How to send TTL output AND acquire AI voltage data using USB-6211

Question

Hello,
I am relatively new to Labview, so please bear with me.  I have a research application involving 2 pressure transducers and a high-speed camera.  I wish to acquire analog voltage data from the 2 pressure transducers.  However, at the start of the acquisition, I will need to send a single TTL output to trigger the camera.  This TTL pulse must be sent out at exactly the same time that the AI acquisition begins, in order to ensure that my 2 pressure measurements and camera images are 'synchronized' in time.
Is this possible on the USB-6211 running with LabView 8.20?  I currently have a fairly simple LabVIEW vi that uses a software trigger to start an AI acquisition - I have attached it with hopes that it may help anyone willing to assist me.  I would prefer to be able to simply add something to it so that it will output a TTL pulse at the start of the acquisition.  
Thank you in advance.
Regards, Larry
Message Edited by Larry_6211 on 12-19-2008 11:24 AM
Attachments:
USB6211_v1.vi ‏212 KB

Answer 1

Hi All,
I'd like to clear a few things up. First, you'll find that if you try to set the delay from ai start trigger and delay from ai sample clock to 0, you'll get an error. Due to hardware synchronization and delays, the min you can set is two. Note that when I say two, I am referring to two tick of the AI Sample clock timebase, which for most acquisitions is the 20MHz timebase. I modified a shipping example so you can play around with those delays if you want to - I find that exporting the signals and looking at them with a scope helps me visualize what is going on. The Manual has some good timing diagrams as well but it looks like you've already hit that. The defaults would give you a delay of  250ns from the start trigger - is this too high for your situation? What is an acceptable delay? I tend to think that "exactly the same time" is a measure of how precise rather than an absolute (think delays in cable length making a difference.)
With all that in mind, I see a few options:
Start your camera off of the AI start trigger (an internal signal) and just know it is 250 ns before your first convert. 
Export the convert clock to use as a trigger. This assumes your camera can ignore the next set of convert clocks.
More complicated option: Internally you have an ai start trigger, sample clock and convert clock. From your start trigger to the first convert is 250ns but if you export your convert clock you're going to get future convert clocks as well. One option would be to generate a single triggered pulse using a counter (start with the  Gen Dig Pulse-Dig Start.vi example) with the AI start trigger as the trigger for the counter, an initial delay of 250 ns, and a high time of whatever you want it to be. This should give you a singe pulse at very close to same time (on the order of path delays) as your first convert clock. 
Hope this helps, 
Andrew S
MIO DAQ Product Support Engineer
Getting Started with NI-DAQmx
Measurement Fundamentals
Attachments:
Acq&Graph Voltage-Int Clk.vi ‏37 KB