Introduction to Industrial Automation

Figure 6.7  Extension of a central PLC unit with modular structure.

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Basic Operating Principles of PLCs 

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are also called discrete I/O modules by many manufacturers, while both terms refer to the inputs 

and outputs of ON and OFF states. Many manufacturers, instead of the English term “I/O 

module” are utilizing the equivalent terms “I/O board”, “I/O card”, and “I/O block”. In Figure 

6.8, a common type of a module with digital outputs is presented. In this case, the external 

view and the general construction is similar for other types of I/O modules that also perform 

their function independently. Inside the plastic protective cover are the electronic circuits for 

the I/Os, while the terminal block is usually associated with plug-in contacts on the electronic 

board. The removability of the terminal block is a significant property in cases of module dam-

age, since it enables the rapid replacement of the module without requiring time-consuming 

rewiring. Furthermore, PLCs have the ability to connect different groups of I/O devices in a 

very short time at the same I/O module, provided that the devices are already connected to the 

corresponding terminal blocks. In general, the cable connection terminals accept conductors up 

to 2.5 mm

2

, while in Figure 6.9, a module used for the control of stepper motors from an Allen-

Bradley industrial PLC is presented.

Digital

outputs

 

16Χ24 V DC

0,5 A

 

0

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7

Ventilation

openings

Removable

terminal block

Plastic housing of

electronic card

Connection-to-base

adapter

LEDs indicating the On/

Off status of outputs