AB, PLC-5 indexed addressing

[lhopper]

On the AB PLC-5 (540E) does the the Index Register (S:24) get reset at the beginning of each scan?

 

[OkiePC]

I don't believe so. It should be left at the same value it was after the last time it was written. The user program can write to this address, and also there are several instructions that use this address. If you're using indexed addressing in your program, be sure to write the value you want just before the instruction that uses it.

 

[lhopper]

Thanks!

I could not find this in any of the manuals. I have been checking code (by others) and found several locations where indexed addressing is used without clearing or setting the index register. MOV and COP instructions.
I think this is causing some problems...!

I read one manual that says if the index offset exceeds the File boundary, that a major fault will occur. Does this apply only to the PLC-5 and not the 540E?

 

[OkiePC]

I don't think that applies to the PLC-5. According to the manual, it does not check to see if you cross a file boundary, but you will get a major error if you go outside the last file.

The controller does not check indexed addresses to make sure that the addresses do not cross data-table file boundaries (e.g., N7 to F8). You could even modify the status file, which is physically the last data table file. But if the indexed address exceeds the data-table area of memory, the controller initiates a run-time error and sets a major fault.

You will always see the "#" symbol in a COP instruction, and the instruction itself is one that will automatically set the index register. Any MOV instructions with the "#" symbol are using it, so the index register needs to be set up first, preferably immediately preceding those instructions.