What Is PLC Communication Protocols And Explain Its Types

What Is PLC Communication Protocols And Explain Its Types

The Communication protocols is a set of rules that sends or receives data between two or more communicating devices.

The communication protocol acts as an agreement between two or more communicating devices.By using the communication protocols, two devices can connect and communicate with each other.Without communication protocol, devices can only be connected but not communicated.

Some Old Communication Methods :

These old, communication methods still have their place, as they allow quick and easy connections among a single supplier’s products. For simple systems with no plans to expand or connect to other components, communication is an option and will continue to be for quite some time.

Many PLC suppliers still support these older physical, wired connections—most often based on RS-232C, RS-422 and RS-485 connectivity—and the related proprietary communication protocols.For example, some AutomationDirect PLCs use a serial RS-232C or RS-422 physical layer network with the DirectNET protocol for communications.

This protocol can be used for PLC-to-PLC communication over a point-to-point or multipoint network using standard PLC instructions, or to talk to AutomationDirect’s C-more HMIs.

When the control system requires venturing outside a single supplier, communication standards come to the forefront, starting at the physical layer.

Some common protocols used in the industrial area include:

  • Modbus RTU
  • EtherNet/IP
  • Ethernet TCP/IP
  • Modbus TCP/IP
  • Profinet

Perhaps the most common industrial serial communication protocol is Modbus RTU, developed by Modicon, which usually runs on an RS-485 network. This and other popular serial protocols are supported by a wide variety of suppliers, and are very familiar to a wide group of automation professionals. But performance is limited, making serial protocols a poor choice for high speed and other demanding applications.

Image result for common protocols used in plc images

Because of performance and other advantages, Ethernet has emerged as the dominant standard for the physical layer of many industrial protocols such as EtherNet/IP, Ethernet TCP/IP, Modbus TCP/IP and Profinet. And unlike serial protocols, multiple Ethernet protocols can run on the same Ethernet physical layer.

Using Ethernet, it’s not too difficult to interconnect several devices such as PLCs, HMIs, field I/O and valve banks. Plus, the communication remains fast while talking to several dissimilar devices on the same cable, due to the very high speed of Ethernet as compared to older serial networks.

Here’s an example of a typical Ethernet network connecting a wide variety of control systems and other components:

The communication protocols are depended upon three fundamental parts as mentioned below.

  1. Baud rate
  2. Network length
  3. Number of nodes

Baud Rate: Baud rate is also known as “Communication speed”. The rate of data transmission on the network is called Baud Rate.In simple term, it is calculated as “number of data transferred per second”.The unit of Baud Rate is bits/second (b/s).

Characteristics of PLC Communication Protocols

Sr. No. Protocols
Baud Rate Network Length Number of Nodes
1 Ethernet 100 Mb/s Few Kilometers
(based on the type of Ethernet)
2 Profibus 5- 12 Mb/s  15 Km 127
3  RS-232 19.2 Kb/s 10 m 1
4 RS-485 10 Mb/s 1.2 Km 32
5 MPI 19.2 – 38.4  Kb/s 50 m 32
6 PPI 187.5 Kb/s 500 m 1
7 DH 230.4 Kb/s 3.048 Km 64
8 Control Net 5 Mb/s 30 Km 99
9Device Net 500 Kb/s 0.487 m 64

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