MODBUS: A Guide to Industrial Communication Protocol

Introduction

In the world of industrial automation and control systems, seamless and efficient communication between devices is paramount. One protocol that has stood the test of time and proven its worth is MODBUS. Originally developed by Modicon in 1979, it has evolved into a widely adopted communication protocol in various industries, including manufacturing, energy, and transportation. In this article, we will delve into the basics of MODBUS, its types, applications, and advantages, to help you grasp the essence of this powerful communication protocol.

1. What is MODBUS?

MODBUS is a serial communication protocol widely used for transmitting information between electronic devices in industrial environments. It runs on a master-slave architecture, where a single “master” device initiates the communication with one or more “slave” devices. These slave devices can be sensors, actuators, Programmable Logic Controllers (PLCs), or any other equipment that communicates over MODBUS.

The protocol uses character serial communication lines, Ethernet, or the Internet protocol suite as a transport layer. It supports communication to and from multiple devices connected to the same cable or Ethernet network. For example, there can be a device that measures temperature and another device to measure humidity connected to the same cable, both communicating measurements to the same computer “MASTER”, via Modbus.

2. Types of MODBUS

There are two primary variants of MODBUS:

– MODBUS RTU (Remote Terminal Unit): This is the most common. It uses binary data representation and runs over serial communication (RS-232 or RS-485). The data is transmitted in a compact format, making it highly efficient for high-speed applications.

– MODBUS TCP/IP: This version uses the TCP/IP protocol, enabling communication over Ethernet networks. With the widespread adoption of Ethernet in industrial settings, MODBUS TCP/IP has become increasingly popular as it allows for faster data transfer and seamless integration with existing IT infrastructures.

3. How MODBUS Works

3.1 Protocol

MODBUS employs a simple and straightforward communication structure. The master initiates a request, known as a “query,” to a specific slave device. The slave then responds with the requested data or action. The data is typically organized in discrete binary inputs, coils, input registers, and holding registers.

Modbus is a commonly used protocol that facilitates communication between a plant/system supervisory computer and a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. The data types in Modbus are named after the industrial control of factory devices. For example, a single-bit physical output is referred to as a coil, while a single-bit physical input is known as a discrete input or a contact.

3.2 Modbus object types

The following object types may be provided by a Modbus server to a Modbus client device: The addresses are representative of the original Modicon specification. Under the current standard the address can be 0 – 65535 with the object type identified by the command used to read or write the coil or register.

Object typeAccessSizeAddress Space
CoilRead-write1 bit00001 – 09999
Discrete inputRead-only1 bit10001 – 19999
Input registerRead-only16 bits30001 – 39999
Holding registerRead-write16 bits40001 – 49999
MODBUS object types

4. Advantages of MODBUS

MODBUS has several advantages that contribute to its widespread adoption:
– Simplicity: The straightforward structure and ease of implementation make MODBUS an attractive choice for many industrial applications.
– Interoperability: MODBUS is vendor-agnostic, allowing devices from different manufacturers to communicate seamlessly if they support the protocol.
– Efficiency: The compact data format used in MODBUS RTU and the speed of MODBUS TCP/IP enable efficient data exchange, even in high-demand environments.
– Reliability: MODBUS has proven to be dependable over the years, making it a trusted choice for critical industrial processes.

Communications and devices

Each device communicating (i.e., transferring data) on a Modbus is given a unique address.

On Modbus RTU, Modbus ASCII, and Modbus Plus (which are all RS-485 single-cable multi-drop networks), only the node assigned as the client may initiate a command. All other devices are servers and respond to requests and commands. (Note that this client/server naming convention inverts the common English understanding and Latin origin that there are multiple clients and only one server/patron/business. To avoid this confusion, the RS-485 transport layer uses the terms ‘node’ or ‘device’ instead of ‘server’, and the ‘client’ is not a ‘node’).

For protocols that utilize Ethernet, like Modbus TCP, any device can transmit a Modbus command. This means that all devices can act as clients, although typically only one device performs this role.

Many modems and gateways support Modbus, as it is a simple and often-copied protocol. Some of them were specifically designed for this protocol. Different implementations use wireline or wireless communication, such as in the ISM radio band, and even Short Message Service (SMS) or General Packet Radio Service (GPRS). One of the more common designs of wireless networks makes use of mesh networking. Typical problems that designers must overcome include high latency and timing issues.

Conclusion

In conclusion, MODBUS is a robust and versatile communication protocol that plays a crucial role in modern industrial automation and control systems. Its simplicity, interoperability, efficiency, and reliability have made it a go-to choose for countless applications across various industries. As technology continues to advance, MODBUS is likely to evolve further, adapting to the ever-changing needs of the industrial landscape. Embracing MODBUS opens a world of possibilities for enhanced automation and optimization, empowering industries to achieve greater efficiency and productivity.

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