Count Down (Ctd); Relay Ladder; Counter Structure - Allen-Bradley Logix5000 Reference Manual

Count Down (CTD)

Mnemonic Data Type
.CD BOOL
.DN BOOL
.OV BOOL
.UN BOOL
.PRE DINT
.ACC DINT
Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES)
The CTD instruction counts downward.
This instruction is available in structured text and function block as CTUD.
Operands:

Relay Ladder

Operand
Counter
Preset
Accum

COUNTER Structure

Description
The count down enable bit indicates that the CTD instruction is enabled.
The done bit indicates that .ACC ≥ .PRE.
The overflow bit indicates that the counter exceeded the upper limit of 2,147,483,647. The counter then
rolls over to -2,147,483,648 and begins counting up again.
The underflow bit indicates that the counter exceeded the lower limit of -2,147,483,648. The counter
then rolls over to 2,147,483,647 and begins counting down again.
The preset value specifies the value that the accumulated value must reach before the instruction sets
the .DN bit.
The accumulated value specifies the number of transitions the instruction has counted.
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
Type Format
COUNTER Tag
DINT Immediate
DINT Immediate
Description
Counter structure
How low to count
Number of times the counter has counted
Initial value is typically 0
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