Jitter in DI lines

[MogaRaghu]

I am using a PCI-6229 card in an Industrial PC running WIN-XP and LV-2009 platform.

All digital inputs ( 23 of them ) are 24V dc level but converted to 5V level by isolation relays and fed to the card.

The equipment has a 15KW variable frequency drive from Control Tech / UK. What I notice is that when the drive is running all of a sudden all the DI lines change state for a moment and return back to previous state. If I connect all DI lines to LED indicators on screen for observation, i can notice them all blinking together at random intervals. This creates some undesired functions on the machine.

Looks like a power line jitter that happens at random due to the drive. Of course the power for the PC is derived from the same three phase input to which the drive is connected. But there is an isolation transformer to step down 400V ac 3Phase to 230V ac single phase and added to that there is a UPS in the input line to the PC. I wonder despite all this how can the drive affect the DI lines ??

The drive wiring is as per data sheet with shielding , grounding as required. In fact all the AI channels are reading rock steady even when the drive is ON so that I am sure the drive is really not installed badly.  What could be the reason for the momentary jitter in t he DI lines ??

[JB]

All DI lines... all blinking together...

What's about the isolation relays ? I would check their signals with a scope.

[MogaRaghu]

>> What's about the isolation relays ? I would check their signals with a scope.

Already done that and they remain latched based on the field input status.

The glitch is defenitely on the PCI bus as the jitter manifests only when the VFD is ON.