How can I measure time between transitions at digital input

[eapen]

I am using LabVIEW 6.0 to measure the time duration between 2 transitions in a digital input port. I have a set up including an HP 34970A Data Acquisition unit with the 34907A Multifunction Module plug-in. The LabVIEW program is trying to poll the digital input for level changes and then using the Time function(Tick Count) to check the times at the transitions. However this method doesn't work reliably as I have to measure time periods of 500ms.

Is there any good way of mesuring time period with the above setup. I don't have a module which connects to the internal multimeter in the 34970A. The other stuff I have are a HP 34903A(Actuator Module) and a HP 33120A(Function/Waveform Generator).

Any method for m
ore accurate time measurement with the above setup will be appreciated. If not, what else do I need to make accurate time measurements.

[Dennis_Knutson]

Frankly I doubt that you're going to be able to measure time interval with much accuracy. You're periidically polling a static digital input and how often you poll is going to vary. You could try polling as fast as possible and eliminate as many background tasks as you can. If your tolerance for the measurement is plus or minus 100 msce, then maybe you'll get a decent measurement most of the time. If it's plus or minus 5/10 msec, then it will never happen. The pc's clock is just not that accurate and a software timed loop on Windows is not deterministic enough. Get a counter/timer board for something really accurate or use a scope to capture the waveform.