RUT955 Modbus

Summary

Modbus is a serial communications protocol. Simple and robust, it has since become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices. This chapter is an overview of the Modbus TCP functionality.

Modbus TCP

Modbus TCP interface allows the user to set or get router parameters like modem temperature, signal strength, etc. In other words, Modbus TCP allows you to control the router's behavior and get its status information. Modbus daemon acts as slave device. That means it accepts connections from the master (client) and sends out a response or sets some system related parameter. By the default Modbus will only accept connections through LAN interface. In order to accept connections through WAN interface also, Allow Remote Access must be checked.

FIELD NAME	VALUE	DESCRIPTION
Enable	yes | no; Default: no	Toggles Modbus TCP ON or OFF
Port	integer [0..65535]; Default: " "	Specifies which port to use for Modbus connections
Allow Remote Access	yes | no; Default: no	If enabled, allows Modbus access from remote devices (from WAN)

Get Parameters

---

To obtain parameters from the system, the read holding registers command is used. Router parameters are held within registers. Each register contains 2 bytes of information. For simplification the number of registers for storing numbers is 2, while the number of registers for storing text information is 16. The register numbers and corresponding system values are described below:

REQUIRED VALUE	REPRESANTATION	REGISTER NUMBERS	NUMBER OF REGISTERS
System uptime	32 bit unsigned integer	1	2
GSM signal strength (dBm)	32 bit integer	3	2
System temperature in 0.1 degrees Celcius	32 bit integer	5	2
System hostname	Text	7	16
GSM operator name	Text	23	16
Router serial number	Text	39	16
Router MAC address	Text	55	16
Router name	Text	71	16
Current SIM card	Text	87	16
Network registration	Text	103	16
Network type	Text	119	16
Digital input 1	32 bit integer	135	2
Digital input	32 bit integer	137	2
Current WAN IP address	32 bit unsigned integer	139	2
Analog input	32 bit integer	141	2

Set Parameters

---

The Modbus daemon also supports setting of some system parameters. For this task the Write holding register command is used. System related parameters and how to use them are described below. The register number specifies where to start writing the required values. All commands, except “Change APN”, accepts only one input parameter. For the APN the number of input registers may vary. The very first byte of APN command denotes a number of SIM card for which set the APN. This byte should be set to 1 (in order to change APN for SIM card number 1) or to 2 (in order to change APN for SIM card number 2).

VALUE TO SET	DESCRIPTION	REGISTER NUMBER	REGISTER VALUE
Digital output 1 (on/off)	Change the state of the digital output number 1	201	1/0
Digital output 2 (on/off)	Change the state of the digital output number 2	202	1/0
Switch WiFi (on/off)	Allows to switch WiFi on or off	210	1/0
Switch mobile data connection (on/off)	Turns mobile data connection on or off	211	1/0
Switch SIM card (SIM1, SIM2, SIM1->SIM2 and SIM2->SIM1)	Allows to change SIM card in use, 3 possible options are supported	212	0/1/2
Change APN	Allows to change APN	213	APN code
Reboot	Reboots the router	220	1

See also

You can find more information on how to use Modbus with RUT routers here: Monitoring via Modbus