Rockwell Automation Publication 1756-RM003N-EN-P - October 2011 495
Special Instructions (FBC, DDT, DTR, PID) Chapter 13
RESULT Structure
Description: 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.
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.
Select the Search Mode
Arithmetic Status Flags: Not affected
Fault Conditions:
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.
You must test and confirm that the instruction doesn’t
change data that you don’t want it to change.
The FBC instruction operates on contiguous memory. In
some cases, the instruction searches or writes past the
array into other members of the tag. This happens if a
length is too big and the tag is a user-defined data type.
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 rung-condition-in 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 the rung-condition-in goes from false to true, the FSC instruction searches for all
mismatches between the Source and Reference arrays.
A major fault will occur if Fault type Fault code
Result.POS > Size of Result array 4 20
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