SLC 5/05 File Bit Comparison FBC

[Christoph]

Thinking about using the FBC instruction. Have never used it
and have a couple questions. 1st does the reference need
to be the same length as the source. I am looking at about
a 21 word length but reference will allways be 0 in the data
hate to wast all the memory if I dont have to.
2nd does the first control set the length of the
source and second control set the length of reference or result
file.
Your help is Appreciated!

 

[TConnolly]

hate to wast all the memory if I dont have to.

Waste it? As opposed to leaving it unused? Its rare that a PLC program needs every last byte of memory, and the only truly wasted memory is the unused memory.

P.S. I'm not advocating sloppy programming. I'm advocating that we ditch the wasted memory paradigms because a SLC5/05 has a fixed memory size and its not going to be dynamically allocating and releasing memory while the PLC program is executing.

 

[Christoph]

I said wasted because I will not be using the result file word
only the status bits of the conrol word for found.

 

[Gerry]

1st does the reference need
to be the same length as the source.

Yes.

2nd does the first control set the length of the source and second control set the length of reference or result file.

1st control determines the length of the source and reference and also has the IN, DN and FD bits you will be interested in.

The 2nd control determines the length of the result file. If you operate it in the mode to detect one miscompare at a time, then the length can be one.

 

[Christoph]

Thanks, somtimes the help file just is not enough.