Introduction to Industrial Automation

6.2   Modular Construction of a PLC

PLC manufacturing companies have adopted two basic types of PLC constructions, which are the 

compact form and the modular shape. As presented in Figure 6.5a, PLCs of the first category are 

solid structures, usually non-expandable, with a specific number of inputs and outputs. They form 

an integrated structure that includes, in addition to the I/Os, the power supply, microprocessor, 

interface with a programming device, STOP-RUN mode switch, LED indicators, etc. PLCs of 

this category are suitable mainly for small-size automation applications, while medium or large 

PLCs follow the philosophy of modular construction. This means that in this case, PLCs are 

composed of independent modular subunits mounted on a common base. As presented in Figure 

6.5b, PLCs of this class are comprised of a base with specific dimensions and, therefore, they have 

specific space capacity for equipment, such as a power supply module, microprocessor unit, and 

various I/O modules. Furthermore, the modular base has an electrical interface bus (printed cir-

cuit conductors) with appropriate plug-in connectors for the modular units, which achieve func-

tional cooperation of all of the PLC’s components. Some manufacturers, instead of the universal 

bus concept, prefer special connecting outlets where each newly-added module is capable of con-

necting properly with the previous one while, at the same time, providing connections to the next 

one. The major advantage of the modular assembly of a PLC is that the engineer has the ability 

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Basic Operating Principles of PLCs 

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to synthesize a PLC that fully corresponds to the requirements of a particular application. Due to 

the fact that, except for the power supply and the microprocessor unit, there are a wide variety of 

I/O modules (e.g., discrete or analog, with a number of available I/O channels, nominal operat-

ing voltage, etc.), the engineer should consider the type of equipment required for each individual 

application and synthesize the PLC accordingly. In addition, except for the various types of I/O 

modules, there are a large number of special purpose modules that make the task of setting up a 

modular PLC more complex. At this point, it should be mentioned that the minimum setting of a 

modular PLC includes a power unit, a processor unit, and an input and an output module, while 

in Figure 6.6, two typical types of commercial compact and modular PLCs are represented.

PLC

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