. Has 1 argument - function entry label name.
TRYCALL - starts some user function with its own error handling. Next action execution is postponed, execution will now continue from the specified label. Has 2 arguments - function entry label label name and error handler label name.
RETURN - ends user function execution. Next action will not be performed, execution will continue from the previously postponed action (by the previously met CALL or TRYCALL).
WRITE - outputs data to the parameter. Has 2 arguments: parameter name, then variable name or an integer value.
PARAMLOG - writes the parameter to the data log with a comment. Has 2 arguments: parameter name, then string name. The parameter must be in Modbus registers or otherwise mapped to Modbus, as its address is used in logs.
PARAMLOGNOCOMMENT - writes the parameter to the data log without a comment. Has 1 argument: parameter name. The parameter must be in Modbus registers or otherwise mapped to Modbus, as its address is used in logs.
SYSLOG - writes the string to the system log. Has 1 argument: the string name.
SENDSMS - sends the string as an SMS. Has 2 arguments: the addressee string, then the SMS text string name. If the addressee strings results in no other characters than 0, the SMS would not be sent.
Most of the following functions take 2 arguments (e.g. a and b) and perform an operation.
ADD - addition (a + b). The first argument is variable, the second can be either variable or an integer value.
SUB - subtraction (a - b). The first argument is variable, the second can be either variable or an integer value.
MUL - multiplication (a * b). The first argument is variable, the second can be either variable or an integer value.
DIV - division (a / b, integer result). The first argument is variable, the second can be either variable or an integer value.
MOD - remainder of division (a - (a / b * b)). The first argument is variable, the second can be either variable or an integer value.
SQRT - square root (integer). Takes a single variable or an integer value argument.
VAL - direct copy. Takes a single argument: variable or an integer value. As with the other direct copying evaluations, function name VAL can be omitted.
Most of the following functions take 2 arguments (e.g. a and b) and perform an operation.
BITSAND - logical AND of every bit of a with the corresponding bit of b. The first argument is variable, the second can be either variable or an integer value.
BITSOR - logical OR of every bit of a with the corresponding bit of b. The first argument is variable, the second can be either variable or an integer value.
SHR - arithmetic shift to the right of the bits of a, b times right. The first argument is variable, the second can be either variable or an integer value.
SHL - shift to the left of the bits of a, b times left. The first argument is variable, the second can be either variable or an integer value.
BITSBIT - bit b copy out of the bits of a. Is equivalent to SHR a b followed by BITSAND a 1. The first argument is variable, the second can be either variable or integer value.
BITSNOT - bits inversion. Takes a single variable or an integer value argument.
MAX - maximal item of the array. Takes a single argument: array of variables.
MAXIDX - index of the maximal item of the array in range of 0 to array size minus 1. Takes a single argument: array of variables.
MIN - minimal item of the array. Takes a single argument: array of variables.
MINIDX - index of the minimal item of the array in range of 0 to array size minus 1. Takes a single argument: array of variables.
SUM - sum of the array items. Takes a single argument: array of variables.
SELECTBY - takes 2 arguments: array of variables and array of conditions of the same size, and selects the variable corresponding to the first condition which is TRUE.
GETUPDPERIODSEC - Task update period in seconds. No arguments.
GETUPDPERIODMSEC - Task update period in milliseconds. No arguments.
GETUPDNAMEIDX - first integer in the taskfile name. This can be used as a base or a factor for parameters mapping, to create several similar tasks with the same contents. No arguments.
Condition evaluation functions are used in either assertions (PUT actions) or checks (IF actions) to determine a condition value.
They can produce TRUE, FALSE or an unknown result (if referring the other unknown item values, see Unknown values below).
All functions come in direct or inversed form, which differ only in result being inverted or not before use.
The functions are listed in both forms, e.g. ‘EQ’ / ‘NE’ for equality and non-equality (inverted EQ) checks.
AND / NAND - logical AND / inverted logical AND (a && b / !(a && b)). Takes 2 condition arguments.
OR / NOR - logical OR / inverted logical OR (a || b / !(a || b)). Takes 2 condition arguments.
VAL / NOT - direct copy / inverted copy (a / !a). Takes a single argument: condition or boolean value (TRUE or FALSE). As with the other direct copying evaluations, function name VAL can be omitted.
SMSRCVD / NOSMSRCVD - checks whether a new SMS starting with a specified text came from a number starting with specified digits. The first argument is a phone number start, the second is an SMS text start. If the phone number is an empty string, SMS from any abonents are checked.
ISKNOWN / ISNOTKNOWN - checks whether the holding item value is known. Takes a single argument: variable or condition (the values are unknown if they are read from an uninitialized parameters or evaluated from the other unknown items, see Unknown values below).
ERREQ / ERRNE - checks whether the variable holds the error. The first argument is variable, the second is the error code name (see Error handling).
Data processing is performed by variable evaluations which store values of integer type.
However, parameters read and write can cast data types from and to fractional values of single and double precision as defined in IEEE 754 standard.
This casting can be done with a multiplier (depending on the parameter type used), to preserve a certain number of decimal digits of the value fractional part.
Some connected devices may also have integer parameters representing a fractional value (pre-multiplied in a similar way as above).
This allows to operate fractional values as integers.
A care should be taken using operations like multiplication, division or square root.
For example, MUL 2 2 = 4, but if the first argument represents 0.2 value multiplied by 10, and the second - 0.02 value multiplied by 100 (fixed point values with 1 and 2 digit precision respectively), the result represents 0.004 multiplied by 1000 (10*100, as the multipliers were also multiplied).
A square root of the previous value (4 standing for a value 0.004 multiplied by 1000), SQRT 4 produces 2, which is a wrong result, because of the multiplier 1000. For SQRT 1000 is not an integer.
To get a correct result, this value multiplier should be adjusted to a nearest multiplier with even number of zeroes, that is 10000. So 4 should be pre-multiplied by 10 (producing 40 for 0.004 multiplied by 10000). Then the SQRT 40 produces 6 which is a correct result (standing for 0.06 multiplied by 100).
Holding items can be organized in arrays of fixed size.
Arrays should be defined before referencing them in evaluations (e.g. DEF some_array VARS 5 or DEF the_other_array CONDS 2).
There are 3 ways of using the arrays:
array items with an integer value index can be used in most of the functions and assertions instead of a single item name as an argument (or an assertion result, e.g. PUT some_array[3] MUL some_array[2] some_array[1]).
array items with variable index can be used: a) either in assertions as a result for a direct copying or a single-argument functions (e.g. PUT some_array[ar_idx] SQRT distance), b) or as an argument for a direct copying (including NOT function which is just an inverted copying of a condition, e.g. IF NOT c_array[counter]).
certain array evaluation functions take arrays as arguments, performing searching or digesting and producing a single variable result.
Some commands like CALL or RETURN result in error due to misusage, which requires program correction.
The other commands or functions result in error due to either wrong argument values (square root of a negative variable) or external reasons (e.g. parameter could not be read or written).
Error occurrence in the function causes execution to interrupt and jump to the function error handler.
This can happen once per function call: if no handler has been provided, or another error occurs in the error handler itself, the execution continues at the caller function handler.
For the main taskfile function, onerror label is used as a default taskfile error handler.
A pre-defined variable lasterror can be used to read error code and perform error-specific actions.
If no caller handler is available, the error is considered unhandled - the default handler is performed similar to this:
Error during the assertion usually causes the result target to have an unknown value.
Unknown values can also appear without an actual error, e.g. when reading an uninitialized parameter or using the variable which has not been asserted.
Unknown values tend to spread, as most functions referring an unknown variable or condition value will result in an unknown value themselves.
Some functions may still determine the result, notwithstanding of unknown items in their arguments.
SELECTBY array function can get the result early (before reaching any of the unknown items later).
AND logical check can result in FALSE if any of its arguments is FALSE (while the other one can have any value), OR can likewise result in TRUE, etc.
Critical commands may require additional checks (ISKNOWN / ISNOTKNOWN) and direct variable assertions, or parameter initializations, to guard against unknown states.
IF check action treat unknown function result as FALSE, and would skip the next command.
This allows to choose between direct and inverted check function to guard the branch against executing in unknown states.
In this example, the program controls the hysteresis value on the second channel of the TR-101 device, depending on the temperature of the sensor on the first channel.
The program uses clock battery-powered memory as a storage of the temperature limits and corresponding hysteresis parameters.
These parameters are mapped to the MC251 own registers to simplify program configuration.
In the taskfile text:
16 is Modbus ID of the device TR-101;
4 is the address of the channel 1 temperature register;
47 is the address of the channel 2 hysteresis reguster;
5500 is the address for the lower temperature limit;
5501 is the address for the upper temperature limit;
5502 is the address for the hysteresis at crossing the lower temperature limit;
5503 is the address for the hysteresis at crossing the upper temperature limit.
Terminal window
@PROTOCOLVERSION12
@UPDATE20#program will run every 20 seconds
@PARAMLOADRATIO50
DEFmc251MBSELF
#TR-101 has Modbus ID 16 and can read no less than 100 registers per a query,
#but write one register per one query
DEFtr101MBWRSINGLE16100
#INT16 – registers with signed integers, as the temperature can be less than 0
DEFt_lowerINT16MEMBATH5500
DEFt_upperINT16MEMBATH5501
DEFt_chan1INT16tr101H4
#UINT16 – as the hysteresis is no less than 0
DEFh_at_lowerUINT16MEMBATH5502
DEFh_at_upperUINT16MEMBATH5503
DEFh_chan2UINT16tr101H47
DEFtemperaturesVARS3
DEFhysteresiVARS3
DEFchecksCONDS3
#program start
run:
#lower and upper temperature limits, as well as its current value
PUTtemperatures[0]READt_lower
PUTtemperatures[1]READt_upper
PUTtemperatures[2]READt_chan1
# hysteresis for temperatures that are below (or above) than the limits
PUThysteresi[0]READh_at_lower
PUThysteresi[1]READh_at_upper
# current hysteresis value
PUThysteresi[2]READh_chan2
# was the temperature out of the limits?
PUTchecks[0]LEtemperatures[2]temperatures[0]
PUTchecks[1]GEtemperatures[2]temperatures[1]
# in other cases - do not change the hysteresis (current value)
PUTchecks[2]TRUE
# the desired hysteresis is selected from hysteresi array
In this example, a program is described that monitors the temperature measured by OB-215.
When the temperature exceeds -15 degrees for more than 10 minutes, it sends an SMS and starts logging the temperature values.
In the taskfile text:
11 is Modbus ID of the OB-215 device;
6 is the address of the register from which the temperature is read.
Terminal window
@PROTOCOLVERSION12
# the program will run every 15 seconds
@UPDATE15
DEFcontrollerMBSELF
DEFob215MBWRSINGLE114
DEFtemperatureINT16ob215H6
# alarm flag - temperature rise for more than 10 minutes
DEFalarm_temp_highUINT16MEMTEMPH5000
# counter for a delay of 10 minutes (600 seconds)
DEFcounterUINT16MEMTEMPH5001
run:
PUTv_tempREADtemperature
PUTv_alarmREADalarm_temp_high
# v_alarm is auto-defined above as a variable
# (by a PUT v_alarm READ construction)
IFISNOTKNOWNv_alarm
PUTv_alarm0
PUTv_counterREADcounter
IFISNOTKNOWNv_counter
PUTv_counter0
PUTtemp_limit-150
IFGRv_temptemp_limit
GOnoticed_temp_high
WRITEalarm_temp_high0
WRITEcounter0
EXITOK
noticed_temp_high:
PUTcounter_limit10
PUTcounter_limitMULcounter_limit10
IFGEv_countercounter_limit
GOtemp_high_too_long
PUTv_periodGETUPDPERIODSEC
PUTv_counterADDv_counterv_period
WRITEcounterv_counter
EXITOK
temp_high_too_long:
# SMS should be sent only once -
# (the flag v_alarm will become 1 at the next run)
Q: Where should I place my taskfiles on the SD card?
A: Place your taskfiles in the TASKS folder in the root of the SD card. The SD card must be formatted as FAT or FAT32. Files can have any name and extension except .OBJ, .MAP, .CNF (e.g., task.txt), and can also be organized in subfolders.
Q: How do I know if my taskfile has syntax errors?
A: When MC251 loads taskfiles, it checks them for errors. If errors are detected, the file won’t be loaded. You can check the loading status via the Web interface page Tasks, or Modbus registers (see Modbus Register Map, registers 2020-2023). The system will indicate the error type and line number where the error was found.
Q: Will my program survive a power cycle or firmware update?
A: The program stored in internal memory is retained at power downs or restarts. It may be deleted during firmware updates, due to data formats of the logic execution engine updates. However, if you keep the SD card with taskfiles installed, the program will be automatically reloaded after the update.
Q: How do I clear the program from internal memory without resetting all settings?
A: Use the button on the Tasks page of the Web interface. Alternatively, enter Setup Mode via Modbus and write command 40959 to register 120. This clears only the internal logic memory while preserving your device configuration. If the SD card with the taskfiles is installed, the program will be automatically reloaded.
Q: What happens if my task takes longer than the UPDATE interval?
A: If the next run is scheduled before the current one finishes, it will be delayed and executed as soon as possible. If multiple runs are delayed, they will be skipped, which could affect the calculations (like counters). In this case, consider optimizing long operations, splitting the task in two or increasing the UPDATE interval. You can check the tasks update moments and late execution at the Tasks page of the Web interface.
Q: Can I share data between multiple taskfiles?
A: Yes. Use memory-sourced parameters (like MEMTEMP or MEMBAT) mapped to the same Modbus address in different taskfiles. Only one taskfile should write to the shared parameter (the “writer”), while others can read from it.
Q: How do I handle decimal/fractional values if variables are integers?
A: Use fixed-point arithmetic by multiplying fractional values by a power of 10. For example, to work with 2 decimal places, multiply values by 100. Parameter types like F32EP2R automatically handle this conversion when reading/writing. However, be careful with multiplication, division and square root operations as they affect the decimal point position.
Q: What’s the difference between MEMTEMP, MEMBAT, MEMFLASH, and MEMFILE?
A:
MEMTEMP - RAM space, data is lost at powerdown or controller reset, small area size but excellent write speed;
MEMBAT - memory area powered by internal clock battery, very limited size but good write speed;
MEMFLASH - controller settings area, data retains at reset, small size and usually read-only;
MEMFILE - extendable file area, requires memory card with the taskfiles to be present, average speed but excellent area size.
Q: Why does my IF check skip the command even when the condition seems true?
A: The IF check treats unknown values as FALSE. If your variable hasn’t been initialized or a READ operation failed, the value may be unknown. Use ISKNOWN to verify the variable state before checking it.
Q: How do I debug my taskfile program?
A: Use SYSLOG commands to write diagnostic messages to the system log. You can also map intermediate values to Modbus registers using memory parameters (e.g., MEMTEMP H 5000) and read them via Modbus.
Q: Can I use the same Modbus address for different parameter types?
A: Yes, for the parameters of the same size with the same address. Memory-stored parameters in that case will share the same memory area. This is useful for transferring data between taskfiles. However, MEMFILE parameters (stored on SD card) don’t support the sharing feature.
Q: What happens when an error occurs during program execution?
A: Execution jumps to the error handler. If TRYCALL was used, it jumps to the specified handler label. Otherwise, it looks for the nearest upper-level handler, and at last, for the onerror label in the taskfile. If no handler is available, the error is logged and the run exits with the error code.
Q: How do I send SMS only once when an alarm condition is triggered?
A: Use a flag variable stored in persistent memory (like MEMBAT) to track whether the SMS was already sent. Check the flag before sending. Set it after sending and clear the flag after the alarm condition ends. See Example 3 for a working implementation.
Q: How do I check the time interval which the event is active for?
A: Either use a counter (memory-stored parameter to increment at each update period while the event continues and to reset to 0 after the event stops) or a timestamp (variable to store a clock from the MC251 registers at the event start and to be cleared at the event end).
Q: How do I check if the task is run for the first time after reset or after the power-off?
A: A MEMTEMP flag, if checked (and then set to 1 in case it was not set), can tell whether the task run is the first after the power cycle. A MEMBAT or a MEMFILE flag can similarly tell if it is the very first run.
Q: How do I scan an array? How do I make a code part executed a set number of times?
A: Use a variable as a loop index. Check before or after the cycle code. Use 1 or more GO operators to continue the cycle or to end/break it. Do not forget to increment or decrement the index variable inside.
Q: Can I make a repeated code part execution with a complex loop end condition?
A: Yes, using IF and GO. However, it is not recommended without explicit timeouts. The loop which is not limited by a constant can turn infinite or in other way exceed the task configured update time. It could cause this task update skips or other task lags.
Q: Can I call a function from a function? How many nesting levels are there?
A: Yes, you can CALL or TRYCALL a function, which in turn calls some other function. The nesting stack size equals the number of RETURN operators in the taskfile. Because of the limited stack size, the recursion is not recommended.
To update the firmware, Overvis MC251 uses the MC251FW1.FUS, MC251FW2.FUS or MC251FW3.FUS file in the UPGRADES folder on the memory card formatted in FAT or FAT32 format.
The update files can be written to the MC251 memory card in 5 ways:
Automatically via Internet
MC251 periodically checks the updates server and downloads the newer firmware when an Internet connection is available.
Download manually using WEB interface
2.1 Access the device using the browser (see Web Interface);
2.2 If the newer firmware is available on the server, and the download has not started automatically, the Download Update button will appear on the page;
2.3 Press the Download Update button to start the download process;
2.4 The download progress will be displayed on the page;
Fig 1 - The firmware update download process
2.5 After the download completion, the file will be checked. This may take a minute;
Fig 2 - The downloaded firmware update file check
2.6 After the file check, the update prompt with Install Update button will be displayed, confirming that the file suits.
Download manually using MODBUS commands
3.1 connect to the device via MODBUS and enter the setup mode (see Configuration via Modbus);
3.2 to verify the version of the downloaded update file, read the header line in registers 2030 - 2061;
3.3 check if the file download is in progress by reading register 2004;
3.4 to download the newer version from the cloud server, write the value 64893 into the control command register 120;
3.5 control the file download by reading registers 2004 - 2005;
3.6 after download completion, double-check the version of the downloaded file in registers 2030 - 2061.
Upload from the client device using WEB interface via cloud
4.1 access the device WEB interface via cloud using the browser (see Web Interface);
4.2 open Files page (if the files are not displayed there, make sure the memory card is properly installed);
4.3 select the UPGRADES folder (if the folder is absent, create it in the root of the files tree);
4.4 make sure the newer firmware file on the client device is named MC251FW2.FUS;
4.5 press the upload button (the blue upwards arrow button) and select the new firmware file at the client device;
4.6 wait until the file upload finishes;
4.7 open Administration page, the downloaded file check status (and the Install Update button, if the file fits) should be available there, as shown above.
Write file to SD card on a personal computer or other external device and put the card into the MC251 (see below for a way to actually update the firmware from a file with a physical access to MC251).
MC251 firmware can be updated from the file on the memory card in 4 ways:
Automatically
MC251 starts update from the newer firmware file at a specified hour (default is 03:00 AM). The preferred update hour may be configured in the device settings.
Update manually using WEB interface
2.1 Access the device using the browser (see Web Interface);
2.2 Open Administration page;
2.3 If the newer firmware file is available on the memory card, the Install Update button will appear on the page;
Fig 3 - The firmware update prompt
2.4 Press the Install Update button to start the firmware update process.
Update manually using MODBUS commands
3.1 connect to the device via MODBUS and enter the setup mode (see Configuration via Modbus);
3.2 to verify the version of the downloaded update file, read the header line in registers 2030 - 2061;
3.3 to start the update from the file, write the value 65397 into the control command register 120.
Manually
By entering the firmware update mode and selecting the firmware update (see below).
MC251 can be put into firmware update mode during device power up or restart.
The firmware update mode may be entered automatically. This happens after a web-interface command, automatic update hour coming, or in case of a previous update failure.
The firmware update mode can also be entered manually, if the button R has been pressed during startup (the R button is the recessed button located on the front panel of the device).
Table 1 - Manually entering the firmware update mode
No.
Step
R button
Display
Time
Remark
1
Initialization
Pressed
1.5 s
To cancel entering the update mode, release R button
2
Notification about entering the update mode
Pressed
Entering upgrade mode
5 s
To cancel entering the update mode, release R button
3
Offer to enter the update mode
Pressed
To enter upgrade mode release button
2 s
Release the button at this step in order to enter the update mode
4
Protection against incidental pressing
Pressed
Holding the button pressed will cancel entering the update mode
After entering this mode manually, the update file has to be selected.
To cancel the updating, cut off the power supply of MC251 or wait until the update mode is exited automatically due to file selection absence.
Table 2 – Update file selection
No.
Step
R button
Display
Time
Remark
1
Finding the available files
Searching for upgrade files…
(depends on the files found)
2
Offering of the update file
Released
(name and version of the update file)
5 s
To select a file, press and release R button at this step
3
Offering all the update files
Released
(depends on the found files)
Step 2 is repeated for each update file
4
Repeating the offer
Released
(depends on time of step 3)
Steps 2 - 3 are repeated 3 times
5
Protection against unintentional update mode entrance
Released
The absence of selection causes the update mode exit
At automatic update mode entrance (or after the manual selection of the file), the firmware update is started.
Table 3 – Firmware update
No.
Step
R button
Display
Time
Remark
1
Update start
Firmware upgrade
2 – 10 s
2
Update process
(progress bar shows the process of updating)
(depends on the update file)
3
Updating is successfully completed
Firmware upgrade success
3 s
4
Startup of the firmware
The errors and warnings during the updating process are shown on the display.
If the critical error is met, the indication of a critical error is made during an hour, after that the MC251 device automatically restarts.
If an error is a result of an incidental event, the firmware will be restored from the file on the memory card.
Table 4 – Firmware update mode warning and error codes
Code
Warning
Automatic actions
Remark
2
Firmware cannot be started
Initialization of emergency update mode: autostart updating from the selected file or from the first available file (if any)
The warning is a result of other errors and is automatically corrected using the available update files
3
Error during the firmware update process
Similar to No.2, but the file with an error has less priority.
The error is automatically corrected using the available update files
4
The update files are not available
Escape from the update mode and start the available firmware
MC251 can continue operation, but the update file should be put into the UPGRADES folder on the memory card in order to update firmware
5
Emergency - firmware cannot be started
Waiting for manual restart of the device or restart automatically in 1 hour
The error is met after a triple emergency mode entrance as a result of other errors. The correct firmware update file should be put into the UPGRADES folder on the memory card. If the error is repeated, contact the manufacturer
6
Emergency - error during the firmware update process
Waiting for manual restart of the device or restart automatically in 1 hour
The error is met after a triple emergency mode entrance as a result of other errors. The correct firmware update file should be put into the UPGRADES folder on the memory card. If the error is repeated, contact the manufacturer
7
Emergency - no available update files, and the firmware cannot be started
Waiting for manual restart of the device or restart automatically in 1 hour
The error is met after a triple emergency mode entrance as a result of other errors. The correct firmware update file should be put into the UPGRADES folder on the memory card. If the error is repeated, contact the manufacturer
Q: Will I lose my configuration or data during an update?
A: No, the firmware update to a newer version process is designed to preserve your device configuration, Modbus settings, and historical data. However, downgrading the firmware may revert the settings to factory defaults. In either case, backing up critical data is always recommended.
Q: The “Download Update” button is not appearing in the Web Interface.
A: This can happen if:
The device is already running the latest firmware.
The device does not have Internet access. Check your Gateway and DNS settings.
The update server is temporarily unreachable.
The memory card is absent/full/faulty.
Q: The firmware does not update automatically.
A: This can happen if:
The device is already running the latest firmware.
The device does not have Internet access. Check your Gateway and DNS settings.
The update server is temporarily unreachable.
The memory card is absent/full/faulty.
The device is not powered on at an update hour (at nighttime by default)
Q: The device does not see the manually written file on the memory card.
A: The file should be named MC251FW2.FUS and placed into the UPGRADES folder on the FAT32 microSD memory card. Check also the following:
The file is the correct MC251 update file downloaded from the Firmware Downloads.
The memory card is inserted.
Q: The update process seems to hang.
A: The update process typically takes 2-4 minutes. If it takes significantly longer:
Do not power off the device immediately.
Check the device display for any error codes (see Table 4).
If the device is unresponsive for over 10 minutes, try a manual restart. If the internal firmware is corrupted, the device should enter the emergency update mode from the file automatically.
Q: Can I downgrade the firmware?
A: Yes, you can install an older firmware version by manually placing the older firmware file (renamed to MC251FW1.FUS) in the UPGRADES folder on the SD card and initiating the update. However, the device settings are not always retained and may be reverted to factory defaults after firmware upgrade.
This page lists available software utilities for the Overvis MC251 Cloud-first Modbus Gateway with 4G/Ethernet/RS-485.
These tools are designed to help with device configuration, testing, and Modbus TCP communication.
Windows Modbus TCP Client Software (Novatek-Electro)
The Modbus TCP Client is a Windows application designed for basic communication and testing with network devices that support the Modbus TCP protocol.
It enables reading and writing to device holding registers.
This application can also connect to other devices on the network, by using bridge and gateway devices (such as EM-482, EM-483, ET-485 converters, and MC251, MC252, EM-480, EM-481, EM-486 controllers).
Key features include:
Simple Interface: Fast connection to devices for viewing registers and modifying values.
Configuration Management: Save device configurations, including register mappings to a file.
Reusable Write Sets: Save sets of register write operations for repeated use.
Event Logging: Connection logs, data transfers, and errors, which can be exported to a file.
Fig. 1 - The login page
Fig. 2 - The quick setup page
Fig. 3 - The dashboard page
Fig. 4 - The LAN/Internet configuration page
Fig. 5 - The Modbus configuration page
Fig. 6 - The Search Devices tool options
Fig. 7 - The devices search progress
Fig. 8 - The Send Request tool options
Fig. 9 - The Modbus request results
Fig. 10 - The cloud connection page
Fig. 11 - The monitoring setup page
Fig. 12 - The tasks management page
Fig. 13 - The memory card page
Fig. 14 - The administration page
