Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 143)

Chapter 8
Using Basic Instructions
8–24
How Counters Work
The figure below demonstrates how a counter works. The count value must
remain in the range of –32,768 to
+
32,767. If the count value goes above
+
32,767 or below –32,768, a counter status overflow (OV) or underflow
(UN) bit is set.
A counter can be reset to zero using the reset (RES) instruction. (See page
8–27.)
Underflow Overflow
–32,768 +32,767
Count Up
0
Count Down
Counter Accumulator Value
(CTU)
(CTD)
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.
Important: The on and off duration of an incoming signal must not be faster
than the scan time x2 (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 EN
(bit 15) rung conditions are true rung conditions go false OR
a RES instruction having the
same address as the CTU
instruction is enabled
(CU)
(DN)
CTU
COUNT UP
Counter
C5:0
Preset 120
Accum 0
Execution Times (
µ
sec) when:
True False
29.84 26.67
Ladder representation:
Count Up (CTU)