Profibus slave keeps dropping comms?

I have a Prosoft MVI56-PDPMV1 master card and about 10 slave devices on my bus, some ABB drives some Turck modules. For some reason, two of the ABB drives will randomly drop communications with the master for about 2 seconds causing my fault logic to kick in. Just those 2 specific slave nodes.

I was logging the MVI56PDPMV1.Status.Fieldbus.FB_Slave_Data_xfer[x].x bit that shows comm status with the slave and was able to see it drops to FALSE for about 2 seconds.

Any ideas on how to troubleshoot this? Don't see any errors on the master itself or the drives themselves.

Probably the first thing I would check on those two problem drives is that the termination switch on the profibus connector has'nt been accidentally bumped in to the on position.

if it isnt decisvely to the off position and maybe there is vibration or something causing it to momentarily terminate??

Just a base level thing to check.

You could also look in to using Profitrace...
http://www.procentec.com/profitrace2/index.php

I've used it many times for problems such as this.

Was able to verify that the COMM STAT light on the MVI56 master and the online LED on the slave VFD both drop out at the same time. I can actually replicate the issue on the slave device by attempting to switch that VFD to running. Always happens right when it tries to ramp up.

Also verified that all terminators were correctly set.

have you checked the profibus cable shields?
Is the shielding and grounding bar in the connector pressed together?

Open each connector to verify

Are the motor cables long ?
If so, using screened cables may not be enough.
Possibly you need also an output choke to limit noise and reflections.

And, be absolutely anal about terminating the shield properly.
Both on the Profibus cable and the VFD power cables.
There must be a big fat clamp over an un-insulated piece of these cables so that the screen has good contact to ground.
For the Profibus cable, the termination in the Profibus connector is not enough when there are VFDs.

Thanks for the info. Definitely seems like an noise issue. Looks like I've got some digging to do. Maybe I'll get approval to buy Profitrace for this.

Here's the Profibus installation guide, a web based training:

http://www.profibus.com/fileadmin/media/wbt/WBT_Assembly_V10_Dec06/en/Seiten/index.html

Should be worth spending an hour.

Kalle