VFD Error When Down Loading to an AB 1769-L33ERM

When down loading code to the PLC (1769-L33ERM) I get a "loss of communication" error for the VFD's in the controller status. I think this is normal. I have a "loss of VFD communication" error monitor that causes our system to trigger the Fatal error state when it is detected that the VFD's are off line. This code is in place because I have had the VFD's drop communication while our machine was running in production mode.

Is there a way to know that the PLC is in a "down load" condition so that the machine will not go into the Fatal state? Does the controller status show a fault in addition to the loss of communications in the controller status?

Thanks,
Tom Kazakoff

Since you're not giving information about the VFDs and network, I only can tell you about a similar problem in a place I visited and how it was solved.

In that place, the maintenance people wanted to keep running the VFDs in case of a loss of communication. Each VFD had a 20-COMM-C card (ControlNet) and they changed a parameter in the card to keep the last condition in case of loss of communication.

IHMO, if you need to upload an application to your PLC, it's better to stop the VFDs. I prefer to avoid dangerous situations.

I have had the same thing happen to me. The error comes up on the VFD and does not reset itself after the PLC starts talking to the Drive. So when your PLC comes back online it sees the Alarm from the VFD and does what it is supposed to do.

The only thing I can think of is to put some logic in the PLC that clears that alarm on the First Scan or you can up the time delay in the VFD for setting that alarm to a number that will allow you to take the PLC offline, download and get back online before the alarm is set. That may be more dangerous that it is worth so you may want to be careful with the timer.

Do you mean that it actually faults the processor when it detects a VFD offline? If that's the case, my first recommendation would be to change that. Faulting the PLC deliberately is something that I've never found a good reason to do.

That being said, if you (or someone who "knows better") is adamant about keeping it that way, then I would implement a powerup sequence in the PLC, where you don't scan any routines for the first 5-10 seconds to give the VFD's a chance to reconnect, then at the end of that time, send a reset command to the drives, give it another second or two for the reset to work, and then start scanning your other routines. When you first power up, or when you switch from program mode to run mode, you'll clear the fault, but if a drive goes offline after that it will still generate the fault.