RSlogix 500 shift register control variable size

[mahryanne]

Hi,
I have a line where I need my shift register to vary in size depending on the type of product being run. I am using ffu and ffl instructions that work great. Also, I have no problem writing to the .len of the control so I can vary the size. I just need to reset the .pos somehow. If the position is at 10 for example, and I change the length of the shift register to 8. it gets stuck and nothing works.
I tried using a blank control register and just copying the file to my current shift register, but that faulted the PLC.
any ideas would help.

thanks,

Marianne Stevens

 

[OkiePC]

If your POS is greater than or equal to your LEN, then the DN bit will be set.

It would help to see your actual logic, but I would expect that you need to reset the POS (MOV 0 x.pos or CLR x.POS) before your change the LEN, and make sure that both of those operations occur before the instructions that use that control element are scanned.

Of course if you need to set the POS to something other than zero, that is a little different. You would always want to set the POS to a value less than or equal to the LEN though, in any case I can imagine.

See if you can post your code and describe the function of the stack for us.

Paul