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Chapter

10PLC Math Instructions

Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.

Objectives

• Program the add instruction in the PLC ladder logic diagram.

• Program the subtract instruction in the PLC ladder logic diagram.

• Program the multiply instruction in the PLC ladder logic diagram.

• Program the divide instruction in the PLC ladder logic diagram.


Addition

• Add (ADD) instruction has three operands:– Two operands hold source data

(Source A and Source B), which are added together.

– Either both sources can be registers or one source can be a register while the other one is a number.

– Third operand is the destination.


Addition (Cont.)

• Add instruction for Allen-Bradley SLC 500 series PLC displayed in block format.


Description of Status Bits


Subtraction

• Subtract (SUB) instruction has three operands:– Two operands, Source A and Source B,

hold the source data. – Content of Source B is subtracted from

Source A. – Result is placed in the destination.


Subtraction (Cont.)

• Subtract instruction for the Allen-Bradley SLC 500 series PLC displayed in block format.


Multiplication

• Multiply (MUL) instruction has three operands:– Two operands hold the source data

(Source A and Source B).– Either both sources can be registers, or

one source can be a register while the other one is a number.

– Destination must be a register.


Multiplication (Cont.)

• Multiply instructions in SLC 500 series PLCs displayed in block format.


Division

• Divide (DIV) instruction has three registers:– Two registers hold the source data

(Source A and Source B).– Either both sources can be registers or

one source can be a register while the other one is a number.

– Destination must be a register.


Division (Cont.)

• Divide instructions in SLC 500 series PLCs displayed in block format.


Advanced Math Instructions

• Advanced math instructions are used to perform mathematical computations.


Advanced Math Instructions (Cont.)

• Compute (CPT) instruction may be used to solve mathematical equations.


Advanced Math Instructions Example

• Given the following equation, you can use the CPT instruction to solve it.

Y = (A × (ln B) × eC) + (tan D)


Advanced Math Instructions Example (Cont.)

• First assign floating point register locations for the parameters in the equation.

• Note that integer registers such as N7:0 hold integer numbers.

• Floating point registers such as F8:0 hold a real number that has both integer and fractional part.



• Assigned registers are:A = F8:0B = F8:1C = F8:2D = F8:3Y = F8:4



• Normally open contact I:1/0 is closed, equation in CPT instruction block calculated.

• Result is placed in floating point register four (F8:4).


PLC Ladder Logic Diagram


Commonly Used Math Instructions