PLC Communication Protocols

PLC communications has grown and changed over the many years since the controller’s inception decades ago. Industrial needs and technological advances have kept PLC communications evolving, and this post will explore some past and present communication standards, and what the future may hold.

The communication protocol is a set of rules that govern data communication.

In other words, the communication protocol acts as an agreement between two or more communicating devices.

PLCs needs to communicate to a number of different devices. First, there are devices which are part and parcel of the PLC, such as remote I/O. Next, are other PLCs and control devices, such as motor drives and servo controllers. Communication with some type of operator interface device is also often required, as is Internet connectivity. Finally, PLCs often communicate to server level PCs running various manufacturing related applications.

All of these communications require two things:

Physical connection or layer, which is the wiring and connection components, and

Shared protocol, which is the common language allowing each device to understand what the bits and bytes in the communication messages mean.

Back in its infancy, PLC communications were typically proprietary, with each supplier having their own closed connections and protocol. This made communications relatively simple among a single supplier’s products, but devilishly complex if you chose to venture outside the realm.

Following are the different types of communication protocols used in every PLC.

• Ethernet
• Profibus
• RS-232
• RS-485
• Multi-Point Interface (MPI)
• Point to Point (PPI)
• Data Highway (DH)
• Control Net
• Device Net
• USB Adapter
• PC Adapter

These are the foremost protocols used for the PLC and other network connections. These protocols are supported by different PLC software brands.

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

1.    Baud rate

2.    Network length

3.    Number of nodes

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 different types of standard communication protocols support different speed (baud rate), distances (network length) and the number of connecting devices (nodes).

• If you want to communicate with the more number of nodes with more speed, Ethernet is the best choice.
• If your network has a length more than 25 Kilo-meter, you should use Control Net protocol