Allen-Bradley SLC 500 Series User Manual

Publication 1747-RM001G-EN-P - November 2008
7-18 Application Specific Instructions
File Bit Comparison (FBC)
and Diagnostic Detect
(DDT)
The FBC and DDT diagnostic instructions are output instructions that you
use to monitor machine or process operations to detect malfunctions.
Both the FBC and DDT instructions compare bits in a file of real-time
machine or process values (input file) with bits in a reference file, detect
deviations, and record mismatched bit numbers. These instructions record the
position of each mismatch found and place this information in the result file.
If no mismatches are found, the DN bit is set and the result file remains
unchanged.
The difference between the DDT and FBC instruction is that each time the
DDT instruction finds a mismatch, the processor changes the reference bit to
match the source bit. The FBC instruction does not change the reference bit.
Use the DDT instruction to update your reference file to reflect changing
machine or process conditions.
Select the Search Mode
Select whether the diagnostic instruction searches for a mismatch one bit at a
time or whether it searches for all mismatches during one program scan.
One Bit at a Time
With each false-to-true rung transition, the instruction compares the next bit
between the input and reference files. If a mismatch is detected, the instruction
stops and sets the found FD bit. Then the instruction enters the position
number of the mismatch into the result file.
The DDT instruction also changes the status of the reference bit to match the
status of the corresponding input bit. The instructions resets the found bit
when the rung goes false.
EN
DN
FD
IN
ER
FBC
File Bit Comparison
Source #B3:0
Reference #B3:10
Result #N7:0
Control R6:0
Length 48<
Position 0<
Control R6:1
Length 10<
Position 0<
FBC
EN
DN
FD
IN
ER
DDT
Diagnostic Detect
Source #B3:0
Reference #B3:10
Result #N7:0
Control R6:0
Length 48<
Position 0<
Control R6:1
Length 10<
Position 0<
DDT
Fixed SLC
5/01
SLC
5/02
SLC
5/03
SLC
5/04
SLC
5/05
•••
Table 7.7 Available Diagnostic Instructions
If You Want to Detect Malfunctions By Use this
Instruction
Comparing bits in a file of real-time inputs with a reference bit file
that represents correct operation
FBC
Change-of-state diagnostics DDT

Allen-Bradley SLC 500 Series Specifications

General

Brand	Allen-Bradley
Model	SLC 500 Series
Category	Controller
Language	English