Connecting Equipment

Overvis Cloud communicates with field equipment through Modbus TCP. In a typical setup, a communication controller (Modbus TCP gateway) connects to the internet and maintains a link with the Overvis server. Modbus RTU devices — temperature controllers, energy meters, PLCs, sensors — connect to the gateway via RS-485 and are accessed by Overvis through the gateway.

graph LR
    A["Modbus RTU<br/>devices"] -->|"RS-485<br/>Modbus RTU"| B["Communication<br/>controller<br/>(Modbus TCP gateway)"]
    B -->|"Internet<br/>Modbus TCP"| C["Overvis Cloud"]
    C -->|"Web / HTTPS"| D["User<br/>(browser)"]

In Overvis, this entire chain is represented as a network: one communication controller plus all the Modbus RTU devices accessible through it. Each device within a network has a unique Modbus address (unit ID) and a set of parameters — individual register values that Overvis reads from or writes to the device. For details on managing networks, devices, and parameters after connection, see Networks, Devices & Parameters.

This page covers everything needed to establish the connection: registering the gateway in Overvis, wiring RS-485 devices, configuring serial communication settings, resolving conflicts with previously registered devices, and diagnosing connection problems.

Connection Methods

Overvis supports three methods for connecting a Modbus TCP gateway to the platform. The first two (PIN code and activation code) use a reverse connection — the controller initiates an outbound TCP connection to Overvis, which avoids firewall and NAT issues. The third method (direct connection) requires Overvis to connect inbound to the controller, which needs a static public IP and port forwarding.

PIN Code

The PIN code is an 8-character alphanumeric code (e.g. A1B2C3D4) printed on a sticker attached to the device. The sticker also contains a QR code that encodes a URL like https://c.overvis.com/A1B2C3D4.

This is the simplest connection method and the recommended choice for first-time setup of Overvis-compatible controllers (MC252, EM-482, EM-483, and others).

How it works:

The controller connects outbound to Overvis and identifies itself by MAC address. When the user enters the PIN code, Overvis looks up the MAC address associated with that PIN and links it to the user’s network. Since the controller initiates the connection, no port forwarding or static IP is required — the connection works through firewalls, NAT, and dynamic IP addresses.

sequenceDiagram
    participant Controller
    participant Overvis
    participant User

    Controller->>Overvis: Connect & send handshake (MAC address)
    Overvis->>Controller: Confirm handshake
    Note over Controller,Overvis: Reverse connection established, waiting for binding

    User->>Overvis: Enter PIN code on Create Network page
    Overvis->>Overvis: Look up MAC address by PIN
    Overvis->>Overvis: Bind connection to user's network
    Overvis->>User: Network created

    loop Polling
        Overvis->>Controller: Read registers (Modbus TCP)
        Controller->>Overvis: Return data
    end

Steps:

1. Make sure the controller is powered on and connected to the internet (Ethernet or GSM). Verify connectivity on the device’s display or web interface.
2. Locate the sticker on the device. Either scan the QR code with your phone (it opens the Create Network page with the PIN pre-filled) or note the PIN to enter it manually.
3. Log in to Overvis at ocp.overvis.com and navigate to the Create Network page.
4. Select Use the PIN code printed on the device sticker, enter the PIN, and click Check connection.

5. If the check succeeds, Overvis confirms the controller is online and shows its details (controller model and firmware version). If it fails, verify that the controller has internet access and try again.

6. Enter a descriptive network name and click Create network.

Important characteristics:

• The PIN code is reusable — it can be used multiple times, for example after deleting and re-creating a network.
• If the controller is already bound to a network in your account, Overvis will warn you that the existing network will be disconnected and offer to proceed. All data in the old network is preserved.
• If the controller is already bound to a network in another account, the PIN code cannot rebind it. You must use the activation code method instead (see Activation Code) or have the current owner unbind the device (see Device Conflicts and Takeover).

Activation Code

The activation code is an 8-digit numeric code (e.g. 38571098) generated by the Overvis server and sent to the controller after it establishes a reverse connection. The code is displayed on the controller’s screen (if it has one) or in its local web interface.

This method is used when the PIN code sticker is unavailable, or when you need to take over a controller that is already registered under another Overvis account. Since reading the activation code requires physical or local network access to the device, it serves as proof of ownership.

How it works:

The controller connects outbound to Overvis and receives a unique activation code, which it displays on its screen or web interface. The user reads this code and enters it in Overvis to bind the device to their account. The code changes every time the controller re-establishes a connection to the server (until it is bound).

sequenceDiagram
    participant Controller
    participant Overvis
    participant User

    Controller->>Overvis: Connect & send handshake (MAC address)
    Overvis->>Overvis: Generate 8-digit activation code
    Overvis->>Controller: Send activation code
    Controller->>Controller: Display code on screen / web interface
    Note over Controller,Overvis: Reverse connection established, waiting for binding

    User->>Controller: Read activation code from display or web interface
    User->>Overvis: Enter activation code on Create Network page
    Overvis->>Overvis: Verify code, look up MAC address
    Overvis->>Overvis: Bind connection to user's network
    Overvis->>Controller: Confirm binding
    Controller->>Controller: Display "active"
    Overvis->>User: Network created

    loop Polling
        Overvis->>Controller: Read registers (Modbus TCP)
        Controller->>Overvis: Return data
    end

Steps:

1. Make sure the controller is powered on and connected to the internet.

2. Obtain the activation code:

   • Controllers with a display (e.g. MC252): the 8-digit code is shown on the screen after the server connection is established. The display shows the code prefixed with (E) for Ethernet or (G) for GSM (e.g. (E) 38571098).

   

   • Controllers without a display (e.g. EM-482, EM-483, EM-130): access the controller’s local web interface by its IP address. The code can be found on the status/parameters page, typically in a “Data store server activation code” or similar field.

   

3. Log in to Overvis at ocp.overvis.com and navigate to the Create Network page.

4. Select Use the one-time code displayed on the device screen, enter the 8-digit code, and click Check connection.

5. If the check succeeds, enter a network name and click Create network.
6. After successful binding, the controller’s display (MC252) will show active instead of the numeric code, or the web interface status page (EM-483, EM-130) will show “Active”. The activation code stops changing — the device is now permanently bound to this network.

Important characteristics:

• The activation code has higher priority than the PIN code. It can take over a controller that is currently bound to another account, because reading the code proves physical access.
• Once bound, the controller remains associated with this network even after reconnections — Overvis will not generate a new activation code until the device is unbound.
• The activation code is single-use: each code is valid only until it is used or until the controller reconnects (which generates a new code).

Where to find the activation code by device:

Device	Location
MC252	Device display and web interface (Cloud tab)
EM-482	Web interface, status page
EM-483	Web interface, status page
EM-130	Web interface, settings/parameters page (“Data store server activation code”)

Direct Modbus TCP Connection

With a direct connection, Overvis initiates the TCP connection to your controller by its public IP address and port. This method works with any standard Modbus TCP device — it does not require Overvis-specific firmware or integration. However, it is more complex to set up and less secure than the reverse connection methods.

How it works:

Overvis establishes a TCP connection to the specified IP address and port, then communicates using standard Modbus TCP protocol. The connection is initiated by the Overvis server, so the target device must be reachable from the internet.

sequenceDiagram
    participant User
    participant Overvis
    participant Router
    participant Controller

    User->>Overvis: Create network with direct connection<br/>(IP, port, gateway Modbus ID)
    Overvis->>Router: TCP connect to public IP:port
    Router->>Controller: Forward to controller (port forwarding)
    Controller->>Router: Modbus TCP response
    Router->>Overvis: Forward response
    Overvis->>User: Connection verified, network created

    loop Polling
        Overvis->>Router: Read registers request
        Router->>Controller: Forward to controller
        Controller->>Router: Return data
        Router->>Overvis: Forward response
    end

Prerequisites:

• Static public IP address from your ISP. Dynamic IP addresses are not supported — if your IP changes, Overvis will lose the connection.
• Port forwarding configured on your router to redirect external traffic to the controller’s local IP and Modbus TCP port (default: 502).
• DHCP reservation or static IP for the controller on your local network, so port forwarding rules remain valid.

Steps:

1. Obtain a static public IP address from your ISP.
2. Configure your router to forward a chosen external port to the controller’s internal IP address and Modbus TCP port (default 502).
3. Ensure the controller always has the same local IP — either by configuring DHCP reservation on the router or setting a static IP on the controller.
4. Log in to Overvis and navigate to the Create Network page.
5. Select Establish direct Modbus TCP connection.
6. Fill in the required fields:
   • Modbus TCP controller — select the controller model from the dropdown, or choose “Other Modbus TCP device” for third-party equipment.
   • Gateway Modbus Address — the Modbus unit ID of the gateway controller itself (default 111 for MC252; check your device’s documentation).
   • IP Address — the static public IP address of your router.
   • Port — the external port you configured for forwarding.

7. Click Check connection to verify Overvis can reach the device.
8. If the check succeeds, enter a network name and click Create network.

Security

Direct connections expose the controller’s Modbus TCP port to the internet. This carries risks:

• Modbus TCP has no built-in encryption. Data (including passwords) is transmitted in plaintext.
• Any device that can reach the IP:port can send Modbus commands. Restrict access using your router’s firewall rules (IP allowlisting) where possible.
• Set strong access passwords on the controller if it supports them (see Advanced Network Settings).
• Consider using a non-standard external port to reduce exposure to automated scanning.

For Overvis-compatible controllers, the reverse connection methods (PIN or activation code) are significantly more secure because only Overvis can communicate through the tunnel.

When to use direct connection:

• Third-party Modbus TCP devices or gateways that don’t support Overvis reverse connections.
• Devices that already have a static public IP and are set up for remote Modbus TCP access (e.g. by a SCADA system).
• Environments where the controller cannot make outbound connections to the Overvis server.

RS-485: Connecting Modbus RTU Devices

Once a communication controller is registered in Overvis, the next step is connecting the actual field devices — temperature controllers, energy meters, PLCs, sensors, VFDs, and other Modbus RTU equipment — to the controller’s RS-485 port.

Wiring

RS-485 uses a differential signaling pair — two wires carrying the same signal with opposite polarity. Connect the controller and device terminals as follows:

• Controller terminal A (non-inverted, D+) → Device terminal A (D+)
• Controller terminal B (inverted, D−) → Device terminal B (D−)
• Optionally connect the shield/ground terminal (labeled ⊥, GND, or G) between devices if available.

Use shielded twisted-pair cable (Category 1 or better). Connect the shield to ground at one end of the bus according to ANSI/TIA/EIA-485-A-1998.

Terminal labeling is not standardized

Different manufacturers label RS-485 terminals inconsistently. Some devices use A for the non-inverted signal, while others use A for inverted. Labels like D+, +, Data+, TX+/RX+ are common alternatives. Always check your device’s documentation for the actual signal polarity of each terminal.

If communication does not work after wiring, swap A and B — reversed polarity is the single most common RS-485 issue and does not damage equipment.

Bus topology:

RS-485 is a bus topology — all devices are connected in a daisy-chain (device to device), not in a star pattern. Avoid stub cables (T-shaped branches) longer than a few centimeters, as they cause signal reflections.

graph LR
    A["Controller<br/>(master)"] --- B["Device 1<br/>ID: 1"]
    B --- C["Device 2<br/>ID: 2"]
    C --- D["Device 3<br/>ID: 3"]
    D --- E["..."]

Termination:

Long RS-485 buses or buses operating at high baud rates may need termination resistors (typically 120 Ω) at both ends of the bus to reduce signal reflections. Some controllers have built-in termination (e.g. EM-482 has a 1000 Ω internal terminator). If you experience intermittent communication errors on a long bus, adding termination resistors is the first thing to try.

Serial Communication Settings

All devices on the RS-485 bus must use the same serial communication settings. A mismatch in any of these parameters will prevent communication.

Baud rate — the speed of data transmission in bits per second. Common values:

Baud rate	Typical use
1200	Slow/legacy devices
2400	Legacy devices
4800	Older equipment
9600	Most common default
19200	Faster polling
38400	Fast devices
57600	High-speed devices
115200	Maximum for most RS-485 devices

The baud rate must match between the controller and all connected devices. Higher baud rates allow faster polling but may be unreliable on long cable runs or in electrically noisy environments.

Byte format (parity and stop bits):

Each byte transmitted on the RS-485 bus includes optional parity and stop bits for error detection. The byte format must match across all devices. Common options:

Format	Description
8N1 (ABSENT)	8 data bits, no parity, 1 stop bit
8N2 (STOP)	8 data bits, no parity, 2 stop bits
8E1 (EVEN)	8 data bits, even parity, 1 stop bit
8O1 (ODD)	8 data bits, odd parity, 1 stop bit
SPACE	8 data bits, space (zero) parity bit, 1 stop bit
MARK	8 data bits, mark (one) parity bit, 1 stop bit
AUTO-STOP	Automatic detection — sends with 2 stop bits, accepts 1 or 2 stop bits. Available on some Overvis-compatible controllers.

The most common configurations are 9600 baud, 8N1 or 9600 baud, 8E1. Check your device’s documentation for the required settings.

Configuring serial settings on Overvis-compatible controllers:

For controllers connected via reverse connection (PIN or activation code), Overvis can read and write the RS-485 serial settings remotely. After creating the network, the network settings page shows the current Bit rate and Byte format of the controller, and allows you to change them.

For third-party controllers connected via direct connection, configure the serial settings through the controller’s own interface (web interface, front panel, DIP switches, etc.).

Bus Length and Device Count

Maximum cable length:

The RS-485 standard (ANSI/TIA/EIA-485-A) specifies a maximum bus length of 1200 meters (4000 feet). In practice, the achievable distance depends on the baud rate, cable quality, and electrical noise environment:

Baud rate	Practical max distance
9600	up to 1200 m
19200	up to 1200 m
38400	up to 1000 m
57600	up to 700 m
115200	up to 500 m

These are approximate values. Use shorter runs in electrically noisy environments (near VFDs, high-voltage lines, welding equipment). If you need to extend beyond 1200 m, use RS-485 repeaters or a second communication controller with its own Overvis network.

Maximum device count:

The number of devices on a single RS-485 bus depends on the receiver input impedance of the devices:

Receiver input current	Max devices
1 mA (standard)	32
0.5 mA (1/2 unit load)	64
0.25 mA (1/4 unit load)	128
0.125 mA (1/8 unit load)	256

Most modern Modbus RTU devices use 1/4 or 1/8 unit load receivers, allowing 128–256 devices on a single bus. Check the RS-485 transceiver specifications in your devices’ datasheets.

Each device must have a unique Modbus address (unit ID) in the range 0–255. Address 0 is reserved for broadcast in the Modbus protocol (writes to all devices, no response expected).

Modbus RTU and ASCII

Modbus over RS-485 supports two framing modes:

• Modbus RTU — binary framing with CRC-16 error checking. This is the standard mode used by the vast majority of devices. Data is compact and transmission is efficient.
• Modbus ASCII — ASCII-encoded framing with LRC error checking. Each byte is transmitted as two ASCII hex characters, making the data human-readable but doubling the bandwidth requirement. Used by some older or specialized devices.

Both modes use the same register model and function codes — the difference is only in how bytes are encoded on the wire.

Most Overvis-compatible controllers default to Modbus RTU. If your connected device requires Modbus ASCII, configure this in the controller’s serial port settings (web interface or via Modbus registers). Overvis itself communicates with the controller using Modbus TCP, which is mode-agnostic — the controller handles the RTU/ASCII translation on the RS-485 side.

Devices in Overvis

Each physical Modbus RTU device connected to the controller’s RS-485 port is represented as a device in Overvis, identified by its Modbus unit ID within the network.

To add a device, open the network page in Overvis and click Add Device. Specify the device name, Modbus ID, and optionally select a device template to pre-configure all parameters. If no template is available, you can add parameters manually by specifying register addresses, data types, and names.

Information about your device’s Modbus register map, supported function codes, and data types is typically found in the device’s documentation or datasheet, usually in a section titled “Modbus registers”, “Communication protocol”, or “Register map”.

For full details on adding and configuring devices and parameters, see Networks, Devices & Parameters.

Device Conflicts and Takeover

When you try to create a network using a controller that is already registered in Overvis, the behavior depends on which connection method you use and whether the controller belongs to your account or another account.

PIN code: same account

If the PIN code is already bound to a network in your account, Overvis shows a warning:

Connect code ABCD1234 is already bound to the network Warehouse. After you submit this form, this network will be disconnected. All data will be preserved.

You can proceed — the old network will be disconnected and the controller will be bound to the new network. The old network and its data (device parameters, readings history) remain accessible.

PIN code: another account

If the controller is already bound to a network in another Overvis account, the PIN code cannot rebind it. The PIN code has lower priority — it proves that you have the sticker, but not necessarily physical access to the device.

To take over the controller, use the activation code method instead.

Activation code: takeover

The activation code has higher priority than the PIN code because it requires reading a code from the device’s display or web interface, which proves physical access. An activation code can take over a controller from any account — after binding, the device is removed from the previous owner’s network.

Unbinding a controller

To unbind a controller from its current network (so it can be registered again, either by you or another user), use any of the following methods:

• Delete the network in Overvis that the controller is bound to.
• Change the network’s connection settings to a different connection method (e.g. switch from reverse connection to direct connection).
• Reset activation on the device — access the controller’s web interface and click the “Restart Activation” (MC252, EM-483) or “Reset Activation Code” (EM-130) button. This removes the device from the Overvis network and generates a new activation code.

After unbinding, the controller re-connects to Overvis and receives a new activation code, ready for re-registration.

Tip

If you acquired a used controller that was previously connected to someone else’s Overvis account, the simplest approach is: access the controller’s web interface → find the activation reset button → reset → read the new activation code → create a network in your Overvis account using the activation code.

Advanced Network Settings

The network creation and settings page includes an Advanced settings section with options that affect how Overvis communicates with the controller.

Controller access passwords

Some controllers support password-based access protection for reading and/or writing registers. If your controller has passwords configured, add them in the Controller access password(s) section. Overvis sends these passwords to the controller after connecting, exactly as entered.

You can add multiple passwords — for example, separate read and write passwords. The controller determines which level of access each password grants.

Note

Modbus TCP transmits passwords in plaintext. For direct connections, this means passwords are visible to anyone who can intercept the traffic. Reverse connections (PIN/activation code) use a tunnel that limits this exposure.

Maximum inflight requests

Controls how many Modbus requests Overvis sends to this network simultaneously (in parallel). The default is 1 — Overvis waits for each response before sending the next request (sequential mode).

Increasing this value allows Overvis to send multiple requests at once, which can speed up polling when the controller supports it. However, many RS-485 controllers process requests sequentially on the serial bus regardless, so setting a high value may not improve performance and can cause timeouts if the controller’s request queue overflows.

Use the default value of 1 unless you know your controller supports parallel request processing.

Network enabled/disabled

Networks can be disabled to pause all communication without deleting the network or losing data. A disabled network stops all readings, writes, background tracking, and alert processing. Dashboard and visualizations cannot access disabled networks. All existing data and settings are preserved, and the network can be re-enabled at any time.

For more details on network management, see Networks, Devices & Parameters.

Connection Status

After creating a network, the network page shows the live connection status of the gateway controller. The status indicator shows one of the following states:

• Connected — the controller is online and communicating with Overvis. A tooltip shows the connection time (“since …”).
• Disconnected — the controller is not reachable. A tooltip shows the last seen time and the disconnect reason:
  • “The device has not established a connection to the server” — for reverse connections (PIN/activation code), the controller has not connected outbound to Overvis.
  • “Server can’t establish the connection (timeout)” — for direct connections, Overvis tried to connect to the controller’s IP but timed out.
  • “No direct connection” — no direct connection configuration is active.

Troubleshooting

Connection problems fall into three tiers. Start at the top and work down — each tier builds on the previous one being resolved.

graph TD
    A["Data access issue"] --> B{"Is the gateway<br/>online in Overvis?"}
    B -->|No| C["Tier 1:<br/>Gateway not reachable"]
    B -->|Yes| D{"Does the device<br/>respond to reads?"}
    D -->|No| E["Tier 2:<br/>Device not reachable"]
    D -->|Yes| F{"Is the data<br/>correct?"}
    F -->|No| G["Tier 3:<br/>Data is wrong"]
    F -->|Yes| H["Out of scope <br/>of this guide"]

Tier 1: Gateway Not Reachable

Symptom: The network shows as offline in Overvis. No data is being read from any device. The connection status shows “The device has not established a connection to the server” (reverse connections) or “Server can’t establish the connection (timeout)” (direct connections).

This means Overvis cannot communicate with the communication controller. For reverse connections, it means the controller has not established or has lost its outbound connection to Overvis. For direct connections, it means Overvis cannot reach the controller’s IP and port.

Reverse connections (PIN / activation code)

1. Power and basic indicators

Verify the controller is powered on and running. Check the power indicator:

Device	Normal power indicator
MC252	PWR LED stays on; display shows IP addresses
EM-482	PWR LED stays green
EM-483	Power LED on; error indicator (⚠) off
EM-130	On/Off indicator on

If the MC252 PWR LED does not light up, check the 9–30V DC power supply and fuse. If the EM-482 PWR indicator is constantly red or blinking fast in red, the device may be malfunctioning — contact the manufacturer. If the EM-483 error indicator (⚠) is red steady or periodic after power-on, the device has a fault.

2. Network connection (Ethernet / Wi-Fi)

The controller must have a working link to the local network before it can reach Overvis.

• MC252 (Ethernet): Check that the Ethernet cable is connected and the LAN LED is active (blinking during data transfer). The display shows the assigned IP prefixed with (E). If the display shows (E) 0.0.0.0, DHCP is still negotiating — wait 20–60 seconds. If it shows (E) 192.168.0.111, DHCP failed and the device fell back to its factory default static IP. Verify the cable, router DHCP, or configure a static IP in the web interface (LAN/Internet tab).
• EM-482 (Wi-Fi): The WI-FI LED blinks slowly (every 3 seconds) while connecting and turns on steady once connected. If the WI-FI LED keeps blinking for more than 2 minutes, the configured Wi-Fi access point is unavailable — check the SSID, password, access point distance, and that the access point is powered on. If the WI-FI and SRV LEDs are blinking alternately, the device is in Wi-Fi setup mode — either complete the configuration via its web interface, or exit setup mode by pressing the R button for about 3 seconds.
• EM-130 (Wi-Fi): The device must be in Wi-Fi Client mode with the Overvis client enabled. If Wi-Fi is disabled or in Access Point mode, it will not connect to the internet. Configure Wi-Fi client mode through the device’s web interface.

3. Internet access and Overvis server connection

Even with a local network link, the controller must be able to reach the Overvis server over the internet.

• MC252: Check whether the SRV LED is on — it indicates the cloud/server connection is active. In the web interface (Cloud tab), verify the cloud connection is enabled and the server address is correct (default: ocp.overvis.com, port 20502).
• EM-482: The SRV LED should turn on shortly after the WI-FI LED. If SRV remains off for more than 2 minutes, the device cannot reach the internet. Verify the server connection address in the web interface. Check that your network firewall is not blocking outgoing connections.
• EM-130: In the web interface remote access settings, verify the Overvis client is enabled (“Overvis turning on” = Yes) and the server address is correct (default: modbus.overvis.com, port 20502).

4. Firewall and port requirements

The controller initiates an outbound TCP connection to the Overvis server on port 20502. Corporate networks and firewalls may block this. Ask your network administrator to allow outbound TCP traffic to ocp.overvis.com:20502 (or modbus.overvis.com:20502 for EM-130).

5. DNS resolution

The controller needs to resolve the Overvis server hostname. If DNS is not working on the local network, configure a public DNS server (e.g. 8.8.8.8) on the controller. The EM-130 uses 208.67.222.222 and 8.8.8.8 as default DNS servers.

6. GSM/LTE connection (MC252)

For controllers using cellular connectivity:

• Verify the SIM card is fully inserted (until you hear a click) and has active data service.
• Check the antenna is attached to the ANT connector.
• Check GSM signal strength on the MC252 display (should be >0%).
• In the web interface (LAN/Internet tab → GSM section), verify the APN settings match your mobile operator’s requirements.
• If the SIM has a PIN lock, enter it in the GSM settings. If in doubt, try disabling the SIM PIN on a phone first.
• Try relocating the antenna for better signal.

7. Cloud connection not enabled on the controller

Some controllers ship with the cloud connection disabled by default:

• MC252: In the web interface, open the Cloud tab and verify the cloud connection is enabled. Click “Save & Reset” after enabling it.
• EM-130: The Overvis client is disabled by default. Enable it in the remote access settings of the web interface.
• EM-482, EM-483: The cloud connection is enabled by default. If it has been disabled, re-enable it in the web interface.

Direct connections

1. Static IP — verify your public IP has not changed. Check with your ISP or visit a “What is my IP” site from the controller’s network.
2. Port forwarding — verify the router’s port forwarding rule is still active and points to the correct internal IP and port (default Modbus TCP port: 502).
3. Controller’s local IP — has the controller’s local IP changed (DHCP lease expired)? Re-configure DHCP reservation or use a static IP on the controller.
4. Firewall — verify no firewall between Overvis and the router is blocking the configured port.
5. Controller — verify the controller is powered, running, and its Modbus TCP service is active on the expected port. On the MC252, check the Modbus tab in the web interface to confirm Modbus TCP Server is enabled on port 502.

Tier 2: Device Not Reachable Through Gateway

Symptom: The network is online (gateway is connected), but one or more devices show read errors, timeouts, or “no response” for all parameters.

The controller can talk to Overvis, but cannot communicate with the end device on the RS-485 bus.

1. Verify Modbus address

Check that the device’s Modbus address (unit ID) in Overvis matches the address configured on the physical device. The address is typically set via DIP switches, front panel, or device configuration software. Common default addresses vary by manufacturer — check the device manual.

2. Check RS-485 wiring polarity

Swap the A and B wires at the controller or at the device. Reversed polarity is the single most common RS-485 problem and does not cause equipment damage. Different manufacturers label terminals inconsistently — what one calls “A” another may call “B”. Look for D+/D- or +/- labels as a more reliable guide.

3. Match serial communication settings

All devices on the RS-485 bus must share identical serial settings. A mismatch in any parameter causes complete silence (no responses at all) or CRC errors:

• Baud rate — verify the baud rate configured on the controller matches the device’s baud rate. The most common default is 9600.
• Parity / stop bits — verify the byte format (e.g. 8N1 vs 8E1) matches between the controller and all devices. If unsure, try AUTO-STOP parity mode on controllers that support it (MC252, EM-482) — it sends with 2 stop bits and accepts 1 or 2 stop bits.
• Modbus RTU vs ASCII — the vast majority of devices use RTU mode. If your device requires ASCII mode, configure the controller accordingly.

For Overvis-compatible controllers, these settings can be changed through:

• MC252: Web interface (Modbus tab → RS-485 section), or remotely via Overvis network settings.
• EM-482: Web interface during setup, or remotely via Overvis network settings.

4. Use the MC252 device search tool

The MC252 web interface has a built-in Search Devices tool (Modbus tab → Search Devices button) that scans all Modbus addresses and optionally tries different baud rates, parity, and protocol settings. This can identify connected devices and their actual configuration. A thorough search takes 30–60 minutes; use the Fast search option if you know all devices use the same settings.

5. Use the MC252 Send Request tool

The MC252 web interface has a Send Request tool (Modbus tab → Send Request button) that sends a single Modbus request and shows the raw response. Use it to test communication with a specific device address and verify the response. An exception code in the response does not necessarily indicate a device problem — it may just mean the device doesn’t have the requested register.

6. Check cable and bus topology

• Check for damaged cable, loose terminal connections, or broken wires.
• RS-485 must be wired in a daisy-chain (bus topology), not in a star pattern. Avoid stub cables (T-shaped branches) longer than a few centimeters.
• On long buses (>100 m) or at high baud rates (>38400), add 120 Ω termination resistors at both ends of the bus. Note that the EM-482 has a built-in 1000 Ω internal terminator.
• Try a shorter cable as a test to rule out cable issues.

7. Verify device power and state

Make sure the device is powered on and in normal operating mode (not in a configuration, firmware update, or error state). Some devices disable Modbus communication during certain modes.

8. Check for bus conflicts

• Verify no two devices on the same RS-485 bus share the same Modbus address. Duplicate addresses cause collisions and garbled responses.
• If you are near the device limit for your transceiver type (32 for standard 1 mA receivers, 128–256 for low-power receivers), try disconnecting some devices to see if communication improves.

9. Electrical noise

RS-485 communication can be disrupted by nearby variable-frequency drives (VFDs), motors, high-voltage switching, or welding equipment. Route RS-485 cables away from power lines and use shielded twisted-pair cable. Connect the cable shield to ground at one end of the bus.

Tier 3: Device Reachable but Data Is Wrong

Symptom: The device responds, but the values shown in Overvis are incorrect — wrong numbers, always zero, negative values when positive are expected, etc.

The communication path works, but the parameter configuration does not match the device’s register map.

1. Register address offset

Modbus register addressing conventions vary between manufacturers. Some device manuals document addresses starting from 0, others from 1, and some from 40001 (the Modicon convention, where 40001 = holding register 0). If your value is off by one register or consistently shifted, try adjusting the register address by ±1.

2. Register type mismatch

Modbus defines four register types. Using the wrong type reads from a different address space:

Register type	Function codes	Typical use
Holding registers	3 (read), 6/16 (write)	Read/write configuration and data
Input registers	4 (read)	Read-only measurements
Coils	1 (read), 5/15 (write)	Single-bit on/off states
Discrete inputs	2 (read)	Single-bit read-only states

Verify the parameter in Overvis uses the same register type as specified in the device documentation.

3. Data type and byte order

A temperature reading stored as a 32-bit float will appear as garbage if read as two independent 16-bit integers. Common data types:

• 16-bit unsigned integer (1 register)
• 16-bit signed integer (1 register)
• 32-bit float, IEEE 754 (2 registers)
• 32-bit unsigned/signed integer (2 registers)

For multi-register values, check both byte order (big-endian vs little-endian) and word order (AB CD vs CD AB). The MC252 Send Request tool in the web interface can help identify the correct interpretation by showing raw register values.

4. Scaling factor

Many devices store values with an implicit decimal point. For example, a temperature of 23.5°C may be stored as the integer 235 (×10) or 2350 (×100). The EM-130, for instance, represents all decimal values as integers with a conversion factor determined by the number of decimal places (1 decimal place = ×10, 2 = ×100, 3 = ×1000). Check the device documentation for the scaling factor.

5. Consult the device documentation

The device’s Modbus register map is the authoritative source for addresses, types, and scaling. It is usually found in the device manual, a separate “communication protocol” document, or on the manufacturer’s website. For Overvis-compatible devices, the register maps are available in the device documentation:

• MC252 Modbus Interface
• EM-482 Modbus Interface
• EM-483 Register Map
• EM-130 Register Map

Common Overvis Error Messages

When creating a network or checking the connection, Overvis may display these error messages:

Error message	Meaning	What to do
”Invalid PIN code.”	The entered PIN code was not found in the system.	Verify you entered the exact code from the device sticker. Check for look-alike characters (O vs 0, I vs 1).
”PIN code is already used under other account.”	The controller is bound to a network in another Overvis account. The PIN code cannot override this.	Use the activation code method instead, which proves physical access and can take over the device.
”Device is unreachable.”	The controller has not established a connection to Overvis. The PIN code is valid, but the device is offline.	Check that the controller is powered on and has internet access. See Tier 1 troubleshooting.
”Invalid activation code.”	The entered activation code does not match any active code on the server.	The code changes every time the controller reconnects. Read the current code from the device display or web interface and enter it promptly.
”Server can’t establish the connection (timeout)“	For direct connections, Overvis could not reach the specified IP and port.	Verify the IP address, port, port forwarding, and that the controller’s Modbus TCP service is running. See Tier 1 — Direct connections.

Controller-Specific Indicators

Use the controller’s LEDs and display to diagnose the connection state without accessing Overvis.

MC252

Indicator	Normal state	Problem state
PWR	On (power OK)	Off — no power
LAN	On/blinking (Ethernet active)	Off — no cable or link
GSM	Slow blink every 1.5s (registered); fast blink 3x/s (TCP link active)	Off — no SIM, no signal, wrong APN
SRV	On (cloud connection active)	Off — no internet or cloud disabled
485	Blinking during RS-485 communication	Off — no RS-485 traffic
Display	(E) or (G) + IP address	(E) 0.0.0.0 — DHCP pending; (E) 192.168.0.111 — DHCP failed

EM-482

Indicator	Normal state	Problem state
PWR	Green (power OK)	Constant red or fast red blink — device malfunction
WI-FI	On steady (connected)	Slow blink every 3s (>2 min) — access point unavailable
SRV	On (server connected)	Off (>2 min) — no internet or wrong server address
WI-FI + SRV alternating	—	Device is in Wi-Fi setup mode

EM-483

Indicator	Normal state	Problem state
Power	On	Off — no power
⚠ (error)	Off	Red steady/periodic — device fault
485	Blinking during RS-485 activity	Off — no RS-485 traffic
LAN	On (network active)	Off — no Ethernet link

The EM-483 web interface status page shows the server connection state: the activation code (waiting for binding), “Active” (bound to Overvis), or “No Code” (registered via QR/PIN sticker).

EM-130

The EM-130 has a single On/Off indicator and a Wi-Fi button — no display. Diagnose connection state through the web interface. In the remote access settings, check that the Overvis client is enabled and the activation code is shown (indicating the server connection is established).