Page 186 - Programmable Logic Controllers, Fifth Edition - Mobile version

Ladder logic program
L1 Inputs Count Output L2
PB1
up
PB1 UDC Light Light
PB2 Count
down PR: 003

PB2
AC: 000
PB3
Reset
PB3

(a)

1 2 3 4 5 6
On
Count up
O
1 2 3 4
On
Count down
O
65
5 4
Counter 3 4 3 2
accumulated 1 2
value 0 Preset value 0

On
Output
O
On
Reset
O

(b)
Figure 8-22 Generic up/down-counter program. (a) Program. (b) Counting
diagram.

As a result, three additional pulses of the count-up down-counters may count below zero and begin count-
input (PB1) produce an accumulated value of 6 but ing down from the largest preset value that can be set
no change in the output. for the PLC’s counter instruction. For example, a PLC
• If the count-down input (PB2) is now pulsed up/down-counter that has a maximum counter preset
four times, the accumulated count is reduced to limit of 999 may count up as follows: 997, 998, 999,
2 (6 – 4). As a result, the accumulated count 000, 001, 002, and so on. The same counter would count
drops below the preset count and the output light down in the following manner: 002, 001, 000, 999, 998,
switches from on to off. 997, and so on.
• Pulsing the reset input (PB3) at any time will reset the One application for an up/down-counter is to keep
accumulated count to 0 and turn the output light off. count of the cars that enter and leave a parking garage.
Figure 8-23 shows a typical PLC program that could be
Not all counter instructions count in the same manner. used to implement this. The operation of the program can
Some up-counters count only to their preset values, and be summarized as follows:
additional counts are ignored. Other up-counters keep
track of the number of counts received above the coun- • As a car enters, the enter switch triggers the up-
ter’s preset value. Conversely, some down-counters counter output instruction and increments the accu-
will simply count down to zero and no further. Other mulated count by 1.

Programming Counters Chapter 8 167

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