Rockwell Automation 1772-LS_LSP,D17726.8.6 PROG/OPER MANUAL-MINI PLC-2/05 - Manual (Page 73)

Timers and Counters
Chapter 8
8Ć6
The upper four bits in the accumulated value (AC) word are status bits:
Bit 14 - Overflow/underflow bit. It is set when the AC value of the CTU
instruction exceeds 999 or when the AC value of the CTD instruction falls
below 000.
Bit 15 - Count complete bit. it is set when the AC value > PR value.
Bit 16 - Enable bit for CTD instruction. It is set when the rung condition
is true.
Bit 17 - Enable bit for CTU instruction. It is set when the rung condition
is true.
There are three types of counter instructions available with the controller:
up counter
down counter
counter reset
Up Counter
Purpose: An Up Counter instruction increments its accumulated value for each
false-to-true transition of the rung condition.
010
00
CTU
030
PR 150
AC 000
When the rung condition becomes:
True
Accumulated value increments by 1.
Bit 14 is set if the AC > 999.
Bit 15 is set when AC > PR. Incrementing the accumulated value continues
after the preset value is reached.
Bit 17 is set and stays set until the rung goes false.
False
Accumulated value is retained.
Bit 14 - no action is taken.
Bit 15 - no action is taken.
Bit 17 is reset.