Modbus is a widely used communication protocol in industrial automation and control systems. Developed by Modicon (now Schneider Electric) in 1979, Modbus has become a de facto standard in the industry due to its simplicity, reliability, and open nature. It enables communication between electronic devices, allowing data to be transferred among controllers, sensors, and other industrial equipment.

Physical layer

There are several types of Modbus, each designed for different applications and environments. The most common variants are Modbus RTU (Remote Terminal Unit), Modbus ASCII, and Modbus TCP/IP. Modbus RTU and ASCII are typically used in serial communication environments, while Modbus TCP/IP is designed for use in Ethernet networks. Modbus RTU is the most widely used variant due to its compact binary representation of data, which allows for efficient transmission.

Client‑server architecture

Modbus uses a simple client‑server (also known as master‑slave) communication model. In this model, the client (master) initiates requests, and the server (slave) responds to those requests. This straightforward approach allows for easy implementation and troubleshooting. The protocol supports various data types, including discrete inputs (single‑bit, read‑only), coils (single‑bit, read‑write), input registers (16‑bit, read‑only), and holding registers (16‑bit, read‑write). These data types provide flexibility in representing information ranging from simple on/off states to more complex numerical values.

Object types

Although the Modbus protocol manages most communication details, it is essential to understand the “function codes” or “entities” that Modbus supports. Since Modbus lacks discovery features, users must manually configure the relevant registers and coils before devices can communicate.

Input Register

The input register is the most common entity type. It represents a read‑only value supplied by a sensor or device. Input registers store a 16‑bit integer (0–65535) and cannot directly represent larger numbers, floating‑point values, or other data types. To address this, many mappings combine two adjacent registers (32 bits) to hold larger integers or a single‑precision floating‑point value. Consult the device’s Modbus register map to determine the specific mapping used.

Holding Register

A holding register supports both read and write operations. Like an input register, it stores a 16‑bit value, but a client can also write to it. Holding registers are commonly used for configurable parameters that may be adjusted during operation.

Discrete Inputs

A discrete input is a read‑only single bit, typically representing an on/off condition from a sensor or device. Modern hardware often uses other data types, making discrete inputs less common.

Coils

Coils are read‑write bits. Like discrete inputs, they represent single‑bit values, but a client can modify them. Coils are often used to control on/off parameters within a device.

Advantages

One of Modbus’s key strengths is its versatility. It can be employed across a wide range of industries, including manufacturing, energy, building automation, and transportation. The protocol’s simplicity makes it easy to implement on hardware platforms ranging from small microcontrollers to large programmable logic controllers (PLCs). Although it is an older protocol, Modbus remains relevant in modern industrial settings, often serving as a bridge between legacy systems and newer, more advanced communication networks.

Hardware and software

Paragon’s N-series controllers can interface with existing Modbus infrastructure at a site. Two common applications that require Modbus hardware integration are:

• Connecting to legacy Modbus hardware devices - An N-series controller can communicate with legacy Modbus devices and provide gateway translation to numerous other output types. Wireless Modbus can also be added easily with this configuration.
• Converting non‑Modbus sensors and controls to a BAS - Large commercial buildings often have an existing Modbus‑based building automation system (BAS). When non‑Modbus sensors and controls need to be added, an N-series controller can bridge the two systems.