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Counters
Just like mechanical counters, PLC counter instructions keep
track of events. As it counts, a counter instruction compares an
accumulated count value to a preset value to determine when
the desired count has been reached. Counters can be used to
start an operation when a count is reached or to prevent an
operation from occurring until a count has been reached.
S7-200 SIMATIC Counters The S7-200 SIMATIC LAD instruction set includes three types
of counters: Count Up Counter (CTU), Count Down Counter
(CTD), and Count Up/Down Counter (CTUD).
SIMATIC Counters
Timer Number (C0 to C255)
PV
LD
PV
CD
CU
CD
R
CTD
CTUD
Count Up Counter Count Down Counter Count Up/Down Counter
PV
CU
CTU
Cxxx
R
Cxxx Cxxx
The Count Up Counter (CTU) counts up by one each time
the count up (CU) input transitions from off to on. When the
accumulated count equals the preset value (PV) the counter
bit (not shown) turns on. The counter continues to count until
the accumulated count equals the maximum value (32767).
When the reset input (R) turns on or when a Reset instruction is
executed, the accumulated count resets to zero and the counter
bit turns off.
The
Count Down Counter (CTD) counts down by one each
time the count down (CD) input transitions from off to on. When
the count reaches zero, the counter bit turns on. When the
load (LD) input turns on, the counter resets the current value to
equal the preset value (PV), and the counter bit turns off.
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