Modbus-RTU-Relay-C User Guide

Overview

Electrical Safety Precautions

This product should be operated and used by professional electricians or technical personnel. During use, please ensure electrical safety and take protective measures such as anti-leakage and insulation.
Before installing, maintaining, or replacing relay equipment, make sure to turn off the power and unplug the device.
Do not attempt to dismantle relay equipment to avoid damaging the device or risking electric shock.
Please properly install and place the relay equipment. Avoid using it in damp, overly hot, or flammable environments to prevent safety accidents due to improper installation or use.

Specifications

Power Supply	DC 7~36V
Communication Interface	RS485
Communication Baudrate	4800，9600，19200，38400，57600，115200，128000，256000
Default Communication Format	9600，N，8，1
Relay Channels	8 Channels
Relay Type	Magnetic latching relay
Contact Form	1NO 1NC
Contact Load	≤10A 250V AC 或 ≤10A 28V DC
Modbus Protocol	Standard Modbus RTU protocol
RS485 Address Setting	1~255
Indicator	STA: MCU indicator, keep flashing when MCU normally working TXD: Transmit Data indicator, lights up when sending data RRXD: Receive Data indicator, lights up when receiving data

Precautions

The module comes out of the factory in the default state, but due to impacts during transportation or installation, its operational state may change. Therefore, it is necessary to reset the relay state when powering on as needed.
The magnetic latching relay will only supply power when the state is changed, and the relay is usually in a power-off state and only relies on magnetic attachment, so it should be avoided to vibrate the module strongly, and the relay state may be changed when it is impacted.
The module status information can be saved when powered off and restored when powered on. However, if the relay is powered off when the state is changed, the state may be uncertain, and the state may change when it is shocked, so it is recommended to reset the relay status as needed after each power-on.
During use, please keep the product away from strong magnetic fields to avoid changes in the operating voltage and reset voltage.

Hardware Connection

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

Example Demonstration

The demo shows how the following two software operate.
SSCOM serial port debugging assistant is more convenient to operate, free of installation, and more convenient for complete display and analysis of instructions, but the disadvantage is that the data is not intuitive.
Modbus Poll software is directly operated on the register, and the data display is more convenient to observe, but the disadvantage is that the instruction is not displayed completely, so you need to be familiar with the Modbus register operation.
You can test using any method. It is recommended to use the SSCOM serial port debugging assistant software for the first test.

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-Char 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.

Modbus Poll Software

The serial port software is not convenient to observe the data, you can choose 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 be commented 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 RS485 CAN HAT through A and B.
If you are using other 485 devices, make sure to connect A-A, B-B.< br/> Run the following commands to run the demo:

sudo apt-get install unzip
wget https://files.waveshare.com/wiki/Modbus%20RTU%20Relay/Modbus_RTU_Relay_Code.zip
unzip Modbus_RTU_Relay_Code.zip
cd Modbus_RTU_Relay_Code/Python3
sudo python3 main.py

STM32

The STM32 demo is based on the NUCLEO-F103RB and RS485 CAN Shield module.
Find the STM32 demo file directory and open the STM32 project. Please note that the keil5 software is installed before use, and download the demo to the development board.
Normal operation of the relay module will sequentially open and then close the channels. The serial port will output the commands to be sent.

Arduino

The Arduino demo is based on the UNO PLUS and RS485 CAN Shield module.
Use the Arduino IDE to open the demo, select the corresponding development board, and download the demo.
Normal operation of the relay module will sequentially open and then close the channels. The serial port will output the commands to be sent.

Development Protocol

Function Code Introduction

Function Code	Description	Note
01	Read coil status	Read relay status
03	Read holding register	Read the address and version
05	Write single coil	Write single relay
06	Write single register	Set the baud rate and address
0F	Write multiple coils	Write all relays

Register Address Introduction

Address (HEX)	Address storage content	Register value	Permission	Modbus Function Code
0x0000 …… 0x0007	Channel 1~8 relay address	0xFF00: relay on 0x0000: relay off 0x5500: relay toggle	Read/Write	0x01, 0x05, 0x0F
0x00FF	Control all relays	0xFF00: all relays on 0x0000: all relays off 0x5500: all relays toggle	Write	0x05
0x0100 …… 0x0107	Channel 1~8 relay toggle	0xFF00: relay toggle 0x0000: relay unchanged	Write	0x05, 0x0F
0x01FF	All relays toggle	0xFF00: all relays toggle 0x0000: all relays unchanged	Write	0x05
0x0200 …… 0x0207	Channel 1~8 relay flash on	Interval time: data100ms Value: 0x0007, Interval time: 7100MS = 700MS	Write	0x05
0x0400 …… 0x0407	Channel 1~8 relay flash off	Interval time: data100ms Value: 0x0007, Interval time: 7100MS = 700MS	Write	0x05
4x2000	UART Parameter	The high eight bits indicate the parity mode: 0x00~0x02 The low eight bits indicate the baud rate mode: 0x00~0x07	Read/Write	0x03, 0x06
4x4000	Device Address	Directly store Modbus address Device address: 0x0001-0x00FF	Read/Write	0x03, 0x06
4x8000	Software Version	Converting to decimal and then shifting the decimal point two places to the left will represent the software version 0x0064 = 100 = V1.00	Read	0x03

Modbus RTU Command Introduction

Control Single Relay

Send code: 01 05 00 00 FF 00 8C 3A

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
05	05 Command	Relay control
00 00	Address	The register address of the relay to be controlled, 0x0000-0x0007
FF 00	Command	0xFF00: relay on; 0x0000: relay off; 0x5500: relay toggle
8C 3A	CRC16	The CRC16 checksum of the first 6 bytes of data

Return code: 01 05 00 00 FF 00 8C 3A

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
05	05 Command	Relay control
00 00	Address	The register address of the relay to be controlled, 0x0000-0x0007
FF 00	Command	0xFF00: relay on; 0x0000: relay off; 0x5500: relay toggle
8C 3A	CRC16	The CRC16 checksum of the first 6 bytes of data

For example: [Address 1 device]:

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

Control All Relays

Send code: 01 05 00 FF FF 00 BC 0A

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
05	05 Command	Relay control
00 FF	Address	Fixed 0x00FF
FF 00	Command	0xFF00: relay on; 0x0000: relay off; 0x5500: relay toggle
BC 0A	CRC16	The CRC16 checksum of the first 6 bytes of data

Return code: 01 05 00 FF FF 00 BC 0A

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
05	05 Command	Relay control
00 FF	Address	Fixed 0x00FF
FF 00	Command	0xFF00: relay on; 0x0000: relay off; 0x5500: relay toggle
BC 0A	CRC16	The CRC16 checksum of the first 6 bytes of data

For example: [Address 1 device]:

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

Read Relay Status

Send code: 01 01 00 00 00 08 3D CC

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
01	01 Command	Query relay status
00 00	Relay Start Address	The register address of the relay, 0x0000 - 0x0007
00 08	Relay Number	The number of relays to be read, which must not exceed the maximum number of relays
3D CC	CRC16	The CRC16 checksum of the first 6 bytes of data

Receive code: 01 01 01 00 51 88

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
01	01 Command	Query relay status
01	Byte Number	The number of all bytes of the returned status information
00	Query status	Received relay status Bit0: the first relay status; Bit1: the second relay status; And so on, with the idle high bit being zero
51 88	CRC16	The CRC16 checksum of the first 6 bytes of data

For example: [Address 1 device]

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

Write Relay Status

Send code: 01 0F 00 00 00 08 01 FF BE D5

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
0F	0F Command	Write relay status
00 00	Relay Start Address	The register address of the relay to be controlled, 0x0000 - 0x0007
00 08	Relay Number	The number of relays to be operated, which must not exceed the maximum number of relays
01	Byte Number	The byte number of the status
FF	Relay Status	Bit0: the first relay status; Bit1: the second relay status; And so on, with the idle high bit being zero
BE D5	CRC16	The CRC16 checksum of the first 6 bytes of data

Receive code: 01 0F 00 00 00 08 54 0D

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
0F	0F Command	Control all registers
00 00	Relay Start Address	The register address of the relay to be controlled, 0x0000 - 0x0007
00 08	Relay Number	The number of relays to be operated
54 0D	CRC16	The CRC16 checksum of the first 6 bytes of data

For example: [Address 1 device]

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 Flash ON/OFF Command

Send code: 01 05 02 00 00 07 8D B0

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
05	05 Command	Single control command
02	Flash on flash off	02: flash on, 04: flash off
00	Relay Address	The address of the relay to be controlled, 0x00~0x07
00 07	Interval Time	The interval time: data100ms Value: 0x0007, Interval time: 7100MS = 700MS The maximum setting for the flash-on flash-off time is 0x7FFF
8D B0	CRC16	The CRC16 checksum of the first 6 bytes of data

Receive code: 01 05 02 00 00 07 8D B0

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
05	05 Command	Single control command
02	Flash on flash off	02: flash on, 04: flash off
00	Relay Address	The address of the relay to be controlled, 0x00~0x07
00 07	Interval Time	The interval time: data100ms Value: 0x0007, Interval time: 7100MS = 700MS The maximum setting for the flash-on flash-off time is 0x7FFF
8D B0	CRC16	The CRC16 checksum of the first 6 bytes of data

For example: [Address 1 device]

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 Baudrate Command

Send code: 01 06 20 00 00 05 42 09

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
06	06 command	Set the baud rate and device address
20 00	Baud rate register	0x2000: set the baud rate
00	Parity Method	x00: no parity, 0x01: even parity; 0x02: odd parity
05	Baud Rate Value	Corresponding baud rate 0x00: 4800 0x01: 9600 0x02: 19200 0x03: 38400 0x04: 57600 0x05: 115200 0x06: 128000 0x07: 256000
42 09	CRC16	The CRC16 checksum of the first 6 bytes of data

Receive code: 01 06 20 00 00 05 42 09

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
06	06 command	Set the baud rate and device address
20 00	Baud rate register	0x2000: set the baud rate
00	Parity Method	x00: no parity, 0x01: odd parity; 0x02: even parity
05	Baud Rate	Corresponding baud rate 0x00: 4800 0x01: 9600 0x02: 19200 0x03: 38400 0x04: 57600 0x05: 115200 0x06: 128000 0x07: 256000
42 09	CRC16	The CRC16 checksum of the first 6 bytes of data

For example: [Address 1 device]

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 Device Address Command

Send code: 01 06 40 00 00 01 5D CA

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
06	06 command	Set the baud rate and device address
40 00	Device address register	0x4000: set the device address
00 01	Device Address	Set the device address, 0x0001-0x00FF
5D CA	CRC16	The CRC16 checksum of the first 6 bytes of data

Receive code: 01 06 40 00 00 01 5D CA

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
06	06 command	Set the baud rate and device address
40 00	Device address register	0x4000: set the device address
00 01	Device Address	Set the device address, 0x0001-0x00FF
5D CA	CRC16	The CRC16 checksum of the first 6 bytes of data

For example: [Address 1 device]

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

Read Device Address Command

Send code: 00 03 40 00 00 01 90 1B

Field	Description	Note
00	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
03	03 Command	Read the device address
40 00	Device address register	0x4000: read the device address
00 01	Byte Number	Fixed 0x0001
90 1B	CRC16	The CRC16 checksum of the first 6 bytes of data

Receive code: 00 03 02 00 01 44 44

Field	Description	Note
00	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
03	03 Command	Read the software version and device address
02	Byte Number	The number of bytes returned
00 01	Device Address	Set the device address, 0x0001-0x00FF
44 44	CRC16	The CRC16 checksum of the first 6 bytes of data

For example: [Address 2 device]

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

Read Software Version Command

Send code: 01 03 80 00 00 01 AD CA

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
03	03 Command	Read the software version and device address
80 00	Software version register	0x4000: read the device address, 0x8000: read software version
00 01	Byte Number	Fixed 0x0001
AD CA	CRC16	The CRC16 checksum of the first 6 bytes of data

Receive code: 01 03 02 01 2C B8 09

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
03	03 Command	Read the software version and device address
02	Byte Number	The number of bytes returned
01 2C	Software Version	Converting to decimal and then shifting the decimal point two places to the left will represent the software version 0x012C = 300 = V3.00
B8 09	CRC16	The CRC16 checksum of the first 6 bytes of data

For example:

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

Exception Function Code

When the received command is incorrect or the device is abnormal, an exception response will be returned in the following format:

Receive: 01 85 03 02 91

Field	Description	Note
01	Device Address	0x00 indicates the broadcast address, 0x01-0xFF indicates the device address
85	Exception Function Code	Exception function code = Request function code + 0x80
03	Byte Number	Exception Code
02 91	CRC16	The CRC16 checksum of the first 6 bytes of data

An exception code is a single-byte value that indicates the type of error. Several commonly used exception codes defined by the Modbus protocol:

Exception Code	Name	Description
0x01	Illegal Function	The requested function code is not supported
0x02	Illegal Data Address	The requested data address is incorrect
0x03	Illegal Data Value	The requested data value or operation cannot be executed
0x04	Server Failure	Server equipment failure
0x05	Response	The request has been received and is being processed
0x06	Device Busy	The device is currently busy and cannot perform the requested operation

Resources

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