Overview

Electrical Safety Precautions

Hardware Connection

• Connect the USB TO 485 to the target boards via cables, A-A and B-B connected as shown below:
  

Example Demonstration

SSCOM Serial Port Debugging Assistant

• Download SSCOM Serial port debugging assistant and open it on the computer, open the corresponding port number, and set the baud rate as 9600. Click Multi-Strings to open the Send Multi-Char window, and click the function to send the corresponding command.
  

• If you need to send other commands, choose SendHEX. For checksum validation, select ModbusCRC16. After entering the first six bytes of the command, clicking SEND will automatically add the CRC check code.
  

• For detailed control commands, please see the development protocol.

From September 2024, the V3 development protocol will be adopted, which will be compatible with the V2 version, and some new features will be added.

Modbus Poll Software

• It is not convenient to use the SSCOM software for observing the data, you can select Modbus Poll software to read the data. Download and install the Modbus Poll software.
• Open the software, select Setup->Read/Write Definition, select the actual device address for Slave ID, select 01 Read Coils function code for Function, and change Quantity to 8 channels. Click OK to confirm.

• Select Connection->Connect..., choose the corresponding serial port, set the baud rate to 9600, and select 8 Data bits and None Parity. Click OK to connect.

• After the connection is normal, you can check the current relay status. Select the corresponding channel, then double-click the status value to pop up the send page. Choose On or Off, then click Send to control the relay opening and closing.

Demo Test

Note: RS485 can not be directly connected to the serial port of the Raspberry Pi, otherwise it may burn the device, you need to add 485 level conversion. For Raspberry Pi, it is recommended to work with the RS485 CAN HAT module. For NUCLEO-F103RB and Arduino, it is recommended to work with the RS485 CAN Shield module.

Raspberry Pi

Open the Raspberry Pi terminal and enter the following command to enter the configuration interface

sudo raspi-config
Select Interfacing Options -> Serial, disable shell access, and enable the hardware serial port

Then restart Raspberry Pi:

sudo reboot

Open the /boot/config.txt file, find the following configuration statement to enable the serial port, if not, you can add it to the end of the file.

enable_uart=1

For Raspberry Pi 3B users, the serial port is used for Bluetooth and needs to comment out:

#dtoverlay=pi3-miniuart-bt

Then restart Raspberry Pi:

sudo reboot

Insert the RS485 CAN HAT into the Raspberry Pi, and connect the Modbus RTU Relay module to the RS485 CAN HAT through A and B.
If you are using other 485 devices, make sure to connect A-A, B-B.
Run the following commands to run the demo:

sudo apt-get install unzip
wget https://files.waveshare.com/wiki/Modbus-RTU-Relay/Modbus_RTU_Relay_Code.zip
unzip Modbus_RTU_Relay_Code.zip
cd Modbus_RTU_Relay_Code/Python3
sudo python3 main.py

STM32

Note: The STM32 demo is based on the NUCLEO-F103RB and RS485 CAN Shield module.

1. Download Demo, find the STM32 project file Modbus Relay.uvprojx in the path Modbus_RTU_Relay_Code\STM32\MDK-ARM, and double-click to open the STM32 project file. Note that you should ensure Keil5 software is installed on your computer before using it.

2. ​​Connect the STM32 to a computer via the STM32 download and debug probe.​ Compile and download the demo to the STM32.

3. Install the RS485 CAN Shield module on the STM32. Connect the RS485_A on the RS485 CAN Shield module to the RS485_A on the Modbus RTU Relay via a wire, and connect the RS485_B on the RS485 CAN Shield module to the RS485_B on the Modbus RTU Relay via a wire. Then power on the Modbus RTU Relay and the STM32 sequentially.

4. After powering on, the serial port will output the sent commands, which can be observed using a serial port assistant. At the same time, the relays will turn on in sequence (e.g., 1→2→3→4), and after all of them are turned on, they will then turn off in sequence (e.g., 1→2→3→4). You can observe whether the relay is normally engaged through the LED indicator light.

Arduino

Note: The Arduino demo is based on the UNO PLUS and RS485 CAN Shield module.
1. Download Demo, find the Arduino project file Modbus_RTU_Relay.ino in the path Modbus_RTU_Relay_Code\Arduino\Modbus_RTU_Relay, and double-click to open the Arduino project file. Note that you should ensure Arduino IDE software is installed on your computer before using it.

2. Connect the Arduino to the computer via a USB cable. In the Arduino IDE software, select the Arduino board model under Tools->Board. Choose the COM port that the Arduino is connected to under Tools->Port.
3. After seeing the prompt to connect to the computer in the lower right corner, click to compile and flash the program, and wait for the flashing to complete.

4. Install the RS485 CAN Shield module on the Arduino. Connect the RS485_A on the RS485 CAN Shield module to the RS485_A on the Modbus RTU Relay via a wire, and connect the RS485_B on the RS485 CAN Shield module to the RS485_B on the Modbus RTU Relay via a wire. Then power on the Modbus RTU Relay and the Arduino sequentially.

5. After powering on, the serial port will output the sent commands, which can be observed using a serial port assistant. At the same time, the relays will turn on in sequence (e.g., 1→2→3→4), and after all of them are turned on, they will then turn off in sequence (e.g., 1→2→3→4). You can observe whether the relay is normally engaged through the LED indicator light.

Note: The data in the figure below is in ASCII code containing spaces and line breaks, and is not the actual data sent.

PLC

Note: The PLC demo is based on SIMATIC S7-200 SMART.

1. Download Demo, find the PLC project file modbus_rtu.smart in the path Modbus_RTU_Relay_Code\PLC, and double-click to open the PLC project file. Note that you should ensure STEP 7-MicroWIN SMART software is installed on your computer before using it.

2. Connect the PLC to the computer via a network cable. Click to download

3. Select the communication interface in the communication popup, find your device, and click Confirm

4. In the download pop-up window, check the boxes for program blocks, data blocks, and system blocks, and then click Download.

5. Connect the 485_A on the PLC module to RS485_A on the Modbus RTU Relay with a wire, and connect the 485_B on the PLC module to RS485_B on the Modbus RTU Relay with a wire. Then power on the Modbus RTU Relay.

6. After powering on the Modbus RTU Relay, in the STEP 7-MicroWIN SMART software, find the "PLC" tab in the upper menu bar, switch to this tab, and click the green "RUN" button in the lower "Operation" area to perform the operation to make the PLC enter the running state

7. After the PLC runs, the relays will turn on in sequence (e.g., 1→2→3→4), and after all of them are turned on, they will then turn off in sequence (e.g., 1→2→3→4). You can observe whether the relay is normally engaged through the LED indicator light.

Development Protocol V3

Relay 0 on: 01 05 00 00 FF 00 8C 3A
Relay 0 off: 01 05 00 00 00 00 CD CA
Relay 1 on: 01 05 00 01 FF 00 DD FA
Relay 1 off: 01 05 00 01 00 00 9C 0A
Relay 2 on: 01 05 00 02 FF 00 2D FA
Relay 2 off: 01 05 00 02 00 00 6C 0A
Relay 3 on: 01 05 00 03 FF 00 7C 3A
Relay 3 off: 01 05 00 03 00 00 3D CA
Relay 0 toggle: 01 05 00 00 55 00 F2 9A
Relay 1 toggle: 01 05 00 01 55 00 A3 5A
Relay 2 toggle: 01 05 00 02 55 00 53 5A
Relay 3 toggle: 01 05 00 03 55 00 02 9A

All relays on: 01 05 00 FF FF 00 BC 0A
All relays off: 01 05 00 FF 00 00 FD FA
All relays toggle: 01 05 00 FF 55 00 C2 AA

Send: 01 01 00 00 00 08 3D CC
Receive: 01 01 01 00 51 88       //all relays off
Send: 01 01 00 00 00 08 3D CC
Receive: 01 01 01 01 90 48       //Relay 0 is on, others are off
Send: 01 01 00 00 00 08 3D CC
Receive: 01 01 01 41 91 B8       //Relay 0 and 6 are on, others are off

All relays on: 01 0F 00 00 00 08 01 FF BE D5
All relays off: 01 0F 00 00 00 08 01 00 FE 95
0-1 on; 2-7 off: 01 0F 00 00 00 08 01 03 BE 94

Relay 0 flash on: 01 05 02 00 00 07 8D B0   //700MS = 7*100MS = 700MS
Relay 1 flash on: 01 05 02 01 00 08 9C 74   //800MS
Relay 0 flash off: 01 05 04 00 00 05 0C F9  //500MS
Relay 1 flash off: 01 05 04 01 00 06 1D 38  //600MS

Set the baud rate as 4800: 01 06 20 00 00 00 82 0A
Set the baud rate as 9600: 01 06 20 00 00 01 43 CA
Set the baud rate as 115200: 01 06 20 00 00 05 42 09

Set the device address as 0x01: 00 06 40 00 00 01 5C 1B
Set the device address as 0x02: 00 06 40 00 00 02 1C 1A
Set the device address as 0x03: 00 06 40 00 00 03 DD DA

Send: 00 03 40 00 00 01 90 1B
Receive: 00 03 02 00 02 04 45     //Address: 0x02

Send: 01 03 80 00 00 01 AD CA
Receive: 01 03 02 01 2C B8 09       //0x012C = 300 =V3.00