Help With Multiple Digital Grounds

[mschmit]

I have a PXI-6259 (M-Series) hooked up to a SCB-68 terminal box.  I have 3 different electronic devices which produce digital signals that I want to monitor with the 6259.  However, If I combine the digital grounds for these DI input lines, I get a ground loop problem which causes my electronics to act erratically.

Is there a way to set a PFI line to use another PFI line to be the reference ground for that channel, sort of like a differential input?  Is there any other way to go about separating my digital grounds so as to prevent these ground loop problems?

Thanks.

Mike

[Brian_Beal]

The best way to go is to use isolation modules.  I have always used the OPTO22 G4 series.  You will need either a 4 or 16 channel terminal board rack, and whatever digital input modules you need for your type of signal.

[mschmit]

That's what I was afraid of... additional hardware.   Any other suggestions?

Could I use the PFIs as analog input in differential format?  I have 4 channels with one ground, 4 with another, and 6 with the last.  I think the 6259 has 16 AI channels as I recall correctly.  Can I reference a single PFI line for the differential input on mulitple AI channels?  I think this is possible...

[AnalogKid2DigitalMan]

Mike:

Can you post an image or a schematic or how the devices are powered and connected up to the DAQ card?

[mschmit]

Here's a schematic of how I have ti wired up right now.  Each axis (encoder) has its own electronics, although they appear to have the same power supply.  Each axis has it's own digital ground.  I think that I'm getting ground loops by connecting all digital grounds to the DGND of the SCB-68 which causes problems with the circuitry that reads the encoders.

I am trying to be as passive as possible and not affect these digital lines which I've tapped into on the equipment.

Message Edited by mschmit on 05-07-2007 11:02 AM

[tbob]

You could try using shielded twisted pair wire with a shield.  ChA and ChA would be one twisted pair, B and B would be a second twisted pair, and the shield would be DGND.  Use this for all axes.  Ground only at one end, preferably at the end closest to the power supply.