Introduction to Industrial Automation

Figure 6.29  Operational interconnection between a CPU and the other units of a PLC.

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Basic Operating Principles of PLCs 

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in the form of a program, the executable instructions. Based on these instructions, the operation of 

the microprocessor is performed. The arithmetic logical unit (ALU) executes all the arithmetic and 

logical operations, while the program counter supervises and controls the program instructions step by 

step. When the user’s program contains jumping instructions (e.g., GOTO instructions), the program 

counter records the execution path of the program at every time instant. In the internal registers of the 

CPU, data are being stored based on instructions from the executable program, and are utilized from 

the ALU when required. In the writing and reading memory, the user’s program is stored in addition 

to specific operational parameters. All the operational steps of the microprocessor are driven from a 

clock that guarantees the proper and timely execution of the instructions.

The operating system is also responsible for the execution of various diagnostic functions from 

the CPU. These diagnostics can be categorized into two forms: the ones that the system is execut-

ing itself in every initialization of the device, and those that are user-initiated by specific instruc-

tions. Typical diagnostic functions during the initial operation of PLCs are checking the proper 

operation of the random-access memory (RAM), the microprocessor, the battery, and the power 

supply. The user is able to initiate specific diagnostic tests that are mainly related to the memory 

or other units, e.g., the communication ports.