Introduction to Industrial Automation

6.8   Central Processing Unit

The overall operation of a PLC is realized and orchestrated from its central processing unit (CPU). 

The heart of the CPU is the microprocessor, while in many cases the terms “central processing 

unit” and “microprocessor” are used interchangeably. However, the correct terminology indicates 

that the microprocessor is the specific integrated circuit that the PLC is utilizing, while the CPU, 

except for the microprocessor, contains additional necessary components and electronic circuits 

for the full operation of the PLC. Figure 6.29 indicates the overall architecture of the main units 

of the PLC and their interconnections within the CPU in a rather simplified way. This figure does 

not contain the power supply that simply supplies all the necessary voltages for the proper opera-

tion of all the units and components. The CPU also contains the digital circuits that store or recall 

data from memory, as well as the necessary circuits for the communication of the microproces-

sor with the PLC’s programming unit. The operation of the microprocessor is controlled from 

a program called “operating system” or “executable program”. The executable program is stored 

permanently in a memory unit that it is always of a ROM type (read only memory) since, from 

the moment that it is developed by the manufacturer and stored in the memory, modifications 

are unnecessary and not allowed. As a definition of the executable program, is a special program, 

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  Introduction to Industrial Automation

written in assembly language, which is able to drive the microprocessor to perform the internal 

functionalities such as processing, control of other internal units, and communication.

More specifically, the CPU of a typical PLC is able to perform the following basic operations:

 1. 

I/O operations. These operations allow the PLC to communicate with the external world and 

include mainly the scanning of the inputs and the updating of the outputs.

 2. 

Logical and arithmetic operations. These operations contain all the logical functions of 

Boolean algebra (AND, OR, NAND, etc.) and basic arithmetic operations (such as addi-

tion, subtraction, multiplication, etc.). In the same category, all the specific operations such 

as timing, counting, and comparison can also be included.

 3. 

Reading, writing, and special handling operations. These operations contain actions that are 

applied on the content of the memory locations and may concern data or instructions.

 4. 

Communication operations. These operations contain specific functions that the PLC should 

execute when it is communicating with external peripheral I/O modules, specific modules, 

another PLC, etc.

In order for the CPU to execute the previous operations, it continuously communicates with 

the memory and the other components through the data bus and the address bus (Figure 6.29). The 

aim of the address bus is to activate, at the proper time instant, I/O points or memory locations that 

will subsequently utilized from the data bus for the transmission of data. The microprocessor selects 

an address which is decoded. In this way, the proper corresponding I/O point or memory location 

is selected through the address bus. The selected I/O point or memory location will then receive or 

transmit data. The data bus, which consists of bidirectional channels, is utilized for the transmission 

of data from the memory to the I/Os and conversely, from the CPU to the memory of registers, tim-

ers, counters, etc. In Figure 6.30, the basic components of a microprocessor are indicated, as well as 

their interconnections with external components. The memory with the operating system contains, 

Address

bus

Data

bus

Signals from controlled system

Signals to controlled system

Input module

CPU

µP

Output module

Programmer

Memory

0

5

6

7

8

0

0

4

a4

GN

D

GN

D

b4

a3

b3

a2

b2

a1

b1

Vo

c1

Vo

c2

3

2

1

0