Page 254 - Programmable Logic Controllers, Fifth Edition - Mobile version

11.1 Math Instructions
MUL (Multiply)—Multiplies source A by source B
and stores the result in the destination.
Math instructions, like data manipulation instructions, en-
able the programmable controller to take on more of the DIV (Divide)—Divides source A by source B and
qualities of a conventional computer. The PLC’s math func- stores the result in the math register and destination.
tions capability allows it to perform arithmetic functions on SQR (Square Root)—Calculates the square root of the
values stored in memory words or registers. For example, source and places the integer result in the destination.
assume you are using a counter to keep track of the number NEG (Negate)—Changes the sign of the source and
of parts manufactured, and you would like to display how places it in the destination.
many more parts must be produced in order to reach a certain TOD (To BCD)—Converts a 16-bit integer source
quota. This display would require the data in the accumu- value to BCD and stores it in the math register or the
lated value of the counter to be subtracted from the quota re- destination.
quired. Other applications include combining parts counted, FRD (From BCD)—Converts a BCD value in the
subtracting detected defects, and calculating run rates. math register or the source to an integer and stores it
Depending on what type of processor is used, various in the destination.
math instructions can be programmed. The basic four
mathematical functions performed by PLCs are: The basic math instructions are ADD, SUB, MUL, and
DIV. Each of these instructions has three parameter fields.
Addition—The capability to add one piece of data to Namely, Source A, Source B and Destination fields.
another.
Subtraction—The capability to subtract one piece of • The Source A and Source B fields can be an input
rack location, file address, instruction field, or a
data from another. fixed value. For example:
Multiplication—The capability to multiply one piece Input Location I:1
of data by another. File Address N7:5
Division—The capability to divide one piece of data Instruction Field C5:2.ACC
by another. Fixed Value 30
Math instructions use the contents of two words or regis- • The Destination fields can be an output location,
ters and perform the desired function. The PLC instructions file address, or an instruction field. For example:
for data manipulation (data transfer and data compare) are Output location O:2
used with the math symbols to perform math functions. File Address N7:8
Math instructions are all output instructions. These instruc- Instruction Field T4:1.PRE
tions can be conditional or unconditional. With conditional Figure 11-2 shows the CPT (compute) instruction used
arithmetic instructions the input logic determines when the with SLC 500 controllers. When CPT instruction is executed,
instruction executes. Unconditional arithmetic instructions then copy, arithmetic, logical, or conversion operation resid-
execute with each scan. ing in the expression field of this instruction is performed
Figure 11-1 shows the Compute/Math menu tab for the and the result is sent to the destination. The execution time
SLC 500 PLC and its associated RSLogix software. The of a CPT instruction is longer than that of a single arithmetic
commands can be summarized as follows: operation and uses more instruction words.

CPT (Compute)—Evaluates an expression and stores The main advantage of the compute instruction is that
the result in the destination. it allows you to enter quite complex expressions in one in-
ADD (Add)—Adds source A to source B and stores struction. Figure 11-3 shows a ladder rung used to convert
the result in the destination. a Fahrenheit temperature to a Celsius temperature using
a single RSLogix 5000 compute instruction. The CPT
SUB (Subtract)—Subtracts source B from source A
and stores the result in the destination.
CPT
Compute

CPT ADD SUB MUL DIV SQR NEG TOD FRD Destination

Input/Output Compare Compute/Math Move/Logic Expression

Figure 11-1 Compute/Math menu tab. Figure 11-2 SLC 500 CPT (compute) instruction.

Math Instructions Chapter 11 235

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