Introduction to Industrial Automation

Figure 6.2  Conventional automation in comparison to programmable automation.

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Basic Operating Principles of PLCs 

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as a direct result of a command execution, means the corresponding writing of a logical “1” or “0” 

at the output image table. In our example, the output 4 should be energized by the registration of a 

logical “1” in the corresponding memory position. Subsequently, the rest of the program’s branches 

are executed, if more exist. When the whole program is executed, then all the states of the outputs 

have been stored in the output image table, digital “1” or “0”. Subsequently, the microprocessor 

transfers the output image table at the PLC’s output component. Thus, in output 4 of the output 

component, a logical “1” TTL signal is transformed to a power signal that can energize a switching 

component, through which an output device is energized, or the relay C in our case. Subsequently, 

the microprocessor repeats the sampling of the inputs, executes the program from the beginning, 

and updates the outputs and repeats again the same cycle, as indicated in Figure 6.1c. This con-

tinuous cyclic operation of the PLC is known as the “scanning” mode. The time for a full scanning 

indicates the operational speed of the PLC, and should vary from a few milliseconds or less. If the 

variation of an input’s state is faster that the scanning time, then these variations are not detectable 

Output

image

table

Scanning

of inputs

Input 3 

=Logic “0” 

=Logic “1” 

C

R

N

Input

device

A

(b)

(c)

Output

device

Conversion to

program 

C

N

R

RS

(a)

R

N

RS

Program

execution

Updating

of outputs

Output 4

Input 3

Inputs

module

Output 4 

Outputs

module

User

program

Input

image

table