Introduction to Industrial Automation

6.7.4   Communication  Modules

When there is a need for a PLC to communicate with another PLC, or with other devices such 

as monitors, printers, or remote I/O modules, or in general with another network of PLCs for the 

exchange of specific information, a communication module is needed for completing this task. 

The simple, compact-type PLCs are usually equipped with a standardized communication port 

for connecting to a PC, and some of them have an additional communication port for connecting 

to a specific industrial network. The communication modules in general are equipped with their 

dedicated microprocessor, which is called a communication processor (CP). This communication 

processor is responsible for implementing the communication protocol for communicating with 

the external devices, either independent or networked and with the central processing unit of 

the PLC. The last, according to the executed program, sends/receives alphanumeric data to/from 

external, independent devices through the communication processor.