File Bit Comparison (FBC)

This information applies to the
CompactLogix
5370,
ControlLogix
5570,
Compact GuardLogix
5370,
GuardLogix
5570,
Compact GuardLogix
5380,
CompactLogix
5380,
CompactLogix
5480,
ControlLogix
5580, and
GuardLogix
5580 controllers.
The FBC instruction compares bits in a Source array with bits in a Reference array.
When enabled, the FBC instruction compares the bits in the Source array with the bits in the Reference array and records the bit number of each mismatch in the Result array.
IMPORTANT: The FBC instruction operates on contiguous memory. You must test and confirm that the instruction doesn’t change data that you don’t want it to change.
The difference between the DDT and FBC instructions is that each time the DDT instruction finds a mismatch, the instruction changes the reference bit to match the source bit. The FBC instruction does not change the reference bit.
If the instruction tries to read past the end of an array, the instruction sets the .ER bit and generates a major fault.
Available Languages
Ladder Diagram
Operands
There are data conversion rules for mixed data types within an instruction. See Data Conversion.
Ladder Diagram
Operand
Type
Format
Description
Source
DINT
array tag
Array to compare to the reference
do not
use CONTROL.POS in the subscript
Reference
DINT
array tag
Array to compare to the source
do not use CONTROL.POS in the subscript
Result
DINT
array tag
Array to store the result
do not use CONTROL.POS in the subscripts
Cmp. Control
CONTROL
structure
Control structure for the compare
Length
DINT
immediate
Number of bits to compare
Position
DINT
immediate
Current position in the source
initial value is typically 0
Result control
CONTROL
structure
Control structure for the results
Length
DINT
immediate
number of storage locations in the result
Position
DINT
immediate
Current position in the result
initial value is typically 0
CAUTION: 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 injury to personnel.
COMPARE Structure
Mnemonic
Data Type
Description
.EN
BOOL
The enable bit indicates the FBC instruction is enabled.
.DN
BOOL
The done bit is set when the FBC instruction compares the last bit in the Source and Reference arrays.
.FD
BOOL
The found bit is set each time the FBC instruction records a mismatch (one-at-a-time operation) or after recording all mismatches (all-per-scan operation).
.IN
BOOL
The inhibit bit indicates the FBC search mode.
0 = all mode
1 = one mismatch at a time mode
.ER
BOOL
The error bit is set either POS or LEN are invalid.
.LEN
DINT
The length value identifies the number of bits to compare.
.POS
DINT
The position value identifies the current bit.
RESULT Structure
Mnemonic
Data Type
Description
.DN
BOOL
The done bit is set when the Result array is full.
.LEN
DINT
The length value identifies the number of storage locations in the Result array.
.POS
DINT
The position value identifies the current position in the Result array.
Select the search mode
If you want to detect:
Select this mode:
One mismatch at a time
Set the .IN bit in the compare CONTROL structure.
Each time the EnableIn goes from false to true, the FBC instruction searches for the next mismatch between the Source and Reference arrays. Upon finding a mismatch, the instruction sets the .FD bit, records the position of the mismatch, and stops executing.
All mismatches
Clear the .IN bit in the compare CONTROL structure.
Each time EnableIn goes from false to true, the FBC instruction searches for all mismatches between the Source and Reference arrays.
Affects Math Status Flags
No
Major/Minor Faults
A major fault will occur if:
Fault type
Fault code
result.POS > size of result array
4
20
See Common Attributes for operand related faults.
Execution
Ladder Diagram
Condition/State
Action Taken
Prescan
See FBC Flow Chart (Prescan)
Rung-condition-in is false
See FBC Flow Chart (False)
Rung-condition-in is true
See FBC Flow Chart (True)
Postscan
N/A
FBC Flow Chart (Prescan)
FBC Flow Chart (False)
FBC Flow Chart (True)
FBC Flow Chart (True) - continued
Example
Ladder Diagram
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