Problem with cRIO9411 DI modules

[CGMENG]

I'm using one channel on each of three cRIO9411 DI modules to synchronize operations on three cRIO chassis.  The synchronization did not work and upon investigating further I found a 1.5VDC constant voltage on each of the input lines with no connections.  The trigger is sourced from a cRIO 9472 using a 24VDC linear PS.  It would appear that the line input state never goes low since the voltage is always at 1.5VDC.   I shorted the DI input through a DMM to ground and measured 0.49mA.  I though about using a pull down resistor but the value would have to be so low that when the 24VDC pulse is on the current through the resistor would be significant.

Is this normal for the 9411?  If so, what is the appropriate way to take care of this issue?

[Doug M]

CGMENG,

An NI 9401 might be better suited for synchronization since it can run much faster but also allows both digital input and output in the same module. 

In your current configuration, I would first do some simple tests to make sure the modules seem to be functioning properly, such as connecting a battery that is > 2V to the 9411 and making sure it reads the correct value. When you said you ran the channel through a DMM to ground, did you wire the positive, negative, and ground lines together? Does the cRIO module read "off" in this configuration?

Were you wiring the COM lines from the 9472 and 9411 together?  I would test each module separately making sure each works, then try setting up simple triggering/synchronization between chassis.

Best Regards,
Doug Mumford
Applications Engineer
National Instruments

[CGMENG]

The 9401 is not appropriate for my application for a number of reasons. 1. I use the DO, 9472, to drive some 24DVC relays for other purposes (all of which have diode protection), 2. My trigger source is 24VDC.

The COM of the 9472 and 9411 are tied together and I also testing it with both tied to earth ground, but this is not the issue.  With only the COM on the power input of the 9411 connected, each DI terminal, a & b, have a 1.49VDC measured from the terminal to the COM.  I though of using a pull down resistor but it would require an extremely low value to get below the 0.8VDC low logic level and when the 24VDC is applied the 0.250Watt limit would be exceeded.  Yes, I can always get a bigger resistor, but this doesn't seem like the best approach.

I tested each module separately and they all have the same characteristics.  Were is the 1.49VDC coming from??

 

I could set up a simple test but as you know there is no 'quick' test with FPGA.  I will probably proceed down this route anyway.

 

[Doug M]

CGMENG,

Yeah, something sounds a little fishy.  If you are tying all the lines to ground it seems like you should measure 0 V, nevertheless, what I am really interested in is what LabVIEW is actually reading in as the state of the line.  You should be able to create a simple VI that just allows you to read the inputs of the 9411 with front panel indicators and toggle lines on the 9472, and compile this in a couple minutes.  I'm trying to understand whether what is happening when you say that the synchronization isn't working, like is the 9411 not reading a false when the line is actually low?  What happens if you connect the 9472 to the 9411 and toggle the values while reading the voltage with your DMM? What voltages are you reading for high and low with the 9472 connected and toggling?