PF525 slow response when indexing due to active bit

they're inside a periodic task that allows time for the IO to update before next scan

That's what I call "working by coincidence"; because the task is executed less frequently, then the probability that the I/O timeout occurs between the Start and Stop bit logic execution is much lower.

I first encountered this phenomenon on a PowerFlex 40P installation with ControlNet, on a corn chip packaging line. Because the stop/start action of the drives was very frequent, the opportunity for them to "not start when requested" was also frequent.
 
A bit of a brain burp today - task priorities. Initial research found these: On page 25 of Logix 5000 design considerations 1756-rm094-en-p.pdf 1769 CompactLogix are shown to operate IO at a priority of 6. If this is correct, VFD logic ‘without the bits’ would be fine in a periodic task with a priority of 5 or better on at least the 1769 CompactLogix platform.

The Logix 5000 Tasks, Programs, Routines manual 1756-pm005-en-p.pdf seems to have contrary information on pages 14-15. I am deciphering it to state IO at priority 6 applies to FlexLogix and DriveLogix, not CompactLogix and no mention of ControlLogix. Can anyone clarify this?

For OP Ben, using the drive ready status bit instead of drive active for the start OTE might enable the faster response desired without waiting for the drive to fully stop. Flying start parameters 545 & 546 might be applicable if the motor will restart before stopping but maybe not for a small inertia application.
 
So if I understand your issue correctly...correct me if I'm wrong

You are running, decide to stop, and sometimes need to restart while the drive is ramping down to the previous stop.

If you use the "active" bit to hold the stop command true while stopping, the drive may not see the stop go false before seeing the start go true - the result is that the drive would just keep stopping and would never start.

So one solution that you are using is to keep the drive running and just control the speed command of the drive. Yes, that is legitimate. You may need to add external fans to the motors for cooling, but I suspect you can get away without that.

If you keep that same logic for controlling speed, what if you issue a brief stop command (using the active bit) only once the drive has reached zero speed?
 
That is correct.
I have used frequency to control the drive and if at zero frequency for 1 second stop the drive.

I was thinking that might be a parameter I was missing that would allow it to start again instantly when start command is high.
That with a bit of software tweak would sort it.

That doesn’t seem the case and what I have works and so far the drive and motor is performing ok and to the touch the motor isn’t getting hotter than normal
 

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