Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 49)

Fundamental Instruction Set
Chapter 5
5Ć12
The upper four bits in the accumulated value (AC) word are status bits:
Bit 14 - Overflow/underflow bit. it is set to one 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 to on when the AC value >PR value.
Bit 16 - Enable bit for CTD instruction. It is set on when the rung condition
is true.
Bit 17 - Enable bit for CTU instruction. It is set on when the rung condition
is true.
There are three types of counter instructions available with the controller:
Up counter
Down counter
Counter reset
We will look at these counters in detail.
Figure 5.4
Counter Instructions with Their Default Values
110
00
CTU
030
PR 000 Up-Count Event
AC 000
110
01
CTD
030
PR 000 Down-Count Event
AC 000
110
02
CTR
030
PR 000 Counter Reset Event
AC 000
Up Counter Instruction
Symbol: -(CTU)-
Purpose: Increments its accumulated value for each false-to-true transition of
the rung condition.
Syntax: Programmed as an output instruction.
Function: When the rung condition becomes: