Introduction to Industrial Automation

Figure 6.33  Grouping and addressing of digital inputs according to an octal system.

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  Introduction to Industrial Automation

The overall design of the PLC I/O hardware needs a priori information on the relevant techni-

cal characteristics of all the available PLCs in the market that could satisfy the required control 

and  functional characteristics of the control system. Every particular application dictates partially 

the way to design the proper I/O hardware; however the design engineer should make a significant 

number of the decisions. Since these decisions concern the I/O hardware, they also indirectly 

affect the automation software program to be developed, and the appropriate or inappropriate I/O 

system design, which has a direct effect on the ease or the difficulty in developing the required 

automation software. At the same time, the cost of the I/O hardware can often be more than the 

cost of the CPU, which is also the most expensive part of a PLC. For the above reasons, a care-

ful and thorough design of the PLC I/O system is required, which should be based on a detailed 

recording of the operational data and specific requirements of the system to be automated.