2. 10
PLC Math Instructions

3. 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.
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4. 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.
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5. Addition (Cont.)
Add instruction for Allen-Bradley SLC 500 series PLC displayed in block format.
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6. Description of Status Bits
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7. 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.
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8. Subtraction (Cont.)
Subtract instruction for the Allen-Bradley SLC 500 series PLC displayed in block format.
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9. 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.
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10. Multiplication (Cont.)
Multiply instructions in SLC 500 series PLCs displayed in block format.
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11. 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.
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12. Division (Cont.)
Divide instructions in SLC 500 series PLCs displayed in block format.
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13. Advanced Math Instructions
Advanced math instructions are used to perform mathematical computations.
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14. Advanced Math Instructions (Cont.)
Compute (CPT) instruction may be used to solve mathematical equations.
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15. Advanced Math Instructions Example
Given the following equation, you can use the CPT instruction to solve it.
Y = (A × (ln B) × eC) + (tan D)
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16. 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.
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17. Advanced Math Instructions Example (Cont.)
Assigned registers are:
A = F8:0
B = F8:1
C = F8:2
D = F8:3
Y = F8:4
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18. Advanced Math Instructions Example (Cont.)
Normally open contact I:1/0 is closed, equation in CPT instruction block calculated.
Result is placed in floating point register four (F8:4).
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19. PLC Ladder Logic Diagram
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20. Commonly Used Math Instructions
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21. Glossary
Add (ADD) instruction - Instruction that calculates the sum of Source A and Source B.
Divide (DIV) instruction - Instruction that calculates the quotient that results from dividing Source A by Source B.
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22. Glossary
Multiply (MUL) instruction - Instruction that calculates the product of Source A and Source B.
Subtract (SUB) instruction - Instruction that calculates the difference between Source A and Source B.
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