4-x relay board C0135 managed by Modbus RTU protocol (RS-485)

To test a device management by Modbus RTU commands, we use the circuitry of 4 relays controlled by the STM8S103 controller. (Relay MCU 4-way relay-communication board (RS-485).

4-х relays board managed by Modbus RTU commands with USB – RS485 converter

Manage the relay 

To turn a relay on/off we will use Modbus “Write Single Coil (0x05)” function.

To send the Modbus commands we will use QmodMaster application.

The example of how to use the application described in details in previous article where we learned how to work with termo/humidity sensor managed by Modbus RTU.

Turn on 1-st relay

After finishing the command you’ll hear the relay click and LED will light.

Turn on 1-st relay

To turn on the relay you can send any number except 0. Zero – to turn off the relay.

Turn off the 1-st relay. Modbus.

Turn off the 1-st relay

The screenshot with the example of turning off of the 4-st relay.

Включение 4-го реле. Modbus.

Turning off the 4-st relay

Включение 4-го реле. Modbus RTU.

Turning off the 4-st relay

Turning on and off a relay by Modbus commands:

Function code Start address Number of coils Data Description
Write Single Coil (0x05) 0 1 1 Turn on 1-st relay
0 1 0 Turn off 1-st relay
1 1 1 Turn on 1-st relay
1 1 0 Turn off 1-st relay
2 1 1 Turn on 1-st relay
2 1 0 Turn off 1-st relay
3 1 1 Turn on 1-st relay
3 1 0 Turn off 1-st relay
FF 1 1 Turn on all relay
FF 1 0 Turn off all relay
Команда Modbus для включения всех реле.

Modbus command to turn off all relays.

Read relays state

To write registers used 0х05 function – “Discrete Output Coils”. According to the table, the addressing of these registers starts from 1.

Memory Address Type Memory Type
1-9999 Read-Write Discrete Output Coils
10001-19999 Read-Only Discrete Input Contacts
30001-39999 Read-Only Analog Input Registers
40001-49999 Read-Write Analog Output Holding Registers

Accordingly, to get the state “Discrete Output Register” it is necessary to read 4 registers responsible for the state of the relay. The function is responsible for reading these registers 0х01 “Read coils”. State:

  • “0” – a relay is turned off.
  • “1” – a relay is turned on.


In the example turned on the 2-d and 4-th relay.

Modbus RTU <-> TCP converter

I switched the relay board from the USB <-> RS-485 converter to the Modbus RTU <-> TCP converter which was discussed in the article. Everything work ok. Relays managed over Wi-Fi.

Upd. Possibly, the detail documentation for the module by links, but, unfortunately, couldn’t download:

The seller doesn’t respond to my requests.

Useful links

  1. Project on Github conc. board C0135. https://github.com/TG9541/stm8ef-modbus. A lot of useful information.
  2. Others my articles concerning RS485 & Modbus.
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