Introduction to Industrial Automation

Figure 7.11  Graphical symbol of instruction CU in LAD language.

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Basic Programming Principles of PLCs 

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CPU. For example, if we have a light electronic display for listing the current counting of the 

passed bottles, the required display information will be obtained from the CV output. Finally, 

the counter has a digital “0” or “1” output. This output is at logic “1” if the content of C

xx

 ≠ 0. 

When the content is reset, the Q output of C

xx

 becomes a logic “0”. It is possible to encounter 

the opposite logic where for C

xx

 ≠ 0, Q = “0”. This output can be used as a contact-based logic 

instruction anywhere in the LAD program with the form 

C

xx

. This instruction will pro-

vide RLO = “1” when C

xx

 ≠ 0 according to the first definition. The same is valid for the corre-

sponding instruction A Cxx in Boolean language, while a similar graphic symbol and features 

can also be found in FBD language.

Comparison instructions. It is possible, via comparison instructions, to compare two numbers 

in all their basic formats, such as integers, real, etc. The comparison of any kind (≤, ≥, >, <, =, etc.) 

is always performed between the two contents existing in the first two accumulators always in 

the same order, e.g., content of register 1 > content of register 2. The comparison instructions are 

always used in conjunction with the load instruction. With two consecutive loading instructions, 

L N1 and L N2 (Boolean language), the two numbers are entered into the two registers and can 

be compared if a comparison instruction is followed. When the comparison is satisfied, the CPU 

generates an RLO = “1”. In Figure 7.12, the graphical symbol for the comparison of two integer 

numbers in the LAD language (Step7—Siemens) is presented.

The input of the comparator is a discrete variable (“0” or “1”) and enables the comparison when 

it is at logical “1”. The output of the comparator generates a logical “1” when the comparison is 

satisfied, provided that the input is enabled. At the inputs IN1 and IN2 of the comparator, the two 

numbers to be compared are declared, which can be either variables (e.g., T, C, or Q) or constant 

values. In FBD language, a similar graphic symbol is utilized.

Program flow control instructions. These instructions, as their name suggests, help to change 

the execution flow of the instructions within the same program with or without a condition. 

With the JU label instruction (in Boolean language), the program flow switches to the instruction 

characterized by this label tag, bypassing the intermediate instructions, unconditionally and inde-

pendently of the RLO. Conversely, with the instruction JC label, the transition occurs only when 

a condition is satisfied and thus RLO = “1”. In LAD language, the jump instruction 

JMP

Label

 

is always executed, while the instruction 

(

JMP

)

Label

I3.3

 is executed only when the digital input 

I3.3 is energized. Similar graphical symbols for the two above instructions are also found in FBD 

language.

When the CALL instruction (in Boolean language) is called at the point of program where the 

instruction is executed, another program block or POU according to the IEC 61131-3 standard 

terminology is in order to be executed. The POU can be an FB or an FC, in which it is possible to 

program a separate automation function that can either be repeated several times (subroutine) or 

CMP

≥ I

IN1

IN2

Input

Output

CMP

≥ I

IN1

IN2