Error -50103 occurred at DAQmx Start Task.vi

[DB66]

I am trying to run 2 different tasks on my PXI combo chassis. One task is at a lower rate (1K) due to the SCXI 1104C limitations, the other task is on a PXI-6259 card at 10K. I keep getting the 50103 error on the 2nd start task vi. I cannot put these into the same task due to the rate difference. I am beginning to wonder if I can even run 2 different rates on the same PXI. Any help will be appreciated.

[Kevin_Price]

Briefly, no, you can't run 2 different AI rates on that device, nor on most other NI DAQ devices, because there's only one "timing engine" available for the AI subsystem and it's A/D converter.  You can't have 2 distinct hw-timed AI tasks running at the same time.   This isn't *unique* to the fact that you have some SCXI stuff hanging off one of the AI channels of the 6259, though the SCXI limitations are certainly contributing to the problem.

Bottom line: you'll need a 2nd DAQ device if you need 10 kHz sampling for one AI task and are limited to no more than 1 kHz sampling for the other.

-Kevin P

[DB66]

Kevin,

Thanks for the info. That is what I was suspecting. It didn't occur to me, until you mentioned it, that the 6259 is servicing the SCXI portion of the chassis (I now remember that). If I understand you correctly, I should be able to add another card to the main part of the chassis and perform my 10K sampling while doing my 1K sampling of the SCXI cards through the 6259.....right?

Thanks again for the help! 

[Kevin_Price]

Yes, adding another card for the 10 kHz sampling will work.

Note however: with tasks on separate boards running at different rates, you generally need to make some special efforts to sync the boards' sample timing.   There are 2 things you need to do to properly align your 2 boards' sample times:

1. Establish a common t0 for both tasks.  You should probably do this with a shared start trigger.

2. Establish 1 and only 1 master timebase that drives all the boards' sample rates.   Here are 2 options:

a. *IF POSSIBLE* (I'm not very familiar with SCXI usage), configure the SCXI AI task to use the 10 kHz tasks' sample clock as its own "sample clock timebase".  You can then also set a "timebase divisor" to be 10.  This would link the clock timing in hardware because the SCXI task literally derives it's timing signal from the hardware sample clock signal generated by the 10 kHz task.

b. It might happen for you automatically, but you'll need to do some research to know for sure.  Many boards *can* align their own internal timebase clocks to a master 10 MHz clock on the PXI chassis backplane.  In fact, it might be the case that many of them do this automatically.  If they do (or if you can force them to), then you're free to configure the boards the easy way, using their own internal clocks, because the PXI chassis acts as the single master that keeps them aligned.   

   Note: you'd still need a trigger signal to establish a common t0

-Kevin P

[croohcifer]

Everything Kevin said is sound advice.  I would just add that as long as you can share a sample rate, I believe you can scan SCXI and non-SCXI channels on the same DAQ card in the same task.  Is there a reason why the SCXI channels can't be sampled at 10 kS/s, too?  The module you're using is just a low pass filter and should not in any way limit your sample rate.  The SCXI backplane limits the aggregate sample rate of SCXI channels to 333 kS/s, but that's a limitation independent of the modules in use.  If you only want data from these SCXI channels at 1 kS/s for your own reasons, you can always decimate the 10k SCXI channels in software.  This solution is more complicated to implement than just using a second DAQ board, as Kevin suggested, but I wanted to point out that it should be possible.

[Kevin_Price]

I actually think my suggestion with a 2nd DAQ board is *more* complicated, not to mention more expensive.  Because I don't have much familiarity with SCXI, I simply accepted the 1 kHz limit as a hard limit without trying to verify.

If you're able to sample the SCXI stuff at 10 kHz and then decimate down to 1 kHz in software as croohcifer mentioned, I think *that* would be a *much* better solution.

-Kevin P

[DB66]

I tried sampling the SCXI and non-SCXI channels at 10K but I was getting an error saying I was exceeding the sample rate, that's when I veered off in another direction trying to do it in separate tasks with separate clocks. I may go back and investigate that more before I go and buy another module.

[croohcifer]

I think the complexity is mainly around managing which channels are SCXI and which are native DAQ.  If the ordering is dynamic, then your algorithm for parsing the channels and selectively decimating could get tricky if you don't build that robustly.  If your channel definitions are always in the same order or static, it's probably easy enough.  It's just not as flexible as using two board in my opinion.

The 1 kS/s limit may also be real and tied to how many SCXI channels are in use.  (You could conceivably have a couple hundred SCXI channels, in which case you'd run into a 1 kS/s, or thereabouts, limit due to the 333 kS/s aggregate rate.)

[croohcifer]

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@DB66 wrote:

I tried sampling the SCXI and non-SCXI channels at 10K but I was getting an error saying I was exceeding the sample rate, that's when I veered off in another direction trying to do it in separate tasks with separate clocks. I may go back and investigate that more before I go and buy another module.

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Can you let us know how many SCXI channels you're using and how many extra DAQ channels you want to use?  It should be pretty easy to see if you're violating the aggregate sample rate.

[DB66]

I re-tried sampling them all at 10K but still got the "exceeding sample rate" error. Backed it down to 8K, same error. Backed it down to 5K and got a "violate settling time" warning but it allowed me to proceed. As I was doing this it dawned on me that the signals I need to monitor are actually 10KHz signals so I will need to go a little higher anyway. looks like adding an additional card is my only avenue.

Thanks again to all that responded!!!!