Devicenet to 1336 Plus Drives

I have a system controlling two 1336 plus drives via SLC 5/04 and 1747-SDN scanner. The logic issues the start command, and the corresponding output is energized and remains so as long as the drive is commanded to run.

The problem I have is that sometimes, the drives do not start. Online I confirm that the start output is energized, but drives don't run, and there are no faults shown on the network. Using online editing, I added an AFI before the output and confirm that toggling the output will start the drive. So the behavior is as if the initial start command didn't get to the drive (lost packet?),even though the output remains on.

At this point I'm just planning on adding logic to monitor the run contact of the drive and toggle the output if I don't get the input when expected. But, having no experience with Devicenet I wanted to at least post the question here to see if anyone had any other troubleshooting advice that doesn't involve modifying the logic, which for me involves change control paperwork unfortunately.

Thanks for any help,
Rob

Long ago in a deer hunting camp in the woods north of Mequon, Wisconsin, a debate raged over a matter of cardinal importance to the gathered men. Voices were raised. Shouts were heard. A shot rang out ! Then silence.

And that's why Allen-Bradley drives use a False-to-True transition of the Start bit, but require that when the Start bit changes state that all the Stop commands be false.

Kind of like a rifle safety.

This is common on drives networked with ControlLogix because the I/O scan is asynchronous to the ladder scan, but it can still happen on networks like DeviceNet if the STOP bit is simply programmed to be True any time that the START bit is false.

If a packet ends up going out that has both the START and STOP bit true, then the START bit's transition will be ignored, even after the STOP bit goes false.

I program the Start bit to stay true only until the drive has reported that it's running. And I program the Stop bit to be true only until the drive has reported that it is not running.

That makes sense. Thanks very much for the information, and for taking the time to reply.

Rob