Re: OMS VME-8 motor controllers: PAL upgrades

[mark@bessrc.msd.anl.gov (Mark S. Engbretson)]

>The interrupt capability can be useful even with the current board. For
>example, I believe the motor device support routine polls the OMS board at 10
>Hz. This means that if I tell the motor to move, and then want to wait for it
>to get done before doing something else, I will suffer an average latency of
>50ms and a worst-case latency of 100ms. If interupts were used on 'operation
>complete' it would reduce the latency dramatically. The interrupt would cause
>a poll and record processing.

It is potentially a much worse problem than that.  Not only do you have the
latency, but the OMS board is broadcasting its position at each of those 50ms
steps as well. To make a long story short, if you are moving, let's say, a huber
and associated analysis theta/2theta motors, you can easily exceed the 
computation
power of your IOC as it tries to keep up with the resultant message storms.

The lower the latency, the worse the problem.