Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 50)

Fundamental Instruction Set
Chapter 5
5Ć13
True
Accumulated value increments by 1.
Bit 14 is set on if the AC >999.
Bit 15 is set on when AC >PR. Incrementing of the accumulated value can
continue 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 is retained if it was set.
Bit 17 is reset.
The CTU retains its AC value when:
You change the mode select keyswitch to the PROG position.
The rung condition turns false.
A power outage occurs and memory backup is maintained.
NOTE: Bit 14 of the accumulated value word is set when the accumulated
value either overflows or underflows. When a down counter preset is reset to
000, the underflow bit 14 will not be set when the count goes below 0.
DownĆCounter Instruction
Symbol: -(CTD)-
Purpose: Decrements its accumulated value from 999 for each false-to-true
transition of the rung condition. This indicates an underflow condition.
Syntax: programmed as an output instruction.
Function: When the rung condition becomes:
True
Accumulated value decrements by 1.
Bit 14 is set when AC<000.
Bit 15 is reset when AC<PR; counting can continue.
Bit 16 is set and stays set until the rung goes false.
False
Accumulated value is retained.
Bit 14 is retained if it was set.
Bit 15 is retained if it was set.
Bit 16 is reset.