URGENT HELP: Synchronization of Multiple Devices

Question

To whom it may concern,
I am relatively new to labview so I am sorry if I use the wrong terminology.
Here is all of my equipment:
DAQ USBX-6341 board.
Cameras (plugged into digital p0.0 channel/port)
Force Sensor (plugged into analog channel/port 1)
Test Machine (plugged into  COM5)
Currently,I am sending in a trigger to the camera to start taking photos. This trigger takes 2 miliseconds and then the labview program tells the forcesensor and testing machine to start. This creates a delay between the camera, force sensor and testing machine. THis makes it difficult to analyze data.
I want the testing machine, cameras, and force sensor to start taking pictures at the same time. I was told that the best way to do this would be to send in a square wave at my desired sampling rate (ex. 15,000) into the DAQ. This would synchronize everything so that it started capturing data at the same time once a trigger was sent in.
Is this the easiest way to accomplish that goal? Is there an easier way to do this?
Thanks
gcass

Answer 1

Hi gcass,
It sounds that what you're looking for is a deterministic system response. Determinism is the ability to say that at a certain point in your program, a certain function will happen within a certain amount of time, plus or minus some variation (Jitter). On an Operating System that isn't Real Time oriented, deterministic responses i.e. getting your 2ms long trigger to execute and then simultaneously begin the acquisition can't happen; this is because the OS gets in the way because of the way it schedules tasks.
For example, the same program can have two different timed responses for when you're running no programs in the background and when you're running an Internet browser. Even when the OS isn't running any background tasks you still cannot be absolutely certain that some other processor time is being used. Unfortunately, this is the limitation of a conventional OS; the processor's power is shared between multiple competing threads which have equal priorities. Furthermore, USB devices aren't deterministic either due to the serial protocols that they follow, which in their nature are indeterministic.
You can only really implement some coarse timing in a traditional OS, whilst remembering that the absolute maximum output rate an OS could achieve is 1kHz. This is where Real Time Operating Systems come into play; these allow threads to be scheduled in a way that the co-ordination of particular tasks can occur at precise periods in time. In order to achieve the response you're after you'd need to use an RTOS based implementation; this is offered on devices such as the Compact RIO or PXI. You can learn more about these products here.
However...
I may be a little unsure about your application, so please correct me if I'm wrong, but I believe you could synchronise your DAQ tasks by driving different DAQ tasks by the same sample clock; this will allow tasks to operate simultaneously as they're both triggered by the same driving clock. I've answered a forum question with a similar implementation of driving two tasks with a single clock; the same can be done for a third task (You're trigger on the first edge).
So, the difference between these two points depends on just how you want to acquire your data; because there are limitations imposed in applications such as these with indeterministic systems. Your choice depends on how quickly you need to analyse your data; however if you only want to synchronise the output trigger and the clocking then I suggest the former solution. 
I hope this helps!
Alex Thomas, University of Manchester School of EEE LabVIEW Ambassador (CLAD)