6602 hardware timing

[RBeck]

Hello, I'm a new researcher who has just started using NI systems and hardware.  My current task is to develop a single 20 MHz pulse that repeats every 25 microseconds.  Things have been going well and I've been making good progress, but I'm hung up on the spacing issue.  The method I'm currently trying to use is a timed-loop that will have timing based on one of the 6602's internal clocks (hopefully the 100 kHz clock, that would be the easiest to work with).  I'm at the point where I can freely adjust spacing above 5 ms, but my output pulses seem to not get any closer despite lowering the number of ticks counted or changing the clock source.  I think I may be programming incorrectly, but I've searched online for literally two days now and even the most relevant fixes are no help at all.  I've looked at these forums and found some possible solutions, but the labview version I have in my office is too outdated to open the sample files for ideas (2009 SP 1).  I'm at the end of my rope here, do I really need to install a completely new OS for such fast pulses?  I'm just confused because the pulses I need are completely stored on the card and, theoretically, the computer itself shouldn't have to make calculations.  I apologize if this isn't the correct forum, I'm just at the end of my rope here.

I'm using a PXI-6602 with an SCB-68 terminal.   

[John_P1]

Hi RBeck,

If your pulse repeats every 25 microseconds, wouldn't that make it a 40 kHz pulse?  I'll assume that you mean the pulse should be 1/20 MHz wide (i.e. 50 ns).

Anyway, the pulses you need can (and should) be stored on the card and regenerated in hardware but this isn't the way you have currently implemented your program (calling start/clear is software dependant and is non deterministic).

You'll probably want to do something more like this (it makes more sense for me to think of your output pulse in terms of time, unless I'm misunderstanding what you want to do):

Best Regards,

[RBeck]

Thank you so much, John.  I'll try out your solution.  I was picked up a few weeks ago as an undergrad assistant and still learning things as I go.  When I learn more, I'll try and give back to the forum by helping others.