Introduction to Industrial Automation

Figure 6.5  Compact (a) and modular (b) structural form of a PLC.

(a)

(b)

Figure 6.6  Two commercial PLCs in compact form (a) (Schneider Electric) and modular form 

(b) (Siemens).

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  Introduction to Industrial Automation

Oversized PLCs usually have a central processing unit which can support a much larger num-

ber of I/Os than those that may be contained in the modular one-base version of the PLC, even in 

the case that the latter is fully equipped with I/O modules. In such a situation, a relevant question 

is “how to expand the initial modular base of a PLC to a second base in order to utilize a large 

number of I/O modules?” This is achieved by the utilization of two communication units and an 

interconnection cable that enables communication between the CPU and the expansion base, as 

presented in Figure 6.7.