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

TSL Automation Solutions September 2, 2025

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.

Frequently Asked Questions

What is Modbus protocol in industrial automation?

Modbus is a serial communication protocol developed in 1979 by Modicon (now Schneider Electric), widely used in industrial automation for connecting electronic devices — PLCs, sensors, drives, meters, and HMIs. It operates in a master-slave model where one master device polls multiple slave devices for data. Modbus remains the most universally supported industrial protocol due to its simplicity, open standard, and virtually universal adoption across equipment manufacturers.

What is the difference between Modbus RTU, Modbus ASCII, and Modbus TCP?

Modbus RTU uses binary encoding over RS-485 serial — compact, fast, most common for serial Modbus. Modbus ASCII uses ASCII character encoding over RS-232 — less efficient but easier to debug. Modbus TCP wraps the Modbus application protocol in TCP/IP packets over standard Ethernet — the modern version for Ethernet-connected devices. In Indian manufacturing, Modbus RTU (RS-485) is most common in legacy installations; Modbus TCP is standard for new Ethernet-connected devices.

How many devices can connect on a Modbus RTU network?

Modbus RTU over RS-485 supports up to 32 addressable slave devices on a single segment (247 addresses are defined, but RS-485 electrical limits typically restrict to 32 without repeaters). Each slave has a unique address (1–247). The Modbus master polls each slave sequentially — scan time increases with each additional device and the amount of data read. For more than 32 devices, use RS-485 repeaters or switch to Modbus TCP over Ethernet.

What data types can Modbus read and write?

Modbus reads and writes four data types: Coils (discrete outputs — ON/OFF states, 1-bit), Discrete Inputs (read-only digital inputs — sensor states, 1-bit), Holding Registers (16-bit read/write registers for setpoints, output values, configuration), and Input Registers (16-bit read-only registers for process values, measurements). 32-bit values (float, DWORD) are stored across two consecutive 16-bit registers.

Do Cermate HMI panels and Avalue PCs support Modbus?

Yes — all Cermate HMI panels support Modbus RTU (RS-485) and Modbus TCP natively as standard drivers. Avalue industrial PCs support Modbus TCP through SCADA software and OPC UA servers with Modbus drivers (KEPServerEX, Ignition, Matrikon OPC). TSL Automation Solutions provides full Modbus connectivity support for both HMI and PC platforms. Contact our Mumbai team for Modbus configuration assistance.

Tags: what is Modbus Modbus RTU explained Modbus TCP Modbus RS-485 PLC Modbus protocol HMI Modbus connectivity industrial Modbus guide India

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TSL Automation Solutions

Head of Marketing, TSL Automation Solutions

Sanjana covers industrial automation trends, product launches, and technology insights for TSL Automation Solutions, a Mumbai-based distributor of HMI, Panel PC, and embedded computing systems serving manufacturers across India and globally.

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

Table of Contents

What Is Modbus?

Modbus RTU vs Modbus TCP

Modbus RS-485: Multi-Drop Networks

Modbus Register Types

Frequently Asked Questions

Need help choosing the right product?

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