Diagnostic Detect (Ddt); Relay Ladder; Compare Structure - Allen-Bradley Logix5000 Reference Manual

Diagnostic Detect (DDT)

Mnemonic Data Type
.EN BOOL
.DN BOOL
.FD BOOL
.IN BOOL
.ER BOOL
.LEN DINT
.POS DINT
The DDT instruction compares bits in a Source array with bits in a Reference
array to determine changes of state.
Operands:

Relay Ladder

Operand
Source
Reference
Result
Cmp control
Length
Position
Result control CONTROL
Length
Position

COMPARE Structure

Description
The enable bit indicates that the DDT instruction is enabled.
The done bit is set when the DDT instruction compares the last bit in the Source and Reference arrays.
The found bit is set each time the DDT instruction records a mismatch (one-at-a-time operation) or after
recording all mismatches (all-per-scan operation).
The inhibit bit indicates the DDT search mode.
0 = all mode
1 = one mismatch at a time mode
The error bit is set if the compare .POS < 0, the compare .LEN < 0, the result .POS < 0 or the result .LEN
< 0. The instruction stops executing until the program clears the .ER bit.
The length value identifies the number of bits to compare.
The position value identifies the current bit.
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
Special Instructions (FBC, DDT, DTR, PID)
Type Format
DINT Array tag
DINT Array tag
DINT Array tag
CONTROL Structure
DINT Immediate
DINT Immediate
Structure
DINT Immediate
DINT Immediate
ATTENTION: Use different tags for the compare control structure
and the result control structure. Using the same tag for both could
result in unpredictable operation, possibly causing equipment
damage and/or injury to personnel.
Description
Array to compare to the reference
Do not use CONTROL.POS in the subscript
Array to compare to the source
Do not use CONTROL.POS in the subscript
Array to store the results
Do not use CONTROL.POS in the subscript
Control structure for the compare
Number of bits to compare
Current position in the source
Initial value typically 0
Control structure for the results
Number of storage locations in the result
Current position in the result
Initial value typically 0
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