PCI-6221 PFI0 start trigger is reading stray signals

[MikeV]

We have a PCI-6221 attached to a SCB-68 in a control console we've manufactured dozens of times. However, this time we're having problems with the PFI0 start trigger. Operating remote 24V actuators (not wired to DAQ card but operated via manual switches) is triggering the start. We've tried a shielded cable and everything is grounded. There's no stray voltage coming into the SCB-68 box. We've also detached all other analog I/O from the box. It's just the start trigger wires going in. What's unusual is that if we attach a wire to the PFI0 (terminal 11) that does not terminate anywhere, we just hold it in the air, not crossing any other wire, operating our actuator valves still initiates the trigger. If we remove the wire, it does not trigger. It's almost as if it recieves wireless signal. But how it wirelessly recieves +5V to trigger is beyond me. We've also tried using a known working spare PCI-6221, same problem. When viewing the PFI0 in MAX, flipping the switch to the actuator will cause the counter to generate 400+ counts. Normal triggering with our ignition button usually generate 10-20 counts. We are really stumped. Thank you for any help!

[AnalogKid2DigitalMan]

Common issue- are there snubbers or diodes on the actuators to dampen the fast risetime inductive kick/EMI wavefront when power is cycled?

Is there something wired or layed out different in this particular cabinet?

Are the signal wires from the SCB to the panel shielded and has the shield terminated at one end?

Also suggest grounding any unused inputs to prevent stray triggers.

I've used a short length of wire on an analog input card to remotely detect the presence of a person from 3' away since the body modifies the stray 60Hz power field eminateing from electrical conduits and equipment. When triggered it would play the 'Intruder alert" wav file from a Star Trek episode.

-AK2DM

[MikeV · Accepted solution]

Do not have an answer to the first question, will have to research that.

Tha cabinet is wired the same, the only difference is we've added a transformer so that the system can operate over seas, but hasn't actually been hooked up yet, just sitting in the cabinet.

Yes, we've used shielded cable that has been terminated at one end. We've tried grounding on either end, same results. I've also waved the shielded cable in the air and smacked people in the head with it, it made me feel better.

Didn't think about grounding unused inputs. I personally think the PFI0 is being triggering with less than +5V, the scope is reading very small voltage.

But I did find a fix. I had mentioned in the original post that triggering the actuator, just once, was causing 400+ edge detections/counts when viewing in MAX. That's a lot of counts in such a short time. So I programmed in a digital filter using a min. pulse width of ".00256". That eliminated the 400+ short bursts of whatever signal. I used the example filtering VI in this knowledgebase article:

http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/9ece7dbe7df338bd86256f8600734a41?O...

For those with similar problems, there's no physical wiring changes, just incorperate the filter.