Modbus is an open serial communications protocol based on a master-slave architecture. In a typical Modbus network, a single server can communicate with up to 247 clients using a 2-wire or 4-wire topology in accordance with the EIA/TIA-485 standard.Modbus can also be used for point-to-point (1 server to 1 client) communication using RS-232 wires in accordance with the EIA/TIA-232 standard.

 

Since its introduction, the Modbus protocol has evolved and different variants and advanced versions have been developed. The original serial Modbus protocol is more specifically known as Modbus RTU and Modbus ASCII. other variants include (but are not limited to) the following types that support communication over Ethernet:

 

Modbus TCP/IP（Modbus TCP）

 

ND Modbus over TCP/IP

 

Modbus UDP

 

Modbus Plus(non-Ethernet based, Modicon specific, not introduced for few applications)

 

Each variant brings new advantages to the standard, including improvements in reliability, speed or distance. The main focus is on Modbus RTU and Modbus TCP/IP, as they seem to be the most widely used today.

 

When Modbus first appeared in the late 1970s, there weren't a lot of communication alternatives around. If there was an alternative, it was usually proprietary, not open source. Today, there are dozens of industrial communication protocols and a growing trend toward open source.

 

1.OPC Unified Architecture (OPC UA):

 

OPC UA is a cross-platform, cross-language industrial communication standard that supports a high level of security features, including data encryption and user authentication.OPC UA also supports complex data structures and method calls, making it suitable for complex industrial applications.

 

2.Distributed Network Protocol (DNP3):

 

DNP3 is a widely used communication protocol for power and water monitoring systems. It provides robust data transmission security and reliability for distributed control systems.

 

3. Controller Area Network (CAN)：

 

The CAN protocol was originally used in the automotive industry, but is also widely used in industrial automation due to its high reliability and real-time performance.

 

4.DeviceNet:

 

DeviceNet is a CAN-based communication protocol designed for industrial devices that supports plug-and-play functionality and simplifies device integration.

 

5.Ethernet/IP:

 

Developed by the ODVA organization, Ethernet/IP combines the high-speed data transfer capability of Ethernet with the deterministic communication characteristics of industrial control networks.

 

6.Profibus/Profinet:

 

Profibus is a fieldbus system for use in a variety of industrial environments, while Profinet is an open Ethernet-based communication standard that supports real-time Ethernet communication.

 

7.Highway Addressable Remote Transducer (HART):

 

The HART protocol enables the hybrid transmission of analog and digital signals by superimposing a digital signal on the traditional 4-20 mA analog signal.

 

8.EtherCAT (Ethernet for Control Automation Technology):

 

EtherCAT is an Ethernet-based real-time industrial Ethernet communication protocol known for its extremely low update cycles and high synchronization accuracy.

 

9.ControlNet:

 

ControlNet is an industrial Ethernet communication protocol developed by Rockwell Automation that provides deterministic communication and high data throughput.

 

10.Optomux:

 

Optomux is an open industrial communication protocol that supports a wide range of communication media and topologies for flexible industrial network design.

 

When an automation or control engineer needs to connect a plant or facility's control system to a third-party device, there are usually several options.

 

When the third-party device is another control system, whether it is a single device such as a motor drive, or a network of multiple devices such as actuators, instruments, or other devices, often Modbus is an option for interfacing between components.

 

In fact, sometimes Modbus is the only communication option other than a hardwired connection.Modbus is the most widely accepted and simplest form of third-party communication.

 

Almost all engineers in the industrial control industry, without fail, are familiar with the Modbus protocol application. That's why we've spent so many issues covering it, and it's also one of the top 3 communication protocols voted for by your friends.



Developers and manufacturers have found that open protocols allow more devices to be embedded, making the control system more valuable and attractive because it can connect more devices.

While there are many alternatives to Modbus, not all communication protocols are interchangeable. Modbus has been around for a while, of course, but it has several advantages over the newer communication protocols available today.

 

Wide Availability

 

One of the advantages of Modbus is its wide availability for modern control systems. Among the different protocols available on the market today, Modbus is considered to be the most available on the market. Almost any industrial automation device with a communication channel can be configured with Modbus RTU, ASCII or Modbus TCP/IP. in many cases Modbus is even included as standard in the device.

 

Open source

 

It allows anyone to use it without paying royalties. A key advantage of open protocols is that they facilitate interoperability between different devices and systems because they follow the same communication rules.

 

However, "open source" usually refers to software whose source code can be accessed and modified by the public. the Modbus protocol itself, as a standard, is open, but that does not mean that all software or hardware that implements the Modbus protocol is open source. In fact, many companies have developed proprietary Modbus communication libraries or devices, and these may not make the source code publicly available.

 

Nevertheless, there are many open source projects and libraries that provide implementations of the Modbus protocol, and these can be used under an open source license, for example:

 

libmodbus: an open source C library for the Modbus protocol.

pymodbus: A Python library that implements the client and server functions of the Modbus protocol.

 

Protocol simplicity

 

The simplicity of the protocol is another reason why some may choose Modbus as the interface to a new control system. When designing interfaces with other control systems, many instances have been encountered where OPC was immediately recommended as the interface of choice because of its openness. Like Modbus, OPC is available on a wide variety of devices.

 

Modbus is the simpler and more reliable of the two compared to OPC and Modbus.

 

OPC is also server/client based and usually consists of a server installed on a PC running a version of Windows. Modbus, by contrast, is strictly on industrialized equipment and does not rely on the Windows operating system, security and the tedious steps of DCOM configuration.

 

Not sure about the future of Modbus and how long it will be around, but I'm willing to bet that it will be here long after my control system career, at least for the next generation, and that Modbus will probably continue to be the "go-to" interface for many years to come.