Running two cDAQ-9174's...

[chuggins1434]

Hello one and all. 

It seems that I've run into a new problem today (at least for me).  I have two cDAQ-9174's and each has a 9205 AI module and a 9264 AO module in it.  I have set up two DAQ Assistants, one to control each unit.  Apparently you can't set up a single task to use more than one module.  Found that out today. 

To start off I set up a simple VI to read just a few channels on the first cDAQ and then a single channel on the second cDAQ.  Both are running samples on demand.  When I highlight the execution the first unit runs fine, and then the second unit tries to run, but always ends up kicking an error. (50405)  I'm guessing the second task is still running when the first one tries to reinitialize. ..not sure, just speculation.  Is there a way to get them to run without stepping on each others toes??  I tried running them in a flat sequence, but that didn't work any better than just having them in the While loop.   Also tried pulling them outside the while loop... still no joy.  I've searched on here, but didn't find much that was applicable... or I just didn't use the right terms to search.  If anyone has any suggestions, I'd really appreciate a point in the right direction.

Thanks!

Chad

[chuggins1434]

Well, I've figured a work around... put a shift register on the while loop, give it a Boolean True to start and then throw a Not.vi on the signal to the shift out side.  Tie one side of the Not.vi to the Stop inut on the first DAQ Asst. and tie the other side of the Not vi to the other DAQ Asst.  Thus, for each iteration of the loop one of the DAQ Asst. will be stopped and the other can run.  Anyone see an issue with this?  Got a better way?  I'm all ears!  🙂

THX

C

[MikhailK]

Hi chuggins1434,

To run two cDAQs simultaneously you will need to arrange them in a Master-Slave configuration. Here is an example you can have a look at:

Synchronizing Multiple NI CompactDAQ Chassis This example does not use DAQ Assistants but uses basic DAQmx functions. Also, you can learn more about multiple device synchronization in the following article: Synchronization Explained

[chuggins1434]

Mikhail,

Thank you for the response! 

I'm obviously running two 9174's, so I'm looking at the first example and as noted in there... "If you use an NI cDAQ-9174, you need an additional hardware-timed digital module installed in any slot to access the PFI lines."  Ok... which hardware?  This is one of my complaints about some of the NI tutorials (not your fault Mikhail I know), but not enough information.  How about an example?  What are my options here?  (ok, rant mode off)

As far as how I'm running now, it seems to be stable.  Zero errors. (I'm running dual 9174's and each one has a 9205 AI module and a 9264 AO module in it... I ended up needing more than 16 AO's was the reason) I am, however, having a weird response from on of my 9174's with a 9205 module installed.  All of my channels are ~1VDC high.  Everything self checks ok, but nothing I do seems to change the response.  I've changed modules, even a different 9174 chassis altogether.  Even with one of the channels grounded directly I still get ~1VDC response.  I've grounded both units together and still one measures ~1VDC higher than the other.  I've disabled the "good" one, totally unplugged the "good" one, still ~1V where there should be none.  Any thoughts on that?

Thanks!  (sorry for the small rant)

Chad

[MikhailK]

Hi Chad,

---

@chuggins1434 wrote:

"If you use an NI cDAQ-9174, you need an additional hardware-timed digital module installed in any slot to access the PFI lines."  Ok... which hardware?

---

The recommended digital modules are the NI 9401 and NI 9402, but any hardware-timed (also known as correlated) digital I/O module works.

Now, you can try resetting your NI 9205 module in MAX to see if this resolves the 1VDC offset you are getting. What we might be seeing here is the charge accumulation on the multiplexer. Have a look at the following articles for more info on this:

Incorrect Readings when Scanning Unconnected or Open Channels

Troubleshooting Unexpected Voltages, Floating, or Crosstalk on Analog Input Channels