TRB255 Modbus

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The information in this page is updated in accordance with firmware version TRB2_R_00.07.10.2.


Summary

Modbus is a serial communications protocol. Simple and robust, it has become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices.

This manual page provides an overview of the Modbus functionality in TRB255 devices.

If you're having trouble finding this page or some of the parameters described here on your device's WebUI, you should turn on "Advanced WebUI" mode. You can do that by clicking the "Advanced" button, located at the top of the WebUI.

Modbus TCP Server

A Modbus TCP Server listens for connections from a TCP Client (client) and sends out a response or sets some system related parameter in accordance with the given query. This provides the user with the possibility to set or get system parameters.

The figure below is an example of the Modbus TCP window section and the table below provides information on the fields contained in that window:

Status

This section displays Modbus TCP Server status information.

General settings

Field Value Description
Enable off | on; default: off Turns Modbus TCP on or off.
Port integer [0..65535]; default: 502 TCP port used for Modbus communications.
Device ID integer [0..255]; default: 1 The device's Modbus server ID. When set to 0, it will respond to requests addressed to any ID.
Mobile Data type Bytes | Kilobytes | Megabytes; default: Bytes Selects mobile data unit representation type.
Allow remote access off | on; default: off Allows remote Modbus connections by adding an exception to the device's firewall on the port specified in the field above.
Keep persistent connection off | on; default: onn Allows keep the connection open after responding a Modbus TCP client request.
Connection timeout integer [0..60]; default: 0 Sets TCP timeout in seconds after which the connection is forcefully closed.
Enable custom register block off | on; default: off Allows the usage of custom register block.
Register file path path; default: /tmp/regfile Path to file in which the custom register block will be stored. Files inside /tmp or /var are stored in RAM. They vanish after reboot, but do not degrade flash memory. Files elsewhere are stored in flash memory. They remain after reboot, but degrade flash memory (severely, if operations are frequent).
First register number integer [1025..65536]; default: 1025 First register in custom register block
Register count integer [1..64512]; default: 128 Register count in custom register block

Modbus Serial Server

A Modbus Serial Server listens for connections from a serial client and sends out a response or sets some system related parameter in accordance with the given query. This provides the user with the possibility to set or get system parameters.

Status

This section displays Modbus Serial Server instances currently existing on the router.

Modbus Serial Server Configuration

The Modbus Serial Server Configuration section is used to configure serial servers. By default, the list is empty. To add a new server instance, enter the instance name, select serial interface and click the 'Add' button.

After clicking 'Add' you will be redirected to the newly added server instance configuration page.

Field Value Description
Enable off | on; default: off Enables this Modbus Serial Server instance configuration.
Name string; default: none Name of the serial server instance. Used for management purposes only.
Device RS232 | RS485; default: RS232 Specifies which serial port will be used for serial communication.
Device ID integer [0..255]; default: 1 Specifies which serial port will be used for serial communication.
Mobile Data type Bytes | Kilobytes | Megabytes; default: Bytes Selects mobile data unit representation type.
Baud rate (RS485) integer [300..230400]; default:9600 Data rate for serial data transmission (in bits per second (bps)).
Baud rate (RS232) integer [300..115200]; default:9600 Data rate for serial data transmission (in bits per second (bps)).
Data bits 7 |8; default: 8 Number of data bits for each character.
Stop bits 1| 2; default: 1 Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronise with the character stream. Electronic devices usually use one stop bit. Two stop bits are required if slow electromechanical devices are used.
Parity Even | Odd| Mark | Space | None; default: None In serial transmission, parity is a method of detecting errors. An extra data bit is sent with each data character, arranged so that the number of 1 bits in each character, including the parity bit, is always odd or always even. If a byte is received with the wrong number of 1s, then it must have been corrupted. However, an even number of errors can pass the parity check.
  • None (N) - no parity method is used.
  • Odd (O) - the parity bit is set so that the number of "logical ones (1s)" has to be odd.
  • Even (E) - the parity bit is set so that the number of "logical ones (1s)" has to be even.
  • Space (s) - the parity bit will always be a binary 0.
  • Mark (M) - the parity bit will always be a binary 1.
Flow control None | RTS/CTS | Xon/Xoff; default: None In many circumstances a transmitter might be able to send data faster than the receiver is able to process it. To cope with this, serial lines often incorporate a "handshaking" method, usually distinguished between hardware and software handshaking.
  • RTS/CTS - hardware handshaking. RTS and CTS are turned OFF and ON from alternate ends to control data flow, for instance when a buffer is almost full.
  • Xon/Xoff - software handshaking. The Xon and Xoff characters are sent by the receiver to the sender to control when the sender will send data, i.e., these characters go in the opposite direction to the data being sent. The circuit starts in the "sending allowed" state. When the receiver's buffers approach capacity, the receiver sends the Xoff character to tell the sender to stop sending data. Later, after the receiver has emptied its buffers, it sends an Xon character to tell the sender to resume transmission.
RS485: Full Duplex off | on; default: off Enables RS485 full duplex.
Enable custom register block off | on; default: off Allows the usage of custom register block.
Register file path path; default: /tmp/regfile Path to file in which the custom register block will be stored. Files inside /tmp or /var are stored in RAM. They vanish after reboot, but do not degrade flash memory. Files elsewhere are stored in flash memory. They remain after reboot, but degrade flash memory (severely, if operations are frequent).
First register number integer [1025..65536]; default: 1025 First register in custom register block
Register count integer [1..64512]; default: 128 Path to file in which the custom register block will be stored. Files inside /tmp or /var are stored in RAM. They vanish after reboot, but do not degrade flash memory. Files elsewhere are stored in flash memory. They remain after reboot, but degrade flash memory (severely, if operations are frequent).

Modbus Registers

Get Parameters

Modbus parameters are held within registers. Each register contains 2 bytes of information. For simplification, the number of registers for storing numbers is 2 (4 bytes), while the number of registers for storing text information is 16 (32 bytes).

The register numbers and corresponding system values are described in the table below:

required value register address register number number of registers representation
System uptime 1 2 2 32 bit unsigned integer
Mobile signal strength (RSSI in dBm) 3 4 2 32 bit integer
System temperature (in 0.1 °C) 5 6 2 32 bit integer
System hostname 7 8 16 Text
GSM operator name 23 24 16 Text
Router serial number 39 40 16 Text
LAN MAC address 55 56 16 Text
Router name 71 72 16 Text
Currently active SIM card slot 87 88 16 Text
Network registration info 103 104 16 Text
Network type 119 120 16 Text
Current WAN IP address 139 140 2 8 bit unsigned integer
Analog input (PIN 11) value 141 142 2 32 bit unsigned integer
GPS latitude coordinate 143 144 2 32 bit float
GPS longitude coordinate 145 146 2 32 bit float
GPS fix time 147 148 16 Text (YYYY-MM-DD hh:mm:ss) Note: returns UTC datetime.
GPS date and time 163 164 16 Text (YYYY-MM-DD hh:mm:ss) Note: returns formatted datetime, adjusted with systems timezone.
GPS speed 179 180 2 32 bit float
GPS satellite count 181 182 2 32 bit unsigned integer
GPS accuracy 183 184 2 32 bit float
Mobile data received today (SIM1) 185 186 2 32 bit unsigned integer
Mobile data sent today (SIM1) 187 188 2 32 bit unsigned integer
Mobile data received this week (SIM1) 189 190 2 32 bit unsigned integer
Mobile data sent this week (SIM1) 191 192 2 32 bit unsigned integer
Mobile data received this month (SIM1) 193 194 2 32 bit unsigned integer
Mobile data sent this month (SIM1) 195 196 2 32 bit unsigned integer
Mobile data received last month (SIM1) 487 488 4 64 bit unsigned integer (Big Endian Notation)
Mobile data sent last month (SIM1) 491 492 4 64 bit unsigned integer (Big Endian Notation)
Mobile data received last 24h (SIM1) 197 198 2 32 bit unsigned integer
Mobile data sent last 24h (SIM1) 199 200 2 32 bit unsigned integer
Active SIM card 205 206 1 16 bit unsigned integer
Mobile data received last 7 days (SIM1) 292 293 2 32 bit unsigned integer
Mobile data sent 7 days (SIM1) 294 295 2 32 bit unsigned integer
Mobile data received last week (SIM1) 503 504 4 64 bit unsigned integer (Big Endian Notation)
Mobile data sent last week (SIM1) 507 508 4 64 bit unsigned integer (Big Endian Notation)
Mobile data received last 30 days (SIM1) 296 297 2 32 bit unsigned integer
Mobile data sent last 30 days (SIM1) 298 299 2 32 bit unsigned integer
Mobile data received today (SIM2) 300 301 2 32 bit unsigned integer
Mobile data sent today (SIM2) 302 303 2 32 bit unsigned integer
Mobile data received this week (SIM2) 304 305 2 32 bit unsigned integer
Mobile data sent this week (SIM2) 306 307 2 32 bit unsigned integer
Mobile data received this month (SIM2) 308 309 2 32 bit unsigned integer
Mobile data sent this month (SIM2) 310 311 2 32 bit unsigned integer
Mobile data received this month (SIM2) 495 496 4 64 bit unsigned integer (Big Endian Notation)
Mobile data sent this month (SIM2) 499 500 4 64 bit unsigned integer (Big Endian Notation)
Mobile data received last 24h (SIM2) 312 313 2 32 bit unsigned integer
Mobile data sent last 24h (SIM2) 314 315 2 32 bit unsigned integer
Mobile data received 7 days (SIM2) 316 317 2 32 bit unsigned integer
Mobile data sent 7 days (SIM2) 318 319 2 32 bit unsigned integer
Mobile data received last week (SIM2) 511 512 4 64 bit unsigned integer (Big Endian Notation)
Mobile data sent last week (SIM2) 515 516 4 64 bit unsigned integer (Big Endian Notation)
Mobile data received last 30 days (SIM2) 320 321 2 32 bit unsigned integer
Mobile data sent last 30 days (SIM2) 322 323 2 32 bit unsigned integer
PIN 3 status 324 325 1 16 bit unsigned integer
PIN 4 status 325 326 1 16 bit unsigned integer
Modem ID 328 329 8 Text
PIN 2 status 336 337 1 16 bit unsigned integer
IMSI 348 349 16 Text
Unix timestamp 364 365 2 32 bit unsigned integer
Local ISO time 366 367 12 Text
UTC time 378 379 12 Text
LAN IP 394 395 2 8 bit unsigned integer
Add SMS 397 398 90 Text

Set Parameters

The Modbus daemon can also set some device parameters.

value to set register address register number register value description
Hostname 7 8 Hostname (in decimal form) Changes hostname
Device name 71 72 Device name (in decimal form) Changes device name
Switch mobile data connection (ON/OFF*) 204 205 1 | 0 Turns mobile data connection ON or OFF
Switch SIM card 205 206 1 | 2 | 0 Changes the active SIM card slot
  • 1 - switch to SIM1
  • 2 - switch to SIM2
  • 0 - switch from the the SIM card opposite of the one currently in use (SIM1 → SIM2 or SIM2 → SIM1)
Reboot 206 207 1 Reboots the router
Change APN 207 208 APN code Changes APN.
The number of input registers may vary depending on the length of the APN, but the very first byte of the set APN command denotes the number of the SIM card for which to set the APN. This byte should be set to:
  • 1 - to set APN for SIM1
  • 2 - to set APN for SIM2
Switch PIN 3 state 324 325 1|0 Toggles PIN 3 ON or OFF
Switch PIN 4 state 325 326 1|0 Toggles PIN 4 ON or OFF
Switch PIN 3 direction 326 327 1|0 Toggles PIN 3 direction between INPUT (0) or OUTPUT (1)
Switch PIN 4 direction 327 328 1|0 Toggles PIN 4 direction between INPUT (0) or OUTPUT (1)
Switch PIN 2 state 336 337 1|0 Toggles PIN 2 ON or OFF
Change LAN IP 394 395 IPv4 (in decimal form) Changes device LAN IP
Send SMS 396 397 1|0 Sends an SMS with content defined in Add SMS (397) register
Add SMS 397 398 Message (in decimal form) Define SMS content which will be sent using Send SMS (396) register.
The register array is split into two parts that represent the recipient's "phone number" (first 10 registers) and the "SMS message contents" (remaining 80 registers).

Modbus TCP Client

A Modbus Client device can request data from Modbus servers. The Modbus TCP Client section is used to configure Modbus TCP servers and enable Client.

Notice the Global section config. It is used to outright turn the service off or on if any active configurations are present.

Clicking the Cog icon opens a modal window. The global configuration slider can be set and it's state saved.

By default, the server list is empty and client is disabled. To add a new server, click the 'Add' button

After clicking 'Add' you will be redirected to the newly added server's configuration page.

General Status

This section displays Modbus Client general status information.

Server Device Configuration

The Server Device Configuration section is used to configure the parameters of Modbus TCP servers that the Client (this TRB255 device) will be querying with requests. The figure below is an example of the Server Device Configuration and the table below provides information on the fields contained in that section:

Field Value Description
Enabled off | on; default: off Turns communication with the server device on or off.
Name string; default: none Server device's name, used for easier management purposes.
Server ID integer [0..255]; default: none Server ID. Each server in a network is assigned a unique identifier ranging from 1 to 255. When the client requests data from a server, the first byte it sends is the Server ID. When set to 0, the server will respond to requests addressed to any ID.
IP address ip4; default: none Server device's IP address.
Port integer [0..65535]; default: none Server device's Modbus TCP port.
Timeout integer [1..30]; default: 5 Maximum response wait time.
Always reconnect off | on; default: off Create new connection after every Modbus request.
Number of timeouts integer [0..10]; default: 1 Skip pending request and reset connection after number of request failures.
Frequency Period | Schedule; default: Period
Delay integer [0..999]; default: 0 Wait in milliseconds after connection initialization.
Period integer [1..99999]; default: none Interval in seconds for sending requests to this device

Requests Configuration

A Modbus request is a way of obtaining data from Modbus servers. The client sends a request to a server specifying the function code to be performed. The server then sends the requested data back to the Modbus client.

Note: Modbus TCP Client uses Register Number instead of Register Address for pointing to a register. For example, to request the Uptime of a device, you must use 2 in the First Register field.

The Request Configuration list is empty by default. To add a new Request Configuration loon to the Add New Instance section. Enter a custom name into the 'Name' field and click the 'Add' button:

The new Request Configuration should become visible in the list:

Field Value Description
Name string; default: Unnamed Name of this Request Configuration. Used for easier management purposes.
Data type 8bit INT | 8bit UINT | 16bit INT, high byte first | 16bit INT, low byte first | 16bit UINT, high byte first | 16bit UINT, low byte first | 32bit float (various Byte order) | 32bit INT (various Byte order) | 32bit UINT (various Byte order) | 64bit INT (various Byte order) | 64bit UINT (various Byte order) | 64bit float (various Byte order) | ASCII | Hex | Bool | PDU; default: 16bit INT, high byte first Defines how read data will be stored.
Function Read coils (1) | Read input coils (2) | Read holding registers (3) | Read input registers (4) | Set single coil (5) | Set single coil register (6) | Set multiple coils (15) | Set multiple holding registers (16); default: Read holding registers (3) Specifies the type of register being addressed by a Modbus request.
First Register integer [0..65535]; default: 1 First Modbus register from which data will be read.
Register Count / Values integer [1..2000]; default: 1 Number of Modbus registers that will be read during the request.
Remove Brackets off | on; default: off Removes the starting and ending brackets from the request (only for read requests).
off/on slider off | on; default: off Turns the request on or off.
Delete [ X ] - (interactive button) Deletes the request.

Additional note: by default the newly added Request Configurations are turned off. You can use the on/off slider to the right of the Request Configuration to turn it on:

After having configured a request, you should see a new 'Request Configuration Testing' section appear. It is used to check whether the configuration works correctly. Simply click the 'Test' button and a response should appear in the box below. Note: to use test buttons, you need to enable Client section. A successful response to a test may look something like this:

Alarms Configuration

Alarms are a way of setting up automated actions when some Modbus values meet user-defined conditions. When the Modbus TCP Client (this TRB255 device) requests some information from a server device it compares that data to with the parameters set in an Alarm Configuration. If the comparison meets the specified condition (more than, less than, equal to, not equal to), the Client performs a user-specified action, for example, a Modbus write request or switching the state of an output.

The figure below is an example of the Alarms Configuration list. To create a new Alarm, click the 'Add' button.

After adding the Alarm you should be redirected to its configuration page which should look similar to this:

Field Value Description
Enabled off | on; default: off Turns the alarm on or off.
Function code Read Coil Status (1) | Read Input Status (2) | Read Holding Registers (3) | Read Input Registers (4); default: Read Coil Status (1) Modbus function used for this alarm's Modbus request. The Modbus TCP Client (this TRB255 device) perform this request as often as specified in the 'Period' field in Server Device Configuration.
Compared condition data type 8bit INT | 8bit UINT | 16bit INT, high byte first | 16bit INT, low byte first | 16bit UINT, high byte first | 16bit UINT, low byte first | 32bit float (various Byte order) | 32bit INT (various Byte order) | 32bit UINT (various Byte order) | 64bit INT (various Byte order) | 64bit UINT (various Byte order) | 64bit float (various Byte order) | ASCII | Hex | Bool; default: 16bit INT, high byte first Select data type that will be used for checking conditions.
First register number integer [1..65536]; default: none Number of the Modbus coil/input/holding-register/input-register to read from.
Values various; default: none The value against which the read data will be compared.
Condition More than | Less than | Equal to | Not Equal to | Less or equal | More or equal; default: More than When a value is obtained it will be compared against the value specified in the following field. The comparison will be made in accordance with the condition specified in this field.
Action frequency Every trigger | First trigger; default: Every trigger Describes how frequently the specified action will be taken.
Redundancy protection off | on; default: off Protection against executing a configured action too often.
Redundancy protection period integer [1..86400]; default: none Duration to activate redundancy protection for, measured in seconds. This field becomes visible only when 'Redundancy protection' is turned on.
Action Ubus event | SMS | MODBUS Write Request| Trigger output | MQTT message; default: MODBUS Write Request Action that will be taken if the condition is met. Possible actions:
  • SMS - sends and SMS message to a specified recipient(s).
  • Modbus Request - sends a Modbus Write request to a specified server.
  • Trigger output - changes state of selected I/O output pin.
SMS: Message string; default: none SMS message text.
SMS: Phone number phone number; default: none Recipient's phone number.
MODBUS Write Request: IP address ip | host; default: none Modbus server's IP address.
MODBUS Write Request: Port integer [0..65535]; default: none Modbus server's port.
MODBUS Write Request: Timeout integer [1..30]; default: 5 Maximum time to wait for a response.
MODBUS Write Request: ID integer [1..255]; default: none Modbus server ID.
MODBUS Write Request: Modbus function Set Single Coil (5) | Set Single Register (6) | Set Multiple Coils (15) | Set Multiple Registers (16); default: Set Single Coil (5) A function code specifies the type of register being addressed by a Modbus request.
MODBUS Write Request: Executed action data type 8bit INT | 8bit UINT | 16bit INT, high byte first | 16bit INT, low byte first | 16bit UINT, high byte first | 16bit UINT, low byte first | 32bit float (various Byte order) | 32bit INT (various Byte order) | 32bit UNIT (various Byte order) 64bit INT (various Byte order) | 64bit UINT (various Byte order) | 64bit float (various Byte order) | ASCII | Hex | Bool; default: Bool Select data type that will be used for executing action.
MODBUS Write Request: First register number integer [0..65535]; default: none Begins reading from the register specified in this field.
MODBUS Write Request: Values integer [0..65535]; default: none Register/Coil values to be written (multiple values must be separated by space character).
Trigger output: OutputConfigurable Input/Output (2) | Configurable Input/Output (3) | Configurable Input/Output (4); default: Configurable Input/Output (2) Selects which output will be triggered.
Trigger output: I/O Action Turn On | Turn Off | Invert; default: Turn On Selects the action performed on the output.
MQTT message: JSON format string; default: none Below this field you can find special codes that begin with the '%' sign. Each code represents a piece information related to the status of the device. Include these codes in the field for dynamic information reports. Possible values: Local time, Unix time, Router name, Device name, Serial number, Current FW version, LAN IP address, Monitoring status, UTC time in ISO, WAN IP address, New line, Modbus server ID, Modbus server IP, First register number, Register value, Mobile IP addresses, Signal strength, Operator name, Network type, Data connection state, Network state, IMSI, IMEI, Modem model, Modem serial number, SIM pin state, SIM state, RSCP, ECIO, RSRP, SINR, RSRQ, ICCID, CELLID, Neighbour cells, Network info, Network serving, WAN MAC address, Analog Current Loop (6,9), Analog Input (6,9), Input (3), Digital Input (1), Output (4), Isolated Output (3,4,8), Isolated Input (2,7), Relay (5,10)
MQTT message: Hostname host | ip; default: none Broker’s IP address or hostname.
MQTT message: Port integer [0..65535]; default: 1883 Broker's port number.
MQTT message: Keepalive positive integer; default: none The number of seconds after which the broker should send a PING message to the client if no other messages have been exchanged in that time
MQTT message: Topic string; default: none The name of the topic that the broker will subscribe to.
MQTT message: Client ID positive integer; default: none Client ID to send with the data. If empty, a random client ID will be generated
MQTT message: QoS At most once (0) | At least once (1) | Exactly once (2); default: At most once (0) A period of time (in seconds) which has to pass after a trigger event before this Action is executed.
MQTT message: Use root CA off | on; default: off Use root CA for verifying the servers certificates
MQTT message: Use TLS off | on; default: off Turns the use of TLS/SSL for this MQTT connection on or off.
MQTT message: Use credentials off | on; default: off Turns the use of username and password for this MQTT connection on or off.

Modbus Serial Client

The Modbus Serial Client page is used to configure the device as a Modbus RTU Client. Modbus RTU (remote terminal unit) is a serial communication protocol mainly used in communication via serial interfaces.

Notice the Global section config. It is used to outright turn the service off or on if any active configurations are present.

Clicking the Cog icon opens a modal window. The global configuration slider can be set and it's state saved.

General Status

This section displays Modbus Client general status information.

Modbus Serial Device Configuration

This section is used to create Modbus Serial Client's server device instances. You may create a Serial Device instance for each supported serial interface.

By default there are no instances created. To add a new serial device configuration, enter an instance name and click the 'Add' button.

After clicking 'Add' you will be redirected to the newly added device's configuration page.

RS Device Modbus Client Configuration

This section is used to configure the Modbus Serial Client's server device interface settings.

Field Value Description
Enable off | on; default: off Enables this Modbus Serial Device instance configuration.
Name string; default: none Name of the serial device instance. Used for management purposes only.
Device RS232 | RS485; default: RS232 Specifies which serial port will be used for serial communication.
Baud rate (RS485) integer [300..230400]; default:9600 Data rate for serial data transmission (in bits per second (bps)).
Baud rate (RS232) integer [300..115200]; default:9600 Data rate for serial data transmission (in bits per second (bps)).
Data bits 7 |8; default: 8 Number of data bits for each character.
Stop bits 1| 2; default: 1 Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronise with the character stream. Electronic devices usually use one stop bit. Two stop bits are required if slow electromechanical devices are used.
Parity Even | Odd| Mark | Space | None; default: None In serial transmission, parity is a method of detecting errors. An extra data bit is sent with each data character, arranged so that the number of 1 bits in each character, including the parity bit, is always odd or always even. If a byte is received with the wrong number of 1s, then it must have been corrupted. However, an even number of errors can pass the parity check.
  • None (N) - no parity method is used.
  • Odd (O) - the parity bit is set so that the number of "logical ones (1s)" has to be odd.
  • Even (E) - the parity bit is set so that the number of "logical ones (1s)" has to be even.
  • Space (s) - the parity bit will always be a binary 0.
  • Mark (M) - the parity bit will always be a binary 1.
Flow control None | RTS/CTS | Xon/Xoff; default: None In many circumstances a transmitter might be able to send data faster than the receiver is able to process it. To cope with this, serial lines often incorporate a "handshaking" method, usually distinguished between hardware and software handshaking.
  • RTS/CTS - hardware handshaking. RTS and CTS are turned OFF and ON from alternate ends to control data flow, for instance when a buffer is almost full.
  • Xon/Xoff - software handshaking. The Xon and Xoff characters are sent by the receiver to the sender to control when the sender will send data, i.e., these characters go in the opposite direction to the data being sent. The circuit starts in the "sending allowed" state. When the receiver's buffers approach capacity, the receiver sends the Xoff character to tell the sender to stop sending data. Later, after the receiver has emptied its buffers, it sends an Xon character to tell the sender to resume transmission.
RS485: Full Duplex off | on; default: off Enables RS485 full duplex.

Modbus Server Device Configuration

This section is used to create server instances that the Client (this TRB255 device) will be querying with requests.

By default there are no instances created. To add a new server configuration, enter an instance name, select a serial device instance and click the 'Add' button.

After clicking 'Add' you will be redirected to the newly added server's configuration page.

Server Device Configuration

The Server Device Configuration section is used to configure the parameters of Modbus RTU servers that the Client (this TRB255 device) will be querying with requests. The figure below is an example of the Server Device Configuration and the table below provides information on the fields contained in that section:

Field Value Description
Enabled off | on; default: off Turns communication with the server device on or off.
Name string; default: none Server device's name, used for easier management purposes.
Serial device serial device instance; default: none Specifies which serial device will be used on this server.
Server ID integer [0..255]; default: 1 Server ID. Each server in a network is assigned a unique identifier ranging from 1 to 255. When the client requests data from a server, the first byte it sends is the Server ID. When set to 0, the server will respond to requests addressed to any ID.
Number of timeouts integer [0..10]; default: 0 Skip pending request and reset connection after number of request failures.
Frequency Period | Schedule; default: Period
Period integer [1..99999]; default: none Interval at which requests are sent to the server device.
Timeout integer [1..60]; default: 1 Maximum response wait time.
Requests Configuration

A Modbus request is a way of obtaining data from Modbus servers. The client sends a request to a servers specifying the function code to be performed. The server then sends the requested data back to the Modbus client.

Note: Modbus Serial Client uses Register Number instead of Register Address for pointing to a register. For example, to request the Uptime of a device, you must use 2 in the First Register field.

The Request Configuration list is empty by default. To add a new Request Configuration loon to the Add New Instance section. Enter a custom name into the 'Name' field and click the 'Add' button:

The new Request Configuration should become visible in the list:

Field Value Description
Name string; default: Unnamed Name of this Request Configuration. Used for easier management purposes.
Data type 8bit INT | 8bit UINT | 16bit INT, high byte first | 16bit INT, low byte first | 16bit UINT, high byte first | 16bit UINT, low byte first | 32bit float (various Byte order) | 32bit INT (various Byte order) | 32bit UINT (various Byte order) | 64bit INT (various Byte order) | 64bit UINT (various Byte order) | 64bit float (various Byte order) | ASCII | Hex | Bool | PDU; default: 16bit INT, high byte first Defines how read data will be stored.
Function Read coils (1) | Read input coils (2) | Read holding registers (3) | Read input registers (4) | Set single coil (5) | Set single coil register (6) | Set multiple coils (15) | Set multiple holding registers (16); default: Read holding registers (3) Specifies the type of register being addressed by a Modbus request.
First Register integer [0..65535]; default: 1 First Modbus register from which data will be read.
Register Count / Values integer [1..2000]; default: 1 Number of Modbus registers that will be read during the request.
Remove Brackets off | on; default: off Removes the starting and ending brackets from the request (only for read requests).
off/on slider off | on; default: off Turns the request on or off.
Delete [ X ] - (interactive button) Deletes the request.

Additional note: by default the newly added Request Configurations are turned off. You can use the on/off slider to the right of the Request Configuration to turn it on:

After having configured a request, you should see a new 'Request Configuration Testing' section appear. It is used to check whether the configuration works correctly. Simply click the 'Test' button and a response should appear in the box below. Note: to use test buttons, you need to enable Client section. A successful response to a test may look something like this:

Modbus Client Alarms

Alarms are a way of setting up automated actions when some Modbus values meet user-defined conditions. When the Modbus Serial Client (this TRB255 device) requests some information from a server device it compares that data to with the parameters set in an Alarm Configuration. If the comparison meets the specified condition (more than, less than, equal to, not equal to), the Client performs a user-specified action, for example, a Modbus write request or switching the state of an output.

The figure below is an example of the Modbus Client Alarms list. To create a new Alarm, click the 'Add' button.

After this you should be redirected to that Alarm's configuration page which should look similar to this:

Field Value Description
Enabled off | on; default: off Turns the alarm on or off.
Function code Read Coil Status (1) | Read Input Status (2) | Read Holding Registers (3) | Read Input Registers (4); default: Read Coil Status (1) Modbus function used for this alarm's Modbus request. The Modbus TCP Client (this TRB255 device) perform this request as often as specified in the 'Period' field in Server Device Configuration.
Compared condition data type 8bit INT | 8bit UINT | 16bit INT, high byte first | 16bit INT, low byte first | 16bit UINT, high byte first | 16bit UINT, low byte first | 32bit float (various Byte order) | 32bit INT (various Byte order) | 32bit UINT (various Byte order) | 64bit INT (various Byte order) | 64bit UINT (various Byte order) | 64bit float (various Byte order) | ASCII | Hex | Bool; default: 16bit INT, high byte first Select data type that will be used for checking conditions.
First register number integer [1..65536]; default: none Number of the Modbus coil/input/holding-register/input-register to read from.
Values various; default: none The value against which the read data will be compared.
Condition More than | Less than | Equal to | Not Equal to | Less or equal | More or equal; default: More than When a value is obtained it will be compared against the value specified in the following field. The comparison will be made in accordance with the condition specified in this field.
Action frequency Every trigger | First trigger; default: Every trigger Describes how frequently the specified action will be taken.
Redundancy protection off | on; default: off Protection against executing a configured action too often.
Redundancy protection period integer [1..86400]; default: none Duration to activate redundancy protection for, measured in seconds. This field becomes visible only when 'Redundancy protection' is turned on.
Action SMS | MODBUS Write Request| Trigger output; default: MODBUS Write Request Action that will be taken if the condition is met. Possible actions:
  • SMS - sends and SMS message to a specified recipient(s).
  • Modbus Request - sends a Modbus Write request to a specified server.
  • Trigger output - changes state of selected I/O output pin.
SMS: Message string; default: none SMS message text.
SMS: Phone number phone number; default: none Recipient's phone number.
MODBUS Write Request: Timeout integer [1..30]; default: 5 Maximum time to wait for a response.
MODBUS Write Request: ID integer [1..255]; default: none Modbus server ID.
MODBUS Write Request: Modbus function Read Single Coil (5) | Set Single Register (6) | Set Multiple Coils (15) | Set Multiple Registers (16); default: Set Single Coil (5) A function code specifies the type of register being addressed by a Modbus request.
MODBUS Write Request: Executed action data type 8bit INT | 8bit UINT | 16bit INT, high byte first | 16bit INT, low byte first | 16bit UINT, high byte first | 16bit UINT, low byte first | 32bit float (various Byte order) | 32bit INT (various Byte order) | 32bit UNIT (various Byte order) | 64bit INT (various Byte order) | 64bit UINT (various Byte order) | 64bit float (various Byte order) | ASCII | Hex | Bool; default: Bool Select data type that will be used for executing action.
MODBUS Write Request: First register number integer [0..65535]; default: none Begins reading from the register specified in this field.
MODBUS Write Request: Values integer [0..65535]; default: none Register/Coil values to be written (multiple values must be separated by space character).
Trigger output: OutputConfigurable Input/Output (2) | Configurable Input/Output (3) | Configurable Input/Output (4); default: Configurable Input/Output (2) Selects which output will be triggered.
Trigger output: I/O Action Turn On | Turn Off | Invert; default: Turn On Selects the action performed on the output.


MQTT Modbus Gateway

The MQTT Modbus Gateway function is used to transfer Modbus data (send requests, receive responses) over MQTT. When it is enabled, the device (this TRB255) subscribes to a REQUEST topic and publishes on a RESPONSE topic on a specified MQTT broker. It translates received MQTT message payload to a Modbus request and relays it to the specified Modbus TCP server.

When the MQTT Gateway receives a response from the server, it translates it to an MQTT message and publishes it on the RESPONSE topic.

Below is an example of the MQTT Gateway page. Refer to the table for information on MQTT Gateway configuration fields and status.

Field Value Description
Enable off | on; default: off Turns MQTT gateway on or off.
Host ip | host; default: 127.0.0.1 IP address or hostname of an MQTT broker.
Port integer [0..65535]; default: 1883 Port number of the MQTT broker.
Request topic alphanumeric string; default: request MQTT topic for sending requests.
Response topic alphanumeric string; default: response MQTT topic for subscribing to responses.
QoS At most once (0) | At least once (1) | Exactly once (2); default: Exactly once (2) Specifies quality of service.
Username string; default: none Username for authentication to the MQTT broker.
Password string; default: none Password for authentication to the MQTT broker.
Client ID integer; default: none Specifies client ID for MQTT broker.
Keepalive integer; default: 5 Keepalive message to MQTT broker (seconds)
Use TLS/SSL off | on; default: off Turns TLS support on or off
TLS type cert | psk; default: cert Selects the type of TLS encryption
TLS insecure off | on; default: off Disables TLS security
Certificate files from device off | on; default: off Choose this option if you want to use certificate files generated on device.
CA file string; default: none Upload/select certificate authority file.
Certificates file string; default: none Upload/select certificate file.
Key file string; default: none Upload/select certificate key file.
PSK string; default: none Specifies the pre-shared key.
Identity string; default: none Specifies identity.

Serial Gateway Configuration

Serial Gateway Configuration section displays Serial gateway instances currently existing on the router.

By default the list is empty. To create a new gateway instance, enter the ID of serial device, select serial interface and click the 'Add' button.

After this you should be redirected to instance's configuration page which should look similar to this:

Field Value Description
Enable off | on; default: off Enables this Serial Gateway instance configuration.
Name string; default: none Name of the gateway instance. Used for management purposes only.
Device RS232 | RS485; default: RS232 Specifies which serial port will be used for serial communication.
Baud rate (RS485) integer [300..230400]; default:9600 Data rate for serial data transmission (in bits per second (bps)).
Baud rate (RS232) integer [300..115200]; default:9600 Data rate for serial data transmission (in bits per second (bps)).
Data bits 7 |8; default: 8 Number of data bits for each character.
Stop bits 1| 2; default: 1 Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronise with the character stream. Electronic devices usually use one stop bit. Two stop bits are required if slow electromechanical devices are used.
Parity Even | Odd| Mark | Space | None; default: None In serial transmission, parity is a method of detecting errors. An extra data bit is sent with each data character, arranged so that the number of 1 bits in each character, including the parity bit, is always odd or always even. If a byte is received with the wrong number of 1s, then it must have been corrupted. However, an even number of errors can pass the parity check.
  • None (N) - no parity method is used.
  • Odd (O) - the parity bit is set so that the number of "logical ones (1s)" has to be odd.
  • Even (E) - the parity bit is set so that the number of "logical ones (1s)" has to be even.
  • Space (s) - the parity bit will always be a binary 0.
  • Mark (M) - the parity bit will always be a binary 1.
Flow control None | RTS/CTS | Xon/Xoff; default: None In many circumstances a transmitter might be able to send data faster than the receiver is able to process it. To cope with this, serial lines often incorporate a "handshaking" method, usually distinguished between hardware and software handshaking.
  • RTS/CTS - hardware handshaking. RTS and CTS are turned OFF and ON from alternate ends to control data flow, for instance when a buffer is almost full.
  • Xon/Xoff - software handshaking. The Xon and Xoff characters are sent by the receiver to the sender to control when the sender will send data, i.e., these characters go in the opposite direction to the data being sent. The circuit starts in the "sending allowed" state. When the receiver's buffers approach capacity, the receiver sends the Xoff character to tell the sender to stop sending data. Later, after the receiver has emptied its buffers, it sends an Xon character to tell the sender to resume transmission.
RS485: Full Duplex off | on; default: off Enables RS485 full duplex.

Request messages

Note: MQTT Gateway uses Register Number instead of Register Address for pointing to a register. For example, to request the Uptime of a device, you must use 2 in the Register Number field.

Modbus request data sent in the MQTT payload should be generated in accordance with the one of the following formats:

  • TCP:
 0 <COOKIE> <IP_TYPE> <IP> <PORT> <TIMEOUT> <SERVER_ID> <MODBUS_FUNCTION> <FIRST_REGISTER> <REGISTER_COUNT/VALUES>
  • Serial:
 1 <COOKIE> <SERIAL_DEVICE_ID> <TIMEOUT> <SERVER_ID> <MODBUS_FUNCTION> <FIRST_REGISTER> <REGISTER_COUNT/VALUES>
  • MODBUS TCP connection management messages:
 2 <COOKIE> <CONNECTION_INDEX> <ACTION>

 2 <COOKIE> <CONNECTION_INDEX> 0 <IP_TYPE> <IP> <PORT> <TIMEOUT>

Explanation:

  • Cookie - a 64-bit unsigned integer in range [0..264-1]). A cookie is used in order to distinguish which response belongs to which request, each request and the corresponding response contain a matching cookie: a 64-bit unsigned integer.
  • IP type - host IP address type. Possible values:
    • 0 - IPv4 address;
    • 1 - IPv6 address;
    • 2 - hostname that will be resolved to an IP address.
  • IP - IP address of a Modbus TCP server. IPv6 must be presented in full form (e.g., 2001:0db8:0000:0000:0000:8a2e:0370:7334).
  • Port - port number of the Modbus TCP server.
  • Timeout - timeout for Modbus connection, in seconds. Range [1..999].
  • Server ID - Modbus TCP server ID. Range [1..255].
  • Modbus function - Modbus task type that will be executed. Possible values are:
    • 1 - read coils;
    • 2 - read input coils;
    • 3 - read holding registers;
    • 4 - read input registers;
    • 5 - set single coil;
    • 6 - write to a single holding register;
    • 15 - set multiple coils;
    • 16 - write to multiple holding registers.
  • First register - number (not address) of the first register/coil/input (in range [1..65536]) from which the registers/coils/inputs will be read/written to.
  • Register count/value - this value depends on the Modbus function:
    • 1 - coil count (in range [1..2000]); must not exceed the boundary (first coil number + coil count <= 65537);
    • 2 - input count (in range [1..2000]); must not exceed the boundary (first input number + input count <= 65537);
    • 3 - holding register count (in range [0..125]); must not exceed the boundary (first register number + holding register count <= 65537);
    • 4 - input register count (in range [0..125]); must not exceed the boundary (first register number + input register count <= 65537);
    • 5 - coil value (in range [0..1]);
    • 6 - holding register value (in range [0..65535]);
    • 15 - coil count (in range [1..1968]); must not exceed the boundary (first coil number + coil count <= 65537); and coil values separated with commas, without spaces (e.g., 1,2,3,654,21,789); there must be exactly as many values as specified (with coil count); each value must be in the range of [0..1].
    • 16 - register count (in range [1..123]); must not exceed the boundary (first register number + register count <= 65537); and register values separated with commas, without spaces (e.g., 1,2,3,654,21,789); there must be exactly as many values as specified (with register count); each value must be in the range of [0..65535].
  • Serial device ID - a string used to identify a serial device. Must match with Device ID field in MQTT Gateway page Serial gateway configuration section.
  • Connection index - a number used to identify a connection on which an action will be preformed (in range [0..7]).
  • Action - a connection action. Possible values are:
    • 0 - OPEN. This will open a closed connection, reopen an already open connection with the same parameters or close an already open connection and open a new one with new parameters.
    • 1 - CLOSE. This will close an open connection and do nothing to the closed one.
    • 2 - STATUS. This will respond with either OK 1 for an open connection or OK 0 for a closed connection.

Response messages

A special response message can take one of the following forms: 

<COOKIE> OK                              - for functions 6 and 16
<COOKIE> OK <VALUE> <VALUE> <VALUE>...   - for function 3, where <VALUE> <VALUE> <VALUE>... are read register values
<COOKIE> ERROR: ...                      - for failures, where ... is the error description

Examples

Below are a few examples of controlling/monitoring the internal Modbus TCP Server on TRB255.

Reboot the device

  • Request:
    0 65432 0 192.168.1.1 502 5 1 6 206 1
  • Response:
    65432 OK

Retrieve uptime

  • Request:
    0 65432 0 192.168.1.1 502 5 1 3 2 2
  • Response:
    65432 OK 0 5590

If you're using Eclipse Mosquitto (MQTT implementation used on TRB255), Publish/Subscribe commands may look something like this:

Retrieve uptime

  • Request:
    mosquitto_pub -h 192.168.1.1 -p 1883 -t request -m "0 65432 0 192.168.1.1 502 5 1 3 2 2"
  • Response:
    mosquitto_sub -h 192.168.1.1 -p 1883 -t response
65432 OK 0 5590

Modbus TCP over Serial Gateway

The Modbus TCP over Serial gateway serial type allows redirecting TCP data coming to a specified port to an RTU specified by the Server ID. The Server ID can be specified by the user or be obtained directly from the Modbus header.

Status

This section displays Modbus TCP over serial instances currently existing on the router.

Modbus TCP over Serial Gateway Configuration

Modbus TCP over Serial Gateway Configuration section displays gateway instances currently existing on the router.

By default the list is empty. To create a new gateway instance, enter the name of instance, select serial interface and click the 'Add' button.

After this you should be redirected to instance's configuration page which should look similar to this:

Field Value Description
Enable off | on; default: off Enables this Modbus TCP over Serial Gateway instance configuration.
Name string; default: none Name of the gateway instance. Used for management purposes only.
Device RS232 | RS485; default: RS232 Specifies which serial port will be used for serial communication.
Baud rate (RS485) integer [300..230400]; default:9600 Data rate for serial data transmission (in bits per second (bps)).
Baud rate (RS232) integer [300..115200]; default:9600 Data rate for serial data transmission (in bits per second (bps)).
Data bits TRB2M= 7 |8; default: 8 Number of data bits for each character.
Stop bits 1| 2; default: 1 Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronise with the character stream. Electronic devices usually use one stop bit. Two stop bits are required if slow electromechanical devices are used.
Parity Even | Odd| Mark | Space | None; default: None In serial transmission, parity is a method of detecting errors. An extra data bit is sent with each data character, arranged so that the number of 1 bits in each character, including the parity bit, is always odd or always even. If a byte is received with the wrong number of 1s, then it must have been corrupted. However, an even number of errors can pass the parity check.
  • None (N) - no parity method is used.
  • Odd (O) - the parity bit is set so that the number of "logical ones (1s)" has to be odd.
  • Even (E) - the parity bit is set so that the number of "logical ones (1s)" has to be even.
  • Space (s) - the parity bit will always be a binary 0.
  • Mark (M) - the parity bit will always be a binary 1.
Flow control None| RTS/CTS | Xon/Xoff; default: None In many circumstances a transmitter might be able to send data faster than the receiver is able to process it. To cope with this, serial lines often incorporate a "handshaking" method, usually distinguished between hardware and software handshaking.
  • RTS/CTS - hardware handshaking. RTS and CTS are turned OFF and ON from alternate ends to control data flow, for instance when a buffer is almost full.
  • Xon/Xoff - software handshaking. The Xon and Xoff characters are sent by the receiver to the sender to control when the sender will send data, i.e., these characters go in the opposite direction to the data being sent. The circuit starts in the "sending allowed" state. When the receiver's buffers approach capacity, the receiver sends the Xoff character to tell the sender to stop sending data. Later, after the receiver has emptied its buffers, it sends an Xon character to tell the sender to resume transmission.
Listening IP ip; default: none IP address to listen for incoming connections. (0.0.0.0) value may be used to listen for incoming connections on any interface or IP address.
Port integer [0..65535]; default: none Port number to listen for incoming connections.
Server ID configuration type User defined | Obtained from TCP; default: User defined Specifies whether server IDs are user defined or automatically obtained from TCP.
Server ID integer; default: none Specifies the server ID of range of permitted server IDs. The way this field is named and its function depends on the value of the Server ID configuration field.
A range of IDs can be specified by placing a hyphen (-) between two integer numbers. For example, if you permit server IDs in the range of 10 to 20, you would specify it as: 10-20
You can also specify multiple values that are not connected in a range using commas (,). For example, to specify 6, 50 and 100 as permitted server IDs, you would have to use: 6,50,100
Permitted server IDs range of integers; default: 1-247 Read Server ID field description.
CRC verification off | on; default: off Checks if sent serial message is not disturbed.
Echo off | on; default: off Turns RS232 echo on or off. RS232 echo is a loopback test usually used to check whether the RS232 cable is working properly.
RS485: Full Duplex off | on; default: off Enables RS485 full duplex.

IP Filter

The IP Filter section is used for configuring which network is allowed to communicate with the device. You may add a new instance by selecting the Interface and pressing Add.

Then enter the IP address and save.

See also

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