Boy, do I dislike the DAQ Assistant! There's an excellent NI White Paper on DAQmx (which, unfortunately, starts with the DAQ Assistant, but read the rest of it and learn!) that you can find by doing a Web search for "Learn 10 Functions in NI-DAQmx ..." -- there's even a Video of an NI Engineer "reading the paper" for you (but I recommend you read it for yourself).
You can use the hardware clock of the DAQ device to be your timing source. [Oh, dear, I didn't see that you are trying to synchronize 3 different DAQ devices -- that complicates things a little, but there are ways to send synch signals ...]. Here's one scenario that I'm simplifying for the purpose of this discussion:
Assume you are sampling at 300 Hz, and have set your DAQ device to sample continuously at 300 Hz for 300 points. You can probably figure out that this loop will "fire" at 1 Hz, delivering an array of 300 samples. You have a second loop that is set to take a single frame from your camera using IMAQdx (and do something with it -- are you just displaying it, or are you trying to save it as an AVI or a series of PNGs?). Finally, you also want to send a Trigger signal (how often? you don't say, but let's assume every 5 seconds, in synch with the DAQ and Camera).
You set up three loops running in parallel, and set up a mechanism to communicate between the loops. One way to do this, with examples that come with LabVIEW, is to use a Producer/Consumer Design Pattern (there are some Producer/Consumer Templates that you can try that show you how this works). Start with the DAQ loop, which will "fire" once a second as it "produces" 300 points at 300 Hz. You put this in a Producer/Consumer Queue, which gets dequeued in the Consumer Loop where you can (a) write it to a Chart and (b) write it to a disk file.
You need a second (well, really a third) loop to fire once/second and tell the Camera to take a picture and "do something" with it. You can't use the same Queue for this loop, as you can Enqueue Many but Dequeue Once. However, you could put a Notifier in the Producer Loop and fire off a Notification when the Producer Fires. The Camera loop could Wait on Notifier, and take a picture every time it receives one. That takes care of the once/second Camera Loop.
The Trigger is also in its own loop. It can also take the Notifier, since, unlike Queues, there can be many loops waiting on the same Notifier. You only want to send the Trigger every 5 seconds, so count the Notifications (1, 2, 3, 4, 5, ...) and when they are divisible by 5, fire off the Trigger.
How do you stop all of these parallel loops? An easy way is to use a VIG (also called a Functional Global Variable), a VI that "stores" a Boolean, initially False, but can be called to set the Boolean to True. When your "Stop" condition is sensed (perhaps you are using a Stop button), you set the VIG to True. In each of your parallel loops, you wire the output of the VIG to the Stop Indicator of the While Loop. Now, whenever the VIG is "turned on", all of the loops, when reading the VIG, will get the "True" output, and will all stop. No Local Variables, no race conditions.
Bob Schor
