Introduction to Industrial Automation

Figure 6.35  Local and peripheral I/O hardware of a PLC.

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  Introduction to Industrial Automation

than the corresponding peripheral ones. The difference exists only in the means of communication, 

in the communication units (CPs) and connecting cables. The main reason for the differentiation of 

the hardware at a local and peripheral level is the voltage drop that occurs in the interconnecting cable 

when it is long. For this reason, the I/O hardware expansion peripheral units require additional power 

supplies (PSs) (see Figure 6.35), which are not necessary in the local expansion units.

PLC manufacturers do not always follow the layout of Figure 6.35, while, on the contrary, 

there are differences that do not undermine the general principles. In addition to the simple local 

extension of a PLC, there are two basic peripheral extension topologies, the serial and star topol-

ogy presented in Figure 6.36. In each peripheral expansion unit, it is possible to have a local 

extension of the I/O hardware. Local expansion I/Os modules are usually mounted in the same 

electrical enclosure together with the CPU and the power supply. The peripheral I/O hardware is 

installed in separate enclosures at corresponding remote extension points.