Introduction to Industrial Automation

8.3.4   The Communication Task of PLCs

The need for data exchange between PLCs and other digital devices or controllers in an automated 

industrial process has led all manufacturers to supply their controllers with additional commu-

nication capabilities. The hardware for the communication capability of a PLC appears in two 

forms, as is described subsequently. In compact type PLCs, the communication hardware and 

software are embedded in the module of their CPU, with the communication port as the only 

visible part. In modular type PLCs, the communication hardware is available as separate modules 

of communication of various types and features, which have already been mentioned in Section 

6.7. Regardless of the form of the hardware, the communication process of a PLC fits together 

with the classical automation program inside the scanning cycle of the PLC, as shown in Figure 

8.9, where two PLCs communicate over a network. It is obvious that this communication process 

refers to the read/write operations of the variables to be transferred over the network and not to the 

communication task of the network operation that is performed by the communication processor-

module. The transmitted data over the network are stored in the buffers of the communication 

modules according to rules defined by the communication protocol of each network. As shown 

in Figure 8.9, the PLC in each scan cycle updates its memory with data that have arrived over 

the network, executes the corresponding instructions based on the updated values in the PLC’s 

memory, and updates the buffer of the communication module with variable values obtained 

from the execution of the automation program that needs to be transmitted through the network. 

These steps are combined suitably with the rest of the classical steps of the automation program 

execution, including the reading of inputs and writing of outputs in the PLC, which have been 

described in detail in Section 6.1.

The addition of the communication task on the rest of the computing work for the scanning 

cycle increases the duration of the latter. On the other hand, the communication network has its 

own response times, which are not related to the duration of the scanning cycle. For the connected 

system of two PLCs of Figure 8.9, it is defined as a response time R

T

; the time that elapses from the 

moment that an input is activated in the PLC1 until the moment that an output will be activated 

in the PLC2. Obviously, in the automation logic program of the PLC2, the output status depends 

Industrial ethernet

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