Modbus Interface Reference

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.

Request Processing

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.

Uwaga

Ensure there are no duplicate Modbus addresses (identifiers) across the RS-485 bus and the remote Modbus TCP server network. The response is accepted from the first responding device.

Uwaga

Ensure there are no redirection loops. E.g. the remote server address is not the MC251 own address, etc. Redirection loops cause the responses to take longer, tend to cause exceptions or connection losses.

If a valid response is received, the MC251 forwards it back to the client.

Troubleshooting

• 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.

SMS Modbus Access

With an active SIM card, the MC251 can process Modbus requests via SMS.

SMS Format

Incoming SMS messages are checked for a valid Modbus request format.

• Modbus Requests: Must begin with a configured password. If the password matches, the request is processed.
• Other Messages: Stored in the incoming SMS list and can be processed by a task file (see Logic Programming).

Request Syntax: [Password] [Access][DeviceID][Resource][Address] [Value]

• Password: As configured in settings.
• Access: R (Read, FC 1-4) or W (Write, FC 5-6).
• DeviceID: Modbus address of the target device.
• Resource:
  • H: Holding Registers
  • I: Input Registers
  • D: Discrete Inputs
  • C: Flags (Coils)
• Address: Register address.
• Value: (Write requests only) The value to write.

Examples:

• Read register 100 of device 1: abc r1h100 (assuming password is “abc”)
• Write 5000 to register 174 of device 2: stanc12 w2h174 5000 (assuming password is “stanc12”)

SMS Responses

The MC251 sends a response SMS upon processing the Modbus request command.

• Success: Returns the command (without password) and the register value.
  • Example: r1h100 2200 (Read value 2200 from register 100 of device 1).
• Error: Returns the command, EXC., the exception code, and a description.
  • Example: r3h873 EXC.2 ILLEGAL DATA ADDRESS (Register 873 of device 3 is not readable).

Table 1 - Standard Modbus Exception Codes

Code	Exception	Description
1	ILLEGAL FUNCTION	The received function code cannot be processed
2	ILLEGAL DATA ADDRESS	The data address specified in the request is not available
3	ILLEGAL DATA VALUE	The value contained in the request data field is invalid value
4	DEVICE FAILURE	Unrecoverable error has occurred when the addressee has tried to perform the requested action
5	ACKNOWLEDGE	The addressee accepted the request and processed it, but it takes a long time
6	DEVICE BUSY	The addressee is busy processing the command. The client can retry the request later
8	MEMORY PARITY ERROR	The parity error was detected when the addressee has tried to read the extended memory
10	GATEWAY PATHS NOT AVAILABLE	The gateway cannot redirect the request, since there is no path (connection) to the addressee
11	TARGET DEVICE FAILED TO RESPOND TO GATEWAY	The gateway did not receive a response to the forwarded request, because the addressee did not respond in time

Configuration via Modbus

The MC251 can be configured using any Modbus TCP client.

1. Connect: Use the MC251’s IP address (press R button on device to view) and Modbus ID (default 111).
2. Enter Password: Write the password (default on device label) into the password registers (see Current Mode Parameters).
3. Verify Mode: If the password is correct, the Mode register will read 1 (Setup Mode).

Managing Settings

In Setup Mode, you can modify the Changeable Settings registers.

• Save Changes: Write 2 to the Command Register. Verify by comparing changeable parameters with saved parameters.
• Save & Apply: Write 4 to the Command Register. Applies Modbus parameters immediately without restart.
• Cancel Changes: Write 9 to the Command Register. Reverts changeable parameters to saved values.
• Factory Reset: Write 444 to the Command Register. Resets all settings to defaults.
• Restart Device: Write 1 to the Command Register. Required for some settings to take effect.
• Exit Setup Mode: Write 0 to the first password register (100). This clears the password and command registers.

MC251 Parameters

Table 2 - Parameter Data Formats

Parameter	Range of values	Description	Number of occupied registers
Number	0 – 65535	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	34817 – 34825

Device Description Parameters

Table 4 - Device Description Parameters

Parameter	Description	Address
Device type	The code that defines the Modbus device for the manufacturer (47 – MC251)	0
Firmware version	Firmware version of embedded software	1
Check code	CRC32 of firmware of embedded software	2 – 3

Current Mode Parameters

Table 5 - Current Mode Parameters

Parameter	Range of values	Initial value	Description	Address
Entered password	String of characters	0	When entering a valid password, the client is given the appropriate permission (see registers 710 - 749)	100 – 119
			When you enter an empty string, the client rights are reset to the rights level at the time of connection
Control command	0 – 65397, writing in the configuration mode	0	See Table 5.1 for the list of commands.	120

Table 5.1 - Control Commands (Register 120)

Value	Command	Description
0	No activity	No action performed
1	Restart	Restart the MC251 device
2	Save	Save settings changes via Modbus
3	Apply	Apply settings without restarting (Modbus parameters only)
4	Save and apply	Execute “Save” then “Apply” commands
6	Export	Save settings to a file on the memory card (see Memory card)
7	Import	Read settings from a file on the memory card and save them
9	Cancel	Revert to saved settings
51	Apply for Modbus	Apply settings via Modbus and RS-485
81	Synchronize clock	Synchronize clock with NTP server
161	Test SMS	Send a test SMS to the main subscriber number
444	Factory Reset	Reset settings to factory defaults
35381	Start clock setting	Allow access to clock setting registers
35431	Cancel clock setting	Close access to clock setting registers without changes
40959	Update task memory	Erase the logic program (re-read if memory card is inserted)
64893	Download Updates	Download latest firmware from cloud to MC251FW2.FUS
65397	Update Firmware	Program firmware from file MC251FW2.FUS

Current Status Parameters

Table 6 - Current Status Parameters

Parameter	Description	Address
Mode (for details see reg. 122)	0: User’s mode;	121
	1: Setting mode
Tabs of access	See Table 6.1 for the list of access bits.	122
Time, min	Number of minutes since the moment of start-up	123 – 124
Number of TCP clients	Number of occupied connections of TCP	125
Limit of TCP clients	Number of prospective clients of TCP	126
Load of RS-485, query/s	Total number of query/s via RS-485	127
Effective load of RS-485, query/s	Number of responds without errors via RS-485 per second	128
Load of RS-485 per second, %	Load of RS-485 for the last second considering the set rate of RS-485 and time of inactivity	129
Load of RS-485 per minute, %	Load of RS-485 for the last minute 130
Load of RS-485 for 5 minutes, %	Load of RS-485 for the last 5 minutes	131
Load of Modbus TCP, query/s	The number of queries received from clients via Modbus TCP per second	132
Effective load of Modbus TCP, query/s	Number of responds without errors being sent to the client via Modbus TCP per second	133
Load of GSM, kB/s	Load of wireless channel with GSM being switched on	134
Load of Ethernet,100 kB/s	Load of wire channel with Ethernet being switched on	135
Max. number of clients of TCP	Maximal number of simultaneously connected clients via TCP – from the moment of start up	136
Max. load of Modbus TCP, query/s	Maximal number of queries received per second from the clients via Modbus TCP – from the moment of start up	137
Max. load of RS-485, %	Maximal load of RS-485 for 5 minutes – from the moment of start up	138
Max. load of GSM, kB/s	Maximal load of GPRS/LTE – from the moment of start up	139
Current IP-address of Ethernet	IP-address, by which MC251 device is accessible in Ethernet network1	140 – 143
Current MAC-address of Ethernet	MAC-address, by which MC251 is detected in Ethernet network	144 – 149
Unused parameter	The parameter is reserved for compatibility	150 – 164
Time to connect to the data collection server	0 – connection to the data collection server is set;	165
	1 – connection to the data collection server is performed;
	2 – 65534: the number of seconds before reconnecting;
	65535: connection to the server is not used
Number of programmed restarts	Number of restarts in accordance to the user setting – for total operational time	166
Number of critical errors	Number of noted errors (failures) causing the restart of the device – for total operational time	167
Total operation time, min	The number of minutes of operating time - for the total operational time	168 – 169
Current time	Number of seconds since 1st of January of specified year (see reg. 172)	170 – 171
Year of countdown	Year, since 1st of January which is taken for time counting	172
Time zone, min	Time zone, for the time count, number of minutes with sign as to UTC+00	173
Temperature, °C	Temperature inside MC251	174
Power voltage, mV	Bus voltage of 12 V	175
Time to connect to the first remote TCP server, s	0 – connection to the remote server is set;
	1 – connecting to the remote server;
	2 – 65534: the number of seconds before reconnecting;
	65535: connection to the remote TCP server is not used	176
Interface for connecting to the first remote TCP server	0 – the connection is not set;	177
	1 – Ethernet connection is set;
	2 – GSM connection is set
DST increment, min	Effective day saving time increment	178
Unused parameter	The parameter is reserved for compatibility	179 – 209
IP-address of client 1	IP-address of client, 0.0.0.0 – not connected	180 – 183
Port of client 1	Port of client, 0 – not connected	184
IP-address of client 2	IP-address of client, 0.0.0.0 – not connected	185 – 188
Port of client 2	Port of client, 0 – not connected	189
IP-address of client 3	IP-address of client, 0.0.0.0 – not connected	190 – 193
Port of client 3	Port of client, 0 – not connected	194
IP-address of client 4	IP-address of client, 0.0.0.0 – not connected	195 – 198
Port of client 4	Port of client, 0 – not connected	199
Unused parameter	The parameter is reserved for compatibility and is equal to 0	200 – 209
Year (current time)	Current year	210
Month (current time)	Current month	211
Day of the month (current time)	Current day of the month	212
Hour (current time)	Current hour	213
Minute (current time)	Current minute	214
Second (current time)	Current second	215
Day of week (current time)	Current day of week (1 – Monday)	216
Month (winter time)	Current month, excluding summer time	217
Day of the month (winter time)	Current day of the month, excluding summer time	218
Hour (winter time)	Current hour, excluding summer time	219
Unused parameter	The parameter is reserved for compatibility	220 – 229
Time of day, s	Number of seconds from midnight of the current day	230 – 231
Time of sunrise, s	0 – 86399: Number of seconds from midnight to sunrise;	232 – 233
	86400: Sunrise is not observed on this day
Sunset time, s	-1: Sunset is not observed on this day;	234 – 235
	0 – 86399: Number of seconds from midnight to sunset;
Time of day, ms	Number of milliseconds from midnight of the current day	236 – 237
Memory card occupancy, 0.01%	10000 – memory card is missing or full	238
Estimated time spent on 1% of the memory card, days	0 – less than one day;	239
	1 – 9999 – days for 1% of the memory card capacity;
	10,000 – more than 10,000 days;
	65535 – unknown
Time before connecting to the second remote TCP server, s	Similar to register 176	240
Connection interface to the second remote TCP server	Similar to register 177	241
Time before connection to the third remote TCP server, s	Similar to register 176	242
Connection interface to the third remote TCP server	Similar to register 177	243
Current IP-address of GSM2	IP-address obtained from GPRS1 provider	900 – 903
Unused parameter	The parameter is reserved for compatibility	904 – 914
GSM signal level, %2	Signal level and quality of radio communication with GSM provider	915
Unused parameter	The parameter is reserved for compatibility	916 – 940
Memory card capacity, kB	0 – card missing or unformatted in FAT / FAT32	941 – 942
The amount of data that has not been written to the memory card since the start, bytes	0 – there were no data record losses	943 - 944
	4294967295 – more than 4 GB losses
Firmware Download Status Bits	See Table 6.2 for the list of status bits.	2004
	Firmware file download progress (х 0.01%) Proportion of downloaded data size to total file size.	2005
	10000 – File fully downloaded
Loaded programming logic tasks number	The number of the tasks correctly loaded into the internal memory	2020
Code of the first illegal logic task error	The code of the first error met in the first taskfile that could not be loaded	2021
Line number of the first illegal logic task error	The line number where the first error was met in the first taskfile that could not be loaded	2022
Loaded programming logic tasks memory usage, bytes	The amount of RAM reserved for the programming logic tasks execution	2023
Firmware File Header	The string identifier of the version, e.g., “MC251, ver.45”.	2030 – 2061
	A blank string indicates the file is either unverified or invalid

Table 6.1 - Tabs of Access Bits (Register 122)

Bit	Description	Value 0	Value 1
0	Permission to obtain RS-485 reading rights (via password)	Cannot be obtained	Can be obtained
1	Current RS-485 reading permission	No permission	Permission granted
2	Permission to obtain RS-485 write/control rights (via password)	Cannot be obtained	Can be obtained
3	Current RS-485 write/control permission	No permission	Permission granted
4	Permission to obtain MC251 register access (via password)	Cannot be obtained	Can be obtained
5	Current MC251 register access permission	No permission	Permission granted
6	Reserved	-	Always 1
7	Permission to configure MC251	No permission	Permission granted
8	Reserved	Always 0	-
9	Connection right	-	Client has right (Always 1)
12	Permission to set the clock	No permission	Permission granted

Table 6.2 - Firmware Download Status Bits (Register 2004)

Bit	Status	Value 0	Value 1
1	Busy Status	Waiting for command	File download in progress
2	Update Download Error	No error	Download error occurred
3	Server Connection	No connection	Connected to server
4	File Data Reception	No data received	File data received
6	File Download Completion	File not downloaded	File fully downloaded
7	File Validity	Not confirmed	File is valid

Settings Parameters

Table 7 - Settings Parameters

Notatka

The internal structure of all sets of settings is similar to the structure of the set described, except for the initial address.

Parameter	Range of values	Factory setting	Description	Address
Ethernet network
Static IP-address	IP-address	192.168.0.111	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
Unused parameter	0	0	Should be 0 for compatibility	313
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 addres-ses 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
Activate SMS 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
Parameter is not used	0	0	It is not used; it should be equal to 0 for compatibility	335 – 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)	464
Connection to the cloud server
Mode of connection to the cloud server	0 – 8	0	See Table 7.2 for connection modes.	465
Cloud server connection port	0 – 65535	20502	The port to which the party is addressed, making connection between MC251 and the server (see reg. 465)	466
Time of waiting for response from the cloud server, s	0 – 3 600	120	0 – the server silence time is not limited;	467
			1–3600 – max. time of server silence after which the connection will be stopped and must be remade again
Delay time before reconnecting to the cloud server, s	0 – 30 000	15	It is used when connecting to the server. After losing connection to the server, the reconnection will be performed after the specified waiting time	468
Unused parameter	0	0	Should be 0 for compatibility	469 - 473
Cloud server address	Character string	modbus.overvis.com	It is used when connecting to the server, if the server address setting is turned on with text string. Address of the remote server with which the connection is supported.	474 - 509
			The string of up to 36 characters can be indicated as address. This string should not have any spaces
Protection
Password for setup mode access	Character string	specified on the device label	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 protec-tion 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 protec-tion 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	2	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 remote server	IP-address	192.168.0.112	It is used when enabling 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	646
Remote server connection mode	0 – 12	0	See Table 7.3 for connection modes.	647
First Modbus-identifier of the remote server	1 – 255	1	It is used during redirection of queries to the remote server.	648
			Two parameters define the range of Modbus identifiers used on the remote server.
			Queries with addresses in this range (and also the broadcast ones with address 0) are sent to the remote Modbus TCP server
Last Modbus-identifier of the remote server	1 – 255	255	same as above	649
Parameter is not used	0	0	It is not used; it should be equal to 0 for compatibility	650 – 699
Daylight saving time
Daylight saving time transition mode	0 – 200	12	See Table 7.4 for transition modes.	700
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	719
Firmware update server connection mode	0 – 4	0	0 – firmware download is not used;	720
			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 – 4	1	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
Maximum limited size of log files, kV	0 – 65535	65535	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 kV
Minimum stored period in recorder mode, days	0 – 184	30	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
Clock settings 3
Day-light saving time offset, min	-1440 – +1440		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	Connect via Ethernet or GSM (prefer Ethernet)
2	Connect via Ethernet or GSM (prefer GSM)
3	Connect only via Ethernet
4	Connect only via GSM
5	Similar to 1 with virtual identifiers 4
6	Similar to 2 with virtual identifiers 4
7	Similar to 3 with virtual identifiers 4
8	Similar to 4 with virtual identifiers 4
9	Similar to 1 in tunnel mode
10	Similar to 2 in tunnel mode
11	Similar to 3 in tunnel mode
12	Similar to 4 in tunnel mode

Table 7.4 - Daylight Saving Time Transition Modes (Register 700)

Value	Country/Mode
0	Automatic transition not used (manual setting)
1	Brazil
2	Great Britain
3	Germany
4	Greece
5	Jordan
6	Italy
7	Namibia
8	Poland
9	Portugal
10	USA
11	Turkey
12	Ukraine
13	Finland
14	France
15	According to preset days

FAQ

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?

A: Key registers are:

• 458-459: Baud rate (default 9600)
• 461: Byte format/parity (see Table 7.1)
• 463: Protocol mode (0=RTU, 1=ASCII)
• 637: Transmission mode (0=Master, 1=Slave, 2=Tunnel)

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.

Need Help?

For technical support and assistance:

• Email: support@overvis.com
• Support portal: www.overvis.com/support

Footnotes

1. 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

2. The content of these registers is available only in Setting Mode. ↩ ↩²

3. Registers for clock setting are available only in Clock Setting Mode (see registers 120, 122). ↩

4. 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