RUT900 Modbus

The information in this page is updated in accordance with firmware version RUT9XX_R_00.06.09.5.

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 chapter of the user manual provides an overview of the Modbus page for RUT900 devices.

Modbus TCP

Modbus TCP provides users with the possibility to set or get system parameters. The Modbus daemon acts as slave device. That means it accepts connections from a master (client) and sends out a response or sets some system related parameter in accordance with the given query.

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:

Field	Value	Description
Enable	yes | no; default: none	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 slave ID. When set to 0, it will respond to requests addressed to any ID.
Allow Remote Access	yes | no; default: no	Allows remote Modbus connections by adding an exception to the device's firewall on the port specified in the field above.
Keep persistent connection	yes | no; default: no	If enabled, the connection will not be closed after each completed Modbus request.
Connection timeout	integer [1..60]; default: 0	Timeout in seconds after which the connection will be closed. Use 0 to use default value provided by Operating System.
Enable custom register block	yes | no; default: no	Allow custom register block

Get Parameters

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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 number	number of registers	representation
System uptime	1	2	32 bit unsigned integer
Mobile signal strength (RSSI in dBm)	3	2	32 bit integer
System temperature (in 0.1 °C)	5	2	32 bit integer
System hostname	7	16	Text
GSM operator name	23	16	Text
Router serial number	39	16	Text
LAN MAC address	55	16	Text
Router name	71	16	Text
Currently active SIM card slot	87	16	Text
Network registration info	103	16	Text
Network type	119	16	Text
Current WAN IP address	139	16	32 bit unsigned integer

Set Parameters

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The Modbus daemon can also set some device parameters. These parameters and explanations on how to use them are described in the table below:

value to set	register address	register number	register value	description
Switch WiFi (ON/OFF*)	210	211	1 | 0	Turns WiFi ON or OFF
Switch mobile data connection (ON/OFF*)	211	212	1 | 0	Turns mobile data connection ON or OFF
Switch SIM card	212	213	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
Change APN	213	214	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
Reboot	220	221	1	Reboots the router

* All ON/OFF commands only accept 0 and 1 values, which represent the following:

• 1 - ON
• 0 - OFF

Modbus TCP Master

A Modbus master device can request data from Modbus slaves. The Modbus TCP Master section is used to configure Modbus TCP slaves. You can create a maximum of 10 slave configurations.

Slave device configuration

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The figure below is an example of the Slave device configuration and the table below provides information on the fields contained in that section:

Field	Value	Description
Enabled	yes | no; default: no	Turns communication with the slave device on or off.
Name	string; default: none	Slave device's name, used for easier management purposes.
Slave ID	integer [0..255]; default: none	Slave ID. Each slave in a network is assigned a unique identifier ranging from 1 to 255. When the master requests data from a slave, the first byte it sends is the Slave ID. When set to 0, the slave will respond to requests addressed to any ID.
IP address	ip; default: none	Slave device's IP address.
Port	integer [0..65535]; default: none	Slave device's Modbus TCP port.
Period	integer [1..6400]; default: 60	Interval at which requests are sent to the slave device.
Timeout	integer [1..30]; default: 5	Maximum response wait time.

Requests configuration

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A Modbus request is a way of obtaining data from Modbus slaves. The master sends a request to a slave specifying the function code to be performed. The slave then sends the requested data back to the Modbus master. You can create a maximum of 64 request configurations for each slave device.

The figure below is an example of the Requests configuration section and the table below provides information contained in the fields of that section:

Field	Value	Description
Name	string; default: Unnamed Parameter	Request name. Used for easier management purposes.
Data type	Hex | Ascii | 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, Byte order 1,2,3,4 | 32bit float, Byte order 4,3,2,1 | 32bit float, Byte order 2,1,4,3 | 32bit float, Byte order 3,4,1,2; default: 16bit INT, high byte first	How read data will be stored.
Function	1 | 2 | 3 | 4 | 5 | 6 | 15 | 16; default: 3	A function code specifies the type of register being addressed by a Modbus request. The codes represent these functions: 1 - read Coil Status 2 - read Input Status 3 - read Holding Registers 4 - read Input Registers 5 - force Single Coil 6 - preset Single Register 15 - force Multiple Coils 16 - force Multiple Registers
First Register	integer [1..65536]; default: 1	First Modbus register number from which data will be read. Note - RUT9XX Modbus Master uses register numbers, which value is +1 higher than address value.
Number of Registers	integer [1..2000]; default: none	Number of Modbus registers that will be read during the request.
Enabled	yes | no; default: no	Turns the request on or off.
Test	- (interactive button)	Generates a Modbus request according to given parameters in order to test the request configuration. You must first save the configuration before you can use the Test button.
Delete	- (interactive button)	Deletes the request.
Add	- (interactive button)	Adds a new request configuration.

Alarm configuration

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Alarms are a way of setting up automated actions when some Modbus values meet user specified conditions. The figure below is an example of the Alarm configuration page and the table below provides information on fields that it contains:

Field	Value	Description
Enabled	yes | no; default: no	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 in Modbus request.
Register	integer [0..65535]; default: none	Number of the Modbus coil/input/holding register/input register that will be read.
Condition	More than | Less than | Equal to | Not Equal to; default: Equal to	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.
Value	various; default: none	The value against which the read data will be compared.
Action	SMS | Trigger output | Modbus Request; default: SMS	Action that will be taken if the condition is met. Possible actions: SMS - sends and SMS message to a specified recipient(s). Trigger output - changes the state of a specified output(s). Modbus Request - sends a Modbus request to a specified slave.
SMS: Message	string; default: none	SMS message text.
SMS: Phone number	phone number; default: none	Recipient's phone number.
Trigger output: Output	4PIN output; default: 4PIN output	Which output(s) will be triggered.
Trigger output: I/O Action	Turn On | Turn Off | Invert; default: Turn On	Action that will taken on the specified output.
Modbus Request: IP address	ip | host; default: none	Modbus slave's IP address.
Modbus Request: Port	integer [0..65535]; default: none	Modbus slave's port.
Modbus Request: Timeout	integer [1..30]; default: 5	Maximum time to wait for a response.
Modbus Request: ID	integer [1..255]; default: none	Modbus slave ID.
Modbus Request: Modbus function	Read Coil Status (1) | Read Input Status (2) | Read Holding Registers (3) | Read Input Registers (4) | Force Single Coil (5) | Preset Single Register (6) | Force Multiple Coils (15) | Force Multiple Registers (16); default: Force Single Coil (5)	A function code specifies the type of register being addressed by a Modbus request.
Modbus Request: First register	integer [0..65535]; default: none	Begins reading from the register specified in this field.
Modbus Request: Number of registers	integer [0..65535]; default: none	The number of registers that will be read from the first register.

Modbus Data to Server

The Modbus Data to Server function provides you with the possibility to set up senders that transfer data collected from Modbus slaves to remote servers. To add a new data sender, enter the server's address, specify the data sending period and click the "Add" button:

Data sender configuration

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When you add a new data sender, you will be redirected to its configuration window. The figure below is an example of that window and the table below provides information on the fields that it contains:

Field	Value	Description
Enabled	yes | no; Default: no	Turns the data sender ON or OFF
Name	string; Default: none	Data sender's name. used for easier management purposes
Protocol	HTTP(S) | MQTT; Default: HTTP(S)	Data sending protocol
JSON format	json string; Default: {"ID":"%i", "TS":"%t","ST":"%s","VR":"%a"}	Provides the possibility to fully customize the JSON segment
Segment count	1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | All; Default: 1	Max segment count in one JSON string sent to server.
URL / Host / Connection string	host | ip; Default: none	Address of the server to which the data will be sent. Important note: when using HTTPS, remember to add the https:// prefix before the URL.
Period	integer [1..6400]; Default: none	Data sending frequency (in seconds)
HTTP(S): Data filtering	All data | By slave ID | By slave IP; Default: All data	Which data this sender will transfer to the server
HTTP(S): Retry on fail	yes | no; Default: no	Specifies whether the data sender should retry failed attempts
HTTP(S): Custom header	string; Default: no	Adds a custom header(s) to HTTP requests
MQTT: Port	integer [0..65535]; Default: none	Port used to connect to host.
MQTT: Keepalive	integer [1..640]; Default: none	MQTT keepalive period in seconds.
MQTT: Topic	string; Default: none	Write topic to which your data will be sent.
MQTT: QoS	0 | 1 | 2; Default: 0	This field defines the guarantee of delivery for specific message. Possible values are: At most once (0) At least once (1) Exactly once (2)
MQTT: Use TLS	yes | no; Default: no	Turns TLS authentication on or off.

MQTT Gateway

The MQTT Gateway function is used to transfer Modbus data (send requests, receive responses) over MQTT. When it is enabled, the device (this RUT900) 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 slave.

When the MQTT Gateway receives a response from the slave, 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.

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	string; default: request	MQTT topic for sending requests.
Response topic	string; default: response	MQTT topic for subscribing to responses.
Username	string; default: none	Username for authentication to the MQTT broker. Leave empty if you do not use client authentication.
Password	string; default: none	Password for authentication to the MQTT broker. Leave empty if you do not use client authentication.

Request messages

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Modbus request data sent in the MQTT payload should be generated in accordance with the following format:

0 <COOKIE> <IP_TYPE> <IP> <PORT> <TIMEOUT> <SLAVE_ID> <MODBUS_FUNCTION> <REGISTER_NUMBER> <REGISTER_COUNT/VALUE>

Explanation:

• 0 - must be 0, which signifies a textual format (currently the only one implemented).
• Cookie - a 64-bit unsigned integer in range [0..2⁶⁴]). 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 slave. IPv6 must be presented in full form (e.g., 2001:0db8:0000:0000:0000:8a2e:0370:7334).
• Port - port number of the Modbus TCP slave.
• Timeout - timeoutfor Modbus TCP connection, in seconds. Range [1..999].
• Slave ID - Modbus TCP slave ID. Range [1..255].
• Modbus function - Only these are supported at the moment:
  • 3 - read holding registers;
  • 6 - write to a single holding register;
  • 16 - write to multiple holding registers.
• Register number - number of the first register (in range [1..65536]) from which the registers will be read/written to.
• Register count/value - this value depends on the Modbus function:
  • 3 - register count (in range [1..125]); must not exceed the boundary (first register number + register count <= 65537);
  • 6 - register value (in range [0..65535]);
  • 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].

Response messages

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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

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Below are a few examples of controlling/monitoring the internal Modbus TCP Slave on RUT900.

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Reboot the device

• Request:
  

  0 65432 0 192.168.1.1 502 5 1 6 206 1

• Response:
  

  65432 OK

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Retrieve uptime

• Request:
  

  0 65432 0 192.168.1.1 502 5 1 3 2 2

• Response:
  

  65432 OK 0 5590

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If you're using Eclipse Mosquitto (MQTT implementation used on RUT900), 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

See also

• Monitoring via Modbus - detailed examples on how to use Modbus TCP