Relay

Modbus Relay

This project is a fully working MODBUS RTU Relay. The project comes with schematic and PCB, as well as the firmware of course!

The core of the relay is an AVR Tiny1624, an automotive grade MPU designed for harsh environment. Note: The code optimized with -Os is <16KB. The -Og used typically for debug is <32k. Therefore, for debugging, a 3224 device is required. Remembed to edit the makefile to specify your device.

This project can be reused for other Modbus devices. A modbus interface generator is provided that makes adding modbus commands simple. The generator reads from a Python configuration file.

Example of such configuration file:

"device@44": [
        (READ_COILS,            u16(0, 2, alias="addr"),
                                u16(1, 3, alias="qty"),
                                "on_read_coils"),

        (WRITE_SINGLE_COIL,     u16(0, 2, alias="addr"),
                                u16([0xFF00, 0, 0x5500], alias="op"),
                                "on_set_single"),

        (WRITE_MULTIPLE_COILS,  u16(0, alias="from"),
                                u16(3, alias="qty"),
                                u8(1, alias="count"),
                                u8(0, 7, alias="values"),
                                "on_set_multiple"),

        (READ_HOLDING_REGISTERS, u16(0, 2), u16(1, 3), "on_read_info"),
    ]

The handling code is just as simple:

   void on_read_info(uint8_t index, uint8_t qty) {
      Datagram::pack<uint8_t>(qty*2);

      while ( qty-- ) {
         switch(index++) {
         case 0: Datagram::pack( DEVICE_ID ); break;
         case 1: Datagram::pack( HW_VERSION ); break;
         case 2: Datagram::pack( FW_VERSION ); break;
         default:
            Datagram::reply_error(modbus::error_t::illegal_data_value);
         }
      }
   }

Solder side of the board

Out of the box, the relay supports the following modbus commands:

• READ_COILS 1-3
• WRITE_SINGLE_COIL Support for 0x5500 to toggle
• WRITE_MULTIPLE_COILS 1-3
• READ_HOLDING_REGISTERS
  1. Version
  2. FW version
  3. HW version

If compiling using the supplied Dockerfile, the elf is 7KB in size, so a ATTiny824 will work.

The device default address is 44 (Decimal). By default, it runs at 115200 8E1.

KiCAD schematic and PCB are available.

Tested with QModMaster.

I plan to add address and serial configuration modbus command (and eeprom persistence).

How to build

You will need a Linux shell, or WSL shell in Windows. The tool 'gitman' is required as well as Docker. (docker-ce or else).

Steps

# Clone this repo
$ git clone https://https://github.com/adarwoo/modbus_relay.git
$ cd modbus_relay
# Optional - install gitman
$ pipx install gitman
$ gitman update
$ make NDEBUG=1 # Optional, add ARCH=attiny1624 to suit you device

The binary modbus_relay.elf can be found in the directory

Modbus Relay Device User Manual

Overview

The Modbus Relay Device is a configurable relay controller with Modbus RTU communication. This document describes how to configure and operate the device, including its default settings, configuration mode, and reset process.

Factory default settings

The relay comes pre-configured with the following value:

Resetting the relay to factory default setttings

If the relay is un-responsive, it may have been mis-configured. The relay can be reset to it's factory default using the following procedure.

1. Power off the relay
2. Configure your Modbus master configuration tool (such as QModMaster) using a baud rate of 9600, 8 data bits, No parity, and 1 stop bit (9600 8N1).
3. Ready the following message:

4. Power on the relay. All LEDs turn RED for 5 seconds.
5. During that time, send the 'Write multiple register' commmand
6. If the relay receives the command, the LEDs will blink fast for 5 seconds, then turn all on indicating the device was successfully reset
7. Now the device is factory reset and can be reached at its address 44 (0x2C).

Simple relay operation

In normal mode:

• The device responds only to frames addressed to its Device ID.
• The bus operates as configured
• Relays operate based on the configured default positions and command inversion settings.
• Regular coil commands do not respond in configuration mode to prevent mode mixing.

Supported Modbus Functions

• Read Coils (Function Code 1):
  • Addresses 0x0000 to 0x0002 for Relay 0, Relay 1, and Relay 2.
• Write Single Coil (Function Code 5):
  • Write 0xFF00 to turn on, 0x0000 to turn off, and 0x5500 to toggle a relay.
• Write Multiple Coils (Function Code 15):
  • Write a bit array to set relay states (1 = closed, 0 = open).
• Read Holding Registers (Function Code 3):
  • Register 0x0000 (40001): Product ID (0x3701).
  • Register 0x0001 (40002): Device Hardware Version (0xMMmm, where MM = major, mm = minor).
  • Register 0x0002 (40003): Software Version (0xMMmm, where MM = major, mm = minor).
  • Register 0x0008 (40009): Device address
  • Register 0x0009 (40010): Baud rate selection (1/100th of the actual baud rate)
  • Register 0x000A (40011): Parity. 0=None, 1=Odd, 2=Even
  • Register 0x000B (40012): Number of stop bits. 1=1 stop bit, 2=2 stop bits
  • Register 0x000C (40013): Watchdog window size in seconds
  • Register 0x0010 (40017 + 40019): Number of minutes in operation as a 32-bits
  • Register 0x0014 (40019 + 40020): Number of relay 0 movements as a 32-bits
  • Register 0x001A (40021 + 40022): Number of relay 1 movements as a 32-bits
  • Register 0x001C (40023 + 40024): Number of relay 2 movements as a 32-bits

Configuring the device

The factory default communication settings for the relay are:

• Device address is 44
• Baud rate is 9600
• 8 data bits
• No parity
• 1 stop bit (9600 8N1).

Configuration registers available

These can be accessed with a write single register command or the serial settings can be set with 1 write multiple command.

Note: When writting the configuration with a multiple write, all 5 settings must be written in the same transaction.

Reset the device

The register at 0x63 (40100) can be written with 0xDEAD5AFE to reset the device remotely. This is usefull to apply any changes made to the settings. The command will reply, and the leds will flash fast for 2 seconds, then solid (the device is booting).

Using the watchdog

The relay has a command watchdog which will release the relays following a period of inactivity. Everytime a valid modbus command is received (including holding register read etc.), the watchdog is reset. This feature allow for the relays to be atomatically releases if the master was to fail. The period is given in seconds. A value of zero (default) turns off the feature.

Troubleshooting

Cannot Communicate with Device

• Ensure the device is in configuration mode or responding to the correct Device ID.
• Check baud rate, parity, and stop bit settings.

Lost Device ID

1. Send a valid broadcast frame to enter configuration mode.
2. Use the default Device ID (0) to reset the device via the reset register.

Relay Does Not Respond

• Verify relay default positions and inversion settings in the holding registers.
• Check the watchdog timeout setting to ensure it isn’t triggering prematurely.

Notes

• Configuration mode is only accessible during the 2-second boot window.
• For security and reliability, avoid using Device ID 0 for normal operations.
• Always validate CRC and frame format for successful communication.