trigger-digital pattern

[cek]

I would like to use "digital pattern" to control a/d acquisition. i.e. a start pulse on one line and a stop pulse on another. I am using a USB 6251. This menu item is grayed out. Does that mean SE does not support it or my USB device? Any work arounds?

Thanks

CHuck

[Paul_C.]

Hi Chuck,

The USB 6251 and our other M series devices do not have the ability to trigger off a "digital pattern".  Digital Pattern triggering is supported by some of our Digital I/O boards such as the PCI 6533 / 6534.  Is there a reason you require two lines to start and stop the analog input acquisition?  Can you just use one line with on and off states (high and low)?  Also, is there an actual digital pattern you must recognize before turning "on" and "off" the analog input?  I don't know of any way without some external hardware to accomplish a hardware timed digital pattern trigger.  With more information on exactly what type of pattern you are trying to recognize there might be a workaround.  You could accomplish software timed digital pattern triggering by just reading in the digital lines and turning on and off the analog input in software.  However, if you're looking to use a hardware timed trigger to just turn your analog input on and off.  I would recommend looking at using a counter to generate a continuous pulse train to be used as the sample clock of your analog input.  This way you could use the "on" and "off" lines to toggle the gate or use one line to turn the gate on or off.  This would turn the sample clock on and off and allow you to control when samples are taken.  You would just need to set the source of your sample clock to be the output of the counter.  I would take a look at the Gen Dig Pulse Train-Continuous.vi example (Help » Find Examples » Hardware Input and Output » DAQmx » Generating Digital Pulses) and see if it will allow you the control over the sample clock that you require. 

Regards,
Paul C.

[cek]

Thanks for the excellent and complete reply.Good to know how NI supports it's products and customers.

Chuck

[Paul_C.]

Hi Chuck,

Thanks for the feedback.  I also wanted to add that I missed that you were using SE (Signal Express).  For some reason I was thinking SE (Single Ended) from another post.  I do want to mention that you should be able to create a continuous digital pulse train within LabVIEW SignalExpress.  All you need to do is create a counter pulse output step and specify the high time and low time for setting frequency and duty cycle.  Take a look at the picture below for more details.



I hope this helps,
Paul C.

Message Edited by Paul C. on 10-31-2007 12:25 PM

[cek]

Thanks Paul,

I simplified my approach and now do use the two counter outputs from the 6251 under SE. The delay between the start of each counter is sufficient for my purpose. Sometimes, simpler is better. Initially, I wanted to use feedback from the external device to close loop but this approach is still giving me the reliability I needed.

Thanks for your follow,

Chuck