Multi-device analog output based on synchronous routing external sampling clock

[qianlushuai]

Dear everyone,

     I need your help. I want to use multiple PXI-6230 devices to output analog signals simultaneously. This requirement can be met normally when different devices use the same reference clock, sampling clock (internal), and start trigger signal. However, when I applied the external sampling clock, which is derived from a signal generator and is input through a PFI0 terminal of PXIe-6674T, two problems arose. The first one is that after the external standard sampling clock is routed over the 6674T, a lot of interfering burr signals are generated, which causes the 6230 analog output to misbehave. The above phenomenon only appears at the initial stage of the sampling clock input and is random. The second problem is that among the multiple 6230s that are supposed to be operating in sync, the slave devices will always miss the first external sampling clock, resulting in a phase difference with the host output. I have tried to modify the terminal impedance of the PFI0 terminal and the routing path (such as PFI0 to PXI_trigger, then PXI_trigger as sampling clock source),  but the above two problems have not been solved. Therefore, I need your help. 

     Attached is a demo program for testing, and an oscilloscope image that can intuitively represent the above two problems.

     Thanks.

[santo_13]

Since you're using multiple instruments of the same model, you can take advantage of DAQmx channel expansion. DAQmx takes care of synchronizing all the instruments at the sample level and still work with your triggering/external clocking.

Read these articles to know more,

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P8XiSAK&l=en-US

https://www.ni.com/en-us/support/documentation/supplemental/15/channel-expansion-explained.html

[qianlushuai]

Dear Santhosh,

     Thanks for your reply. Single-task multi-channel or multi-device expansion is the solution we tried at the beginning, but unfortunately, the PXI-6230 device does not support multi-device tasks, so we hope to achieve it through multi-task synchronization.

Lushuai Qian

[Kevin_Price]

I have only the slightest experience with NI-Sync and no insight into possible causes of the "burrs" you mention.  I'm focused on the missed first sample clock from your regular DAQmx devices.

I'm assuming the scope signal you call "host" is the AO task whose start trigger is being exported -- is that correct?

I'm getting into speculative territory here, but I *wonder* whether the "ao/StartTrigger" internal signal is one of the ones that has its transitions synced to a timebase.  And I further wonder whether the timebase chosen for this syncing is influenced by the use of an external sample clock.  Perhaps the start trigger signal is directly sync'ed to that external clock?  That could explain why the other 2 tasks don't seem to start until one cycle later.

Again, mere speculation.  But here's a little something I'd try: configure all 3 AO tasks to trigger off a counter output and configure the counter to be triggered by the (routed) external signal.  Then make sure to start all 3 AO tasks before starting the counter task.  This method would avoid any of that timebase syncing I speculated about.  It's at least something new to try...

-Kevin P