RTU Modbus Configuration Tutorial

Modbus is the lingua franca of industrial automation. This guide covers both Modbus RTU (serial) and Modbus TCP (Ethernet) on the FourFaith RTU family.

Modbus Basics

Modbus is a request/response protocol where a master (e.g. SCADA) polls slaves (e.g. RTU) for register values. There are 4 data types: Coils (read/write bits), Discrete Inputs (read-only bits), Input Registers (16-bit read-only), and Holding Registers (16-bit read/write).

Configuring Modbus RTU (Serial)

Connect the RTU's RS485 port to your master device. Set baud rate (default 9600), parity (default None), data bits (8), stop bits (1), and slave address (default 1). Use the FourFaith Toolbox app to configure these via USB or Bluetooth.

Configuring Modbus TCP (Ethernet)

Set a static IP address on the RTU's Ethernet port, enable Modbus TCP server, and configure the TCP port (default 502). The RTU will respond to any master on the network.

Register Map

All FourFaith RTUs use a standardized register map. Input registers 0-7 hold DI status, 100-107 hold AI values (in engineering units, 0.1 resolution). Holding registers 0-15 control DO and configuration parameters. Full map is in the user manual.

Testing with Modbus Poll

Use a free tool like 'Modbus Poll' (Windows) or 'qModMaster' to test your configuration before connecting to SCADA. Verify function codes 1, 2, 3, 4 work as expected.