Page 257 - Programmable Logic Controllers, Fifth Edition - Mobile version

operation of the logic rung can be summarized as follows:
L1 Input SW Ladder logic program SUB instruction used with the SLC 500 controllers. The
SUB
SW SUBTRACT • When input switch SW is closed the rung will be true.
Source A N7:10 • The value stored at the source B address, N7:05
520
Source B N7:05 (322), is subtracted from the value stored at the
322 source A address, N7:10 (520).
Destination N7:20 • The answer (198) is stored at the destination ad-
198
dress, N7:20.
Figure 11-7 SLC 500 SUB (subtract) instruction. • Source A and source B can be either values or ad-
dresses that contain values, but A and B cannot both

Sign Bit (S)—Address S2:0/3, is set to 1 when the be constants.
result is a negative number. The program of Figure 11-8 shows how the SUB func-
tion can be used to indicate a vessel overfill condition.
11.3 Subtraction Instruction This application requires an alarm to sound when a supply
system leaks 5 lb or more of raw material into the ves-
The SUB (subtract) instruction is an output instruction that
subtracts one value from another and stores the result in sel after a preset weight of 500 lb has been reached. The
the destination address. When rung conditions are true, the operation of the program can be summarized as follows:
subtract instruction subtracts source B from source A and • When the start button is pressed, the fill solenoid
stores the result in the destination. Figure 11-7 shows the (rung 1) and filling indicating light (rung 2) are

Inputs Ladder logic program Outputs
Fill
L1 Stop Start Full solenoid L2
1
Fill
Start Fill solenoid
solenoid
Start
Stop Fill
solenoid Full Filling
2
Stop
GEQ Full
3 GREATER THAN OR EQUAL Filling
Source A I:012
Source B 500
I:012 Full
Weight (All 16 bits)
transducer Full SUB
4 SUBTRACT Alarm
Source A I:012
Source B 500
Destination N7:1

Alarm
GEQ
5 GREATER THAN OR EQUAL
Source A N7:1
Source B 5
Figure 11-8 Vessel overfill alarm program.

238 Chapter 11 Math Instructions

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