How to start 2 SCXI modules for multiplex acquisition

[CHAssoc]

We are trying to run an SCXI1100 for multiplexed voltage signals and an SCXI1102 for thermocouples, in a buffered continuous acquisition. In earlier releases of LabVIEW, I would have established a channel list on the configure vi, then acquisition start on continuous sampling, with the read vi inside the loop. This would poll both modules as voltage signals. I would have then applied thermocouple linearisation to the gathered data. In release 8 the thermocouple linearisation has gone and we are forced to establish two different definitions of the signals we wish to acquire, one for the voltages and one for the thermocouples and CJC. My problem now is how to start the two separate acquisition definitions to achieve continuous multiplexed reading. I have tried defining a digital trigger from one acquisition line to start the other, but every source definition tried so far is either reserved or not available.

[devchander]

Hi since all ur SCXI modules seem to be multiplexed to a singel DAQ card, there is no way you can acquire them using two acq loops simultaneously.

Just do this, acquire both voltage and thermocouple channels by creating individual virtual channels by using create Virtual channel VI inside a for loop , configure all channels for a single task, acquire using a single read.

Now the DAQmx read will read both voltage and temperature channels

Any doubts, do get back

Regards

Dev

[CHAssoc]

Dev,

Thanks for your reply. I was sure that attempting to start the acquisition twice was not going to work, but the problem is that you have to define how the Create Virtual Channel vi is to be used before you can wire to it. The appearance of the vi icon and the terminals available change according to what kind of device you are connecting. Therefore, I wasn't quite sure about what you suggested, using a loop structure, because of course the same vi icon would be in each instance of the structure. But you did give me the required clue. What is not made clear in any of the NI documentation is that it is possible to chain the Create Virtual Channel vi's together. Therefore we start by defining a set of voltage channels, which creates a task reference and then on the same task we can use the vi again, but this time as a thermocouple input, and effectively add channels of different kinds to the one acquisition list. This is almost identical to what I described originally as the way to solve the problem before LabVIEW 8, except that the thermocouple linearisation is defined before the continuous acquisiton loop start rather than after it.

Anyway, thanks again for your help. The application now works perfectly and we have a happy customer.

Chris

[devchander]

Hey Chris,

Good to hear that ur client is happy

Yes, That is exactly how you use connect different types of virtual channel VI's, and the data read in DAQmx read for each channel will be in  the order that they were configured.

Regards

Dev