Page 259 - Programmable Logic Controllers, Fifth Edition - Mobile version

L1 Inputs Ladder logic program Outputs L2
MUL
ON/OFF MULTIPLY Heater
Source A I:012
400
Source B 0.0100000
TWS Destination N7:0 PL1
1 4
2 I:012
ADD
3
ADD
Source A I:012
4 0 0 400
Source B N7:0
4
High limit Destination N7:1
404
SUB PL2
I:013 SUBTRACT
Source A I:012
400
Thermocouple Source B N7:0
input 4
Low limit Destination N7:2
396

PL1
LES
LESS THAN
Source A I:013
0
Source B N7:2
396
PL2
GRT
GREATER THAN
Source A I:013
0
N7:1
Source B 404

ON/OFF PL1 PL2 Heater

Heater

Figure 11-11 The MUL instruction used as part of a temperature control program.

• In this example, the set-point temperature is 400°F. 11.5 Division Instruction
• Therefore, the electric heaters will be turned on The divide (DIV) instruction divides the value in source A
when the temperature of the oven drops to less than by the value in source B and stores the result in the desti-
396°F and stay on until the temperature rises above nation and math register. Figure 11-12 shows an example
404°F. of the DIV instruction. The operation of the logic rung
• If the set-point is changed to 100°F, the deadband can be summarized as follows:
remains at ±1 percent, with the lower limit being
99°F and the upper limit being 101°F. • When input switch SW is closed the rung will be true.
• The number stored in word N7:1 represents the • The data in source A (the accumulated value of
upper temperature limit, and the number stored in counter C5:10) is then divided by the data in source
word N7:2 represents the lower limit. B (the constant 2).

240 Chapter 11 Math Instructions

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