Introduction to Industrial Automation

7.5.5   Programming Mathematical Operations

Programming a mathematical operation in an automation application does not present any diffi-

culty. Regardless of the type of mathematical operation, it is only required to “load” the two num-

bers (among them the operation will be applied) in two corresponding registers, and then invoke 

the corresponding instruction to execute the mathematical operation. In most cases, the instruction 

of the arithmetic operation is accompanied by the declaration of the type of numbers such as real, 

integer, etc. The only point that someone needs to be aware of is the issue of the sequence in which 

registers are loaded. The mathematical operation is made with the first number from the content of 

the register 2 (i.e., the first one loaded first) and the second number from the content of the register 1 

(i.e., the second one loaded last). When the mathematical operation refers to a single number (e.g., 

ABS), then it will obviously be performed on the content of the register 1. Figure 7.60 is illustrative 

of how the subtraction of two integers is performed. The result of the mathematical operation is 

always present in register 1, so that by using the T instruction (remember that the transfer instruc-

tion always acts on register 1) the result can be transferred to another location in the memory of the 

PLC. In the subtraction example, the result is transferred to the MW 100, while the other three 

basic numerical operations are dealt in the same way. Examples of automation programs with a need 

for mathematical operations are described in Section 7.5.6. Finally, it should be noted that currently 

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