modbus-relay

Modbus TCP/IP server that translates incoming requests to RTU and relays them to configured serial device.

Caution

🚧 This project is still in WIP 🚧

Demo

• Thessla Green recuperator on /dev/ttyAMA0
• modbus-relay running on Raspberry Pi 3 Model B+, with debug enabled

• Client using rust & tokio-modbus, connects via TCP/IP

Features

• Exposes RTU device via TCP/IP
• Option to flush after every write¹
• Option to manual control of RTS¹
• Simple configuration via JSON in /etc/modbus-relay.json

Tech Stack

• Made with modern C++23
• cmake for building
• ASIO for networking/io
• libmodbus for Modbus
• spdlog for logging
• fmt for formatting
• nlohmann_json for JSON

Roadmap

• Handle errors

• Handle function code 0x01 - Read Coil Status

• Handle function code 0x02 - Read Input Status

• Handle function code 0x03 - Read Holding Registers

• Handle function code 0x04 - Read Input Registers

• Handle function code 0x05 - Force Single Coil

• Handle function code 0x06 - Preset Single Register

• Handle function code 0x0F - Force Multiple Coils

• Handle function code 0x10 - Preset Multiple Registers

• Handle function code 0x10 - Preset Multiple Registers

• Handle multiple connections

• CI/CD

• Debian/ArchLinux packages

• systemd service

Documentation

Configuration file is in /etc/modbus-relay.json. See Usage on how to generate it.

TCP/IP server options:

• tcp_ip - on which interface to listen to, ex. "0.0.0.0"
• tcp_port - on which port to listen to, ex. 5000

RTU client options:

• rtu_device - tty device, ex. "/dev/ttyAMA0"
• rtu_serial_type - serial type, "RS232" | "RS485"
• rtu_baud_rate - baud rate, ex. 9600
• rtu_parity - parity, "None" | "Even" | "Odd"
• rtu_data_bits - data bits, 5 | 6 | 7 | 8
• rtu_stop_bit - stop bit, 1 | 2
• rtu_slave_id - slave id in decimal, ex. 10 (0x0A)
• rtu_rts_enabled - wheter to use RTS, true | false
• rtu_rts_type - RTS type, "None" | "Up" | "Down"
• rtu_rts_delay - delay in us, ex. 3500
• rtu_rts_manual_control - wheter to manually set TIOCM_RTS via ioctl on rtu_device, before and after each write, true | false
• rtu_flush_after_write - wheter to manually call tcflush(rtu_device, TCIOFLUSH) after each write, true | false
• rtu_enable_libmodbus_debug - to enable libmodbus debug logging, true | false

Installation

Install dependencies mentioned in tech stack section

  mkdir -p build
  cd build
  cmake -DCMAKE_BUILD_TYPE=Release ..
  cmake --build .

Usage

To generate config file

sudo sh -c './modbus-relay -dump-default-config > /etc/modbus-relay.json'

Just run it

./modbus-relay

FAQ

Why?

Needed to expose RTU device via TCP/IP, because developing on RPI3B+ was painfully slow and full of OOMs

License

MIT

Authors

• @aljen

Footnotes

1. Needed for a device I was working with ↩ ↩²