NI 9477 DO causing NI 9213 TC noise

[jwrehm]

On my cRIO-9073 I have basic LabVIEW code accepting NI 9213 thermocouple measurements, calculating some simple logic based on desired setpoints, and then sending out an on/off digital signal via NI 9477 to switch a solid state relay which in turn switches a motor on/off for each.  This is all being done with NI Scan Interface for about 17 channels.

Everything functions fine until the Digital Out is set to "TRUE" on one particular channel (10).  When this channel switches it throws noise across all the TC inputs and occasionally freezes all of them.  Most bounce 10 degrees F high or low and channel 10 reads 60-70 F high.  The only way to fix this is to assure that the DO on channel 10 is set to "FALSE" and to reboot the cRIO.  I don't have this problem on any of the other channels!

All of the output voltages seem good when read with a multimeter.  I've checked the physical wiring many times and cannot find any issues.  There is no error out of the scan interface variables in the LabVIEW wiring diagram.  Seems to me like the DO module and the TC input module should operate independently if not for the cRIO backplane they are both connected to.  The cRIO and the NI 9477 DO module (which needs its own PS to send voltage out) do share the same power supply (NI FP-PS-4) although I can't see how this could be the issue.

If anyone has any suggestions I'd love to hear them!  Thanks in advance for your feedback.

- John 🙂

[jareds]

Hi John,

Is each channel of the 9477 connected to a solid state relay?  What are these relays switching?  How much voltage and current is passing through each channel?  Do you have something available that could read the voltage at the TC inputs independent of the cRIO, such as an oscilloscope?  This could confirm if the noise is actually on the line or if it's something internal to the 9213.

[jwrehm]

Jared -

Each channel of the 9477 is connected to a solid state relay that is switching a dedicated circulating pump motor.  Each of these channels from the cRIO is switching the relay with 28 VDC and 1 Amp (or less).  On the other side each relay is using about 120 VAC to drive the pumps.

Like I said 16/17 of these channels give me no problems, its only the one channel that throws everything off.  I don't have a way of testing just the TC module independently.

I'm thinking of changing out the pump motor.  These do go out occasionally causing shorts - although I still have trouble believing this could be whats throwing off all of the TC data.  Is that a waste of time or should I focus elsewhere?

Thanks again.    - John

[Hueter]

If one channel is bahaving different that the other ones sounds like ahardware problem to me. I would recomend calling into NI support so you can see if it needs to be repaired.

Hunter

[JÞB]

The cRIO and the NI 9477 DO module (which needs its own PS to send voltage out) do share the same power supply (NI FP-PS-4) although I can't see how this could be the issue.

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The supply offers 600mA

Each channel of the 9477 can sink upto 1A

The cRIO runs off the same supply

You are starving the cRIO