How to make two start and stop digital triggers work simutanuously and independently

[Davis Zeng]

I have one question on DAQmx start and stop digital triggers. My application needs to create two independent start and stop digital triggers for 4 channel analog inputs from 2 separate NI9215. (Sample Frequency 5KHz/each). My hardware is NI CDAQ-9172 with two NI9215 blocks and two NI9421 DI. I modify 200306.vi sample to perform DAQ from any two channels of each NI9215 without any problem. However, when I combine two of them into one vi (see the attached DAQ test.vi ),  I can only run one loop at a time and must finish the 2nd loop in order to start the 1st loop  (One start trigger at a time and in sequence).  I can not run them simultanuously and indenpendently even they are triggered at the same time. .  

I wonder whether there is any other way to do this or timing and synchronization can solve my problem. Thanks!

[Ana P]

Hi Davis,

I understand that you are trying to run two analog input tasks simultaneously. It is not possible in the cDAQ chassis, the cDAQ User Guide specify that “the NI cDAQ-9172 has one AI timing engine, which means that only one analog input task may be running at a time on a chassis. However, the analog input task can include channels from multiple analog input modules.” Why do you need to start and stop the channels with different triggers? If you need all channels to be sampled simultaneously at some point, one way would be to perform the acquisition in all channels with one task. You can start this task with the earliest start trigger and stop it with the latest stop trigger.  Then after the acquisition is done, select the data that you want from each channel by manipulating the data acquired in the task. I hope this is helpful,

[Davis Zeng]

Hi, Ana:

Thank you for your help. I was confirmed last week that my card only has one trigger circuit. I tried your suggestion and did collect the 4-channel data with the earliest start trigger and stop it with the latest stop trigger. Then I start using each trigger’s start and stop timers to control my interested data arrange for each channel. It seems that there is a little bit more data then those from hardware trigger but I can live with that. The software trigger is not suitable for me as my data may have abnormal signal. As I am still working on this so maybe get back to you if there is another issue.

I appreciate your help.

Davis