Maximum safe voltage for NI 9401

[Walter_Rebsch]

Background:

During a system test, wires were inadvertently reversed and 24V DC was applied to all the COM lines on the 9401 and to DIO 4, 5, and 6.  Thus pins 1,3,4,6,7,9,10,12,13 (all the COM lines are bussed together in our 25pin DSub) and pins 20,22, and 23 were all raised to 24V.  Te other inputs were floating (not connected).  The module is now non-functional.  It self tests ok, but when you turn on the test panels, none of the DIO's will work as either inputs or outputs anymore.  So we blew the module.  It's dead.

What we want to do:

We are considering ways of modifying our wiring harnesses such that if 24V is applied to the wrong wires while it's hooked up, the module will be protected.

My questions:

If we place diodes (probably 40v Schottky's) on the DIO pins, this will theoretically protect our output channels from being driven by some external source, correct?  But will the module still be blown by a voltage on the COM lines?  How much can COM be safely driven away from the power supply ground of the compactDAQ carrier board without blowing the module?  How much current can COM sink without burning it up?  Do you think it was the voltage on the DIO pins that killed it, the COM pins, or both?  Anyway, we want to make sure this doesn't happen again.

Thank you for any suggestions.

Walter

[Walter_Rebsch]

It is probably also worth noting that the 9401 module is rated for the following:

page 10 of user manual:
input protection: +/- 30 V max on one channel at a time (referenced to COM)

page 11 of user manual:
isolation barrier continuous voltage rating: 60 VDC channel to earth ground

page 12 of user manual:
safety max voltage DIO to COM: +/- 30V (has a note that it can be applied between any channel terminal and COM without damaging the module or devices)

But the manual says nothing about what the module can withstand when applying a voltage to COM.

The spec on page 10 tends to imply that the device failed because of the 24V was on 3 pins at once, but i'm still suprised that the device died from only 24V when it is supposedly protected to +/- 30.

I'm also concerned now about what is going to happen in the field if a transient voltage appears on my ground lines.  All the common lines are bussed together and attached to my power supply common at a single point (which is isolated from earth ground).  Do I need to add some sort of protective circuit to COM on the 9401?  In the wiring, numerous 24V signals are in multiconductor cables along with 5V signals.  If a short develops, am I going to loose the board again?  If COM were to be shorted to 24V and the DIO pins were being held a 0V, then the DIO pin would see -24V from it's point of view.  How many channels of that can it take before it pops?