Page 178 - Programmable Logic Controllers, Fifth Edition

8.2 Up-Counter
Figure 8-8 shows the program and timing diagram for
an SLC 500 Count-Up Counter. This control application
The up-counter is an output instruction whose function is designed to turn the red pilot light on and the green pilot
is to increment its accumulated value on false-to-true light off after an accumulated count of 7. The operation of
transitions of its instruction. It thus can be used to count the program can be summarized as follows:
false-to-true transitions of an input instruction and then
trigger an event after a required number of counts or tran- • Operating pushbutton PB1 provides the off-to-on
sitions. The up-counter output instruction will increment transition pulses that are counted by the counter.
by 1 each time the counted event occurs. • The preset value of the counter is set for 7.

L1 Inputs Ladder logic program Outputs L2
I:1/0
CTU
Rung 1 COUNT-UP COUNTER CU
PB1 (Count) Counter C5:1 Red PL
PB1 (Count) Preset 7 DN O:2/0 R
Accumulated 0
I:1/0
C5:1/DN O:2/0
O:2/1 G
Rung 2
Green PL
PB2 (Reset) Counter done bit Red PL
I:1/1 C5:1/DN O:2/1
Rung 3
Counter done bit Green PL

I:1/1 C5:1
Rung 4 RES
PB2 (Reset)

(a)

True 1 2 3 4 5 6 7
Rung 1
(count)
False
Rung 2

Rung 3

Rung 4
(reset)

Preset 6 7
value (7) 5
4
3
Accumulated 2
value 1

(b)
Figure 8-8 Simple up-counter program. (a) Program. (b) Timing diagram.

Programming Counters Chapter 8 159

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