The PLC-5 is really the heir to the PLC-2 application set, so it contains the FSC and FBC instructions that work similarly to the ones in the PLC-2/30. I don't think any of the smaller PLC-2 models had those file instructions, but they were all obsolete before I joined the Company five years ago so I don't know for sure. They aren't in the Mini PLC-2 manuals I have in hardcopy.
The FBC and DDT instructions were recently added to the SLC-5/03 and higher controllers in Series C firmware, to match the PLC-5 instruction set.
But as you've found, a File Search isn't built into the SLC-500 instruction set. So we have to "roll our own".
The point of the FSC is to move through a file of a determined length and compare a source value to each element of the file, and set a flag and pointer when a match is found.
A FOR-NEXT loop would be the conventional way to do this, but the SLC doesnt' have one of those, either.
A subroutine would be my preferred way to separate this function from the rest of the ladder logic. I'm going to run one through my bench SLC and I'll attach it to a later post.