What Is Modbus? The Industrial Protocol That Connects PLCs, HMIs, and Field Devices

What Is Modbus?

Modbus is a serial communication protocol originally developed by Modicon in 1979. It defines how a master device (typically a PLC, SCADA server, or HMI) requests data from and sends commands to one or more slave devices (sensors, drives, meters, field instruments).

Modbus RTU vs Modbus TCP

Variant	Physical Layer	Typical Use
Modbus RTU	RS-232 or RS-485 serial	PLC ↔ HMI, PLC ↔ VFD, PLC ↔ energy meter
Modbus ASCII	RS-232 serial	Legacy systems, low-speed devices
Modbus TCP	Ethernet (TCP/IP)	SCADA ↔ PLC, MES ↔ controllers

Modbus RS-485: Multi-Drop Networks

Modbus RTU over RS-485 supports up to 32 devices on a single two-wire bus (extendable to 247 with repeaters) — making it ideal for connecting multiple field devices to a single PLC COM port. Maximum cable length is 1200 metres at 9600 baud.

Modbus Register Types

• Coils (0x) — digital read/write (relay outputs, digital commands)
• Discrete Inputs (1x) — digital read-only (limit switches, digital sensor states)
• Input Registers (3x) — 16-bit analogue read-only (temperature, pressure, flow)
• Holding Registers (4x) — 16-bit read/write (setpoints, configuration, counters)

All Cermate HMI panels and Avalue industrial PCs support both Modbus RTU and Modbus TCP as standard. Contact TSL Automation Solutions for wiring guidance and protocol configuration support.