Page 260 - Programmable Logic Controllers, Fifth Edition - Mobile version

is then divided by the value stored in source B,
L1 Input SW Ladder logic program • The value stored in source A, address N7:0 (120),
DIV address N7:1 (4).
SW DIVIDE
Source A C5:10.ACC • The answer, 30, is placed in the destination address
N7:5.
Source B 2
• As a result, the equal instruction becomes true, turn-
Destination N7:3 ing output PL1 on.
The program of Figure 11-14 is an example of how
Figure 11-12 SLC 500 DIV (divide) instruction. the DIV function is used as part of a program to convert
Celsius temperature to Fahrenheit. The operation of the
• The result is placed in the destination N7:3. program can be summarized as follows:
• If the remainder is 0.5 or greater, a roundup occurs • The thumbwheel switch connected to the input
in the integer destination. module indicates Celsius temperature.
• The value stored in the math register consists of the • The program is designed to convert the recorded
unrounded quotient (placed in the most significant Celsius temperature in the data table to Fahrenheit
word) and the remainder (placed in the least signifi- values for display.
cant word). • The following conversion formula forms the basis
• Some PLCs support the use of floating point num- for the program:
bers as well as integer (whole number) values. As
)
(
an example, 10 divided by 3 may be expressed as F = × C + 32
9 __
3.333333 (floating-point notation) or 3 with a re- 5
mainder of 1.
• A minor fault bit is set upon detection of a division • In this example, a current temperature reading of
60°C is assumed.
by zero.
• The PLC processor carries out its internal opera-
The program of Figure 11-13 is an example of how the tions using binary numbers and the FRD instruction
DIV instruction calculates the integer value that results is used to convert the 16-bit integer values from the
from dividing source A by source B. The operation of the thumbwheel switch into BCD values.
program can be summarized as follows: • The MUL instruction multiplies the temperature

• When input switch SW is closed the DIV instruc- (60°C) by 9 and stores the product (540) in address
tion is executed. N7:0.

Input Ladder logic program Output
L1 L2
SW
SW PL1
DIV
DIVIDE
Source A N7:0
120
Source B N7:1
4
Destination N7:5
30
PL1
EQU
EQUAL
Source A N7:5
30
Source B 30
Figure 11-13 DIV instruction used to calculate the value that results
from dividing source A by source B.

Math Instructions Chapter 11 241

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