Preface
MicroLogix 1000 Programmable Controllers User Manual
6–18
Count Up (CTU)
The CTU is an instruction that counts false-to-true rung transitions. Rung
transitions can be caused by events occurring in the program (from internal logic or
by external field devices) such as parts traveling past a detector or actuating a limit
switch.
When rung conditions for a CTU instruction have made a false-to-true transition,
the accumulated value is incremented by one count, provided that the rung
containing the CTU instruction is evaluated between these transitions. The ability
of the counter to detect false-to-true transitions depends on the speed (frequency) of
the incoming signal.
Note
The on and off duration of an incoming signal must not be faster than the scan time
2x (assuming a 50% duty cycle).
The accumulated value is retained when the rung conditions again become false.
The accumulated count is retained until cleared by a reset (RES) instruction that has
the same address as the counter reset.
Using Status Bits
This Bit
Is Set When
And Remains Set Until One
of the Following
Count Up Overflow Bit OV (bit
12)
accumulated value wraps
around to –32,768 (from
+32,767) and continues
counting up from there
a RES instruction having the
same address as the CTU
instruction is executed OR
the count is decremented
less than or equal to +32,767
with a CTD instruction
Done Bit DN (bit 13)
accumulated value is equal
to or greater than the preset
value
the accumulated value
becomes less than the preset
Count Up Enable Bit CU
(bit 15)
rung conditions are true
rung conditions go false OR
a RES instruction having the
same address as the CTU
instruction is enabled
The accumulated value is retained after the CTU instruction goes false, or when
power is removed from and then restored to the controller. Also, the on or off status
of counter done, overflow, and underflow bits is retentive. The accumulated value
and control bits are reset when the appropriate RES instruction is enabled. The CU
bits are always set prior to entering the REM Run or REM Test modes.
Execution Times
(
µ
sec) when:
True
False
29.84
26.67
(CU)
(DN)
CTU
COUNT UP
Counter
Preset
Accum
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