Modbus Serial to TCP Gateway

modbus-serial2tcp listens on a Modbus RTU serial line and forwards requests to Modbus TCP servers. Use it to expose RS232/RS485 devices to software that only speaks Modbus/TCP.

Overview

• Acts as the inverse of mbusd: serial input is proxied to one or more TCP servers.
• Built in Go with minimal runtime dependencies.
• Works with common USB-to-serial adapters and physical serial ports.

Features

• Forwards Modbus RTU frames to a TCP server and returns responses to the serial client.
• Configurable serial settings (baud rate, parity, timeout) and slave ID filtering.
• Optional verbose logging for troubleshooting.
• Pre-built binaries for multiple Linux architectures, or build from source.

Installation

Grab a pre-built Linux binary from the Releases page:

• modbus-serial2tcp-amd64-linux (x86_64)
• modbus-serial2tcp-arm64-linux (ARM64)
• modbus-serial2tcp-arm-linux (ARM)

Download the file that matches your platform, rename it if desired (e.g., modbus-s2t), and make it executable:

wget https://github.com/d21d3q/modbus-serial2tcp/releases/latest/download/modbus-serial2tcp-amd64-linux -O modbus-s2t
chmod +x modbus-s2t

Usage

Quick start

Forward requests from /dev/ttyUSB0 at 9600 baud to a Modbus/TCP server running on 192.168.1.100:502 and filter for slave ID 1:

./modbus-s2t \
  --port /dev/ttyUSB0 \
  --speed 9600 \
  --tcp-server 192.168.1.100:502 \
  --slaveid 1

Command-line options

Flag	Short	Description	Default	Required
--port	-p	Serial port path (e.g., /dev/ttyUSB0, COM1)	/dev/ttyUSB0	Yes
--speed	-s	Serial port baud rate	9600	Yes
--tcp-server	-S	TCP server address (ip:port)	–	Yes
--slaveid	-i	Modbus slave ID to filter (1–247)	1	Yes
--parity	-P	Serial parity (N none, E even, O odd)	N	No
--timeout	-t	Request timeout in seconds	1	No
--verbose	-v	Increase log verbosity (repeat for more detail)	0	No

Systemd service

For unattended use, install the included modbus-s2t.service unit:

1. Copy the binary to a system path and make it executable:

   sudo cp modbus-serial2tcp /usr/local/bin/
   sudo chmod +x /usr/local/bin/modbus-serial2tcp

2. Install the service file:

   sudo cp modbus-s2t.service /etc/systemd/system/

3. Edit /etc/systemd/system/modbus-s2t.service to match your serial device and TCP target. Example:

   ExecStart=/usr/local/bin/modbus-serial2tcp --port /dev/ttyUSB0 --speed 9600 --tcp-server 192.168.1.100:502 --slaveid 1

4. Enable and start the service:

   sudo systemctl daemon-reload
   sudo systemctl enable modbus-s2t.service
   sudo systemctl start modbus-s2t.service

5. Check status and logs:

   sudo systemctl status modbus-s2t.service
   sudo journalctl -u modbus-s2t.service -f

Build from source

Requirements:

• Go 1.19 or later
• just (optional, for using the provided recipes)

git clone https://github.com/d21d3q/modbus-serial2tcp.git
cd modbus-serial2tcp

# Build using just (produces binaries for common targets)
just build-all

# Or build manually
go build -o modbus-serial2tcp .

Acknowledgments

• mbserver
• modbus

License

MIT License. See LICENSE.