Modbus RTU Relay Module

Here is a Modbus RTU Relay Module I got some time ago, but waited for time to test it. It is a Modbus 2-channel relay module is expected to be very useful device for making something Modbus compatible. It should be suitable for industrial control equipments and/or component in IoT applications.

Description:
Specification:
Power Source: DC
Contact Load: Low Power
Voltage: 12V

Modbus RTU instruction
Baud rate: 9600 8 NONE 1

This cheap relay module arrived with no user manual, no operating instructions, no datasheets, and no technical support.

Steps:
1. Software settings communication baud rate
2. Set the address (communication using the device address, the default address is 01)

Modbus RTU instructions
Baud rate :9600 8 NONE 1

Modbus RTU Relay Module article describes 5V version of same type of board. The article list also the following commands to control the relays on the board.

Relay 1 ON = 01 05 00 00 FF 00 8C 3A
Relay 2 ON = 01 05 00 01 FF 00 DD FA
Relay 1 OFF = 01 05 00 00 00 00 CD DA
Relay 2 OFF = 01 05 00 01 00 00 9C 0A

After finishing the command I should get get the corresponding relay click and LED feedback and relay indicator lights up when the relay is ON.

I needed a software to send those codes to the device. The typical normal terminal editors like TeraTerm and Putty are not the right tools to do that. I need something where I can feed those HEX codes and send them out when I need. HTerm is a free terminal program for serial communication running on Windows and Linux. It supports sending messages you define (it accept input in HEX format). I installed to Windows version to a computer that run Windows 10.

The software worked well and my card reacted to most of those commands. Everything except relay one off command worked.

The board tells that it is designed to work with 12V power (it has relays with 12V coils in there). For a short test it seemed to work OK when powered with 9V battery. The relays say that they can control 10A 250V AC or 10A 30V DC. The relay outputs have a changeover contacts (closes one circuit and simultaneously opens a second circuit).

According Modbus protocol description the data packets indicate that the default device address is 1 (0×01) and the relays are controlled with Write Single Coil (0×05) command. The write address for first relay is 0 (0×0000) and for second relay it is 1 (0×0001). The value to write to turn on relay is 0xFF00 and to turn relay off the value to write is 0×0000.

Next test was to try a Modbus test software for controlling the relays. I used a free PowerHud Mobbus Tester softwARE from https://www.modbustester.com/ to test Modbus communications. It worked well and could control both relays.

The module also supports two digital inputs. There was lack of exact technical data on voltage levels they are designed to use and the addresses the information should be read from. But after some testing I found out that the inputs can be rad with Read Discrete Input (0×02) command at addresses 0×0000 for IN1 and 0×0001 for IN2.

The inputs are connected to the power input ground

The inputs seemed to react so that a positive voltage of 0 to 0.6V showed state “off” and voltage of 0.7V to 12V showed state “on”. Due the lack of exact technical data I did not try higher voltage than the device nominal operating voltage. The input circuit seems to consists of a transistor driven with few resistors. I measured that the input takes around 2 mA current when you feed control voltage to it from 9V battery.

Information on other board that look similar but might not be exactly similar:

Modbus RTU Relay Module
https://www.electroschematics.com/modbus/

MODBUS RTU 2 CHANNEL RELAY MODULE FIRST TEST
https://www.youtube.com/watch?v=o5cIF3TRwyg

N4ROB02 2CH Micro Size MODBUS RTU RS485 Relay Board
https://www.youtube.com/watch?v=P0LsmAz_VpU

Modbus-Rtu 2 Relay Outputs 2 Switch Inputs TTL/ RS485 Interface Communication
https://www.fruugo.fi/modbus-rtu-2-relay-outputs-2-switch-inputs-ttl-rs485-interface-communication/p-274194316-608528429?language=en&ac=croud&utm_source=organic_shopping&utm_medium=organic

N4ROB02 2CH Micro Size MODBUS RTU RS485 Relay Board
https://www.youtube.com/watch?v=P0LsmAz_VpU

Modbus RTU 2 Way Relay Module TTL/RS485 Interface Communication with Indicator
https://www.ebay.co.uk/itm/144858429900?chn=ps&_ul=GB&_trkparms=ispr%3D1&amdata=enc%3A1n1VKqhPwR8y76HhajGXCxw84&norover=1&mkevt=1&mkrid=710-169684-076987-9&mkcid=2&itemid=144858429900&targetid=293946777986&device=c&mktype=pla&googleloc=1005576&poi=&campaignid=20646531284&mkgroupid=154544387436&rlsatarget=pla-293946777986&abcId=&merchantid=232722533&gad_source=1&gclid=EAIaIQobChMIgLXm4fPEhAMV-VCRBR0IYgy8EAQYCCABEgLCAfD_BwE

LC 2-way Modbus relay module
http://www.chinalctech.com/m/view.php?aid=455

N4ROB02 2CH Micro Size MODBUS RTU RS485 Relay Board
https://www.youtube.com/watch?v=P0LsmAz_VpU