Allen-Bradley MicroLogix 1400 - CTU - Count up CTD - Count down

Publication 1766-RM001A-EN-P - October 2008
Timer and Counter Instructions 185
CTU - Count Up
CTD - Count Down
Instruction Type: output
The CTU and CTD instructions are used to increment or decrement a
counter at each false-to-true rung transition. When the CTU rung makes a
false-to-true transition, the accumulated value is incremented by one
count. The CTD instruction operates the same, except the count is
decremented.
CU
DN
CTU
Count Up
Counter C5:0
Preset 0<
Accum 0<
CTU
CU
DN
CTU
Count Down
Counter C5:0
Preset 0<
Accum 0<
CTD
Execution Time for the CTU and CTD Instructions
Controller CTU - When Rung Is: CTD - When Rung Is:
True False True False
MicroLogix 1400 0.4849 µs 0.3812 µs 0.4350 µs 0.3803 µs
TIP
If the signal is coming from a field device wired to an input on the
controller, the on and off duration of the incoming signal must not be more
than twice the controller scan time (assuming 50% duty cycle). This
condition is needed to enable the counter to detect false-to-true transitions
from the incoming device.
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