What is Modbus Protocol

A Modbus network consists of a programmable controller with intelligent terminals and a computer connected by a joint line or a local dedicated line. The system architecture consists of both hardware and software. It is for various data acquisition and process monitoring applications.

The Modbus network has only one host from which all communications are sent. The network can support up to 247 remote slave controllers, but the actual number of slaves supported is determined by the communication equipment used. With this system, each PC can exchange information with the central host without affecting the execution of each PC’s control tasks.

Modbus protocol is a serial communication protocol, which can be said to be the most widely used communication protocol in industrial automation, with which products of different manufacturers can communicate with each other simply and reliably.

The Modbus protocol is a common language applied to electronic controllers. The Modbus protocol defines a message structure that controllers can recognize and use, regardless of the network they communicate. It describes how a controller requests access to other devices, such as responding to requests from different devices and how errors are detected and logged. Modbus is a request/response protocol.

The current Modbus protocols are Modbus ASCII, Modbus RTU, Modbus TCP, etc. We can say Modbus is like a standard language, and these three are like local dialects.

TCP is mainly for network communication applications. ASCII and RTU are mainly for serial communication applications. Modbus RTU is a compact, binary representation of data, and Modbus ASCII is a human-readable, lengthy representation. Modbus RTU is most commonly used in industrial fields.

Its common function codes are 01.02.03.(04).05.06.0F.10.

• 01.Read the status of the switching output
• 02.Read the status of switch input
• 03.Read register value
• 05.Writing individual switching outputs
• 06.Writing a single register value
• 0F.Write multiple switching outputs
• 10.Writing multiple register values

Why is Modbus more widely used than other protocols?

• Publicly available and copyright free
• Easy to deploy and maintain
• There are no restrictions for vendors to modify bits or bytes locally
• The main advantage of using Modbus is that it is a simple open-source protocol. Modbus is very cheap to develop and requires minimal hardware design. In addition, Modbus supports interoperability and compatibility with a wide range of devices (from different vendors) and systems.

Modbus allows multiple (approximately 240) devices to be connected on the same network to communicate, for example, a device that measures temperature and humidity and sends the results to a computer. In data acquisition and monitoring control systems (SCADA), Modbus is often used to connect a monitoring computer to a remote terminal control (RTU) system.