simultaneous acquisition with different sampling rates on SCXI

[pyro61]

Hi, I'm wondering if it's possible to simultaneously acquire different signals using different sampling rates with the SCXI system.  I've got a chassis contaning an SCXI-1600 which is connected to my computer through USB.  Interfacing with the 1600 in the same chassis are two SCXI modules, an 1100 and an 1102.  The 1100 is connected to 32 BNCs, and the 1102 is connected to 32 thermocouplers.

Is there a way (in LabView) to measure all 64 channels simultaneously, but at different sampling rates?  I'd only like to sample the thermocouplers at about 1 Hz, while I'd like to sample the BNCs as fast as possible.  However, right now I'm limited to sampling all 64 channels at about 1500 Hz each (a total of 100 KS/s, which seems to be the limit for the SCXI-1600).  It would be really nice if I didn't have to sample the thermocouplers so fast, but I'm afraid that may not be possible due to hardware limitations.

Thanks!

Scott

[Otis]

Hi pyro61,

Unfortunately there's not a really great solution for you.  The way an SCXI chassis works is that it multiplexes all of the channels that are in the scan list.  The only way that you can get rid of some of the samples would be to do it in software and programmatically look at only half of the data except for that one point per second that you want.

You can either sample all 64 channels at their maximum rate (200 kS/s / 64 = 3,125 S/s), or you can pick some rate lower than that rate.  If you need to sample the different modules at different rates then you will have to have those different modules in different chassis.  Otherwise you are limited to one rate per chassis.

Note: If you have the modules configured for parallel mode, then you could read multiple modules at multiple rates; however, in your case this will not work because you are only using one DAQ board (the SCXI-1600), the modules you are using are not supported for parallel mode, and because you can only read 8 channels at a time when in parallel mode.

My best recommendation would be to sample as fast as you can, and then discard the unwanted thermocouple data.

Regards,

[pyro61]

Otis, thank you for the prompt reply.

I have one more question, though.  I did read that the maximum sampling rate for the SCXI-1600 was 200 KS/s, however when I configure the DAQ-Assistant with my 64 channels for a sampling rate of greater than 1562.5, it gives an error saying that the maximum sampling rate is 1562.5 Hz.  With 64 channels, this equals 100 KS/s total.  Is there any reason that this could be happening?  It would be very nice to get the maximum 3125 S/s per channel out of the device.

Thanks again,

Scott

[Otis]

Hi Scott,

Unfortunately the SCXI-1600 is the only device (along with B-Series Devices) that cannot be simulated (NI-DAQmx Simulated Devices).  Let me try to set up the system and see if I can get something similar to what you are seeing.

Regards,