Synchronizing Period (2 counter - HF method) measurements

[CJKS]

Hello NI Forum Folks,

I'm running LabVIEW 7.1 and a 6602 DAQ with a BNC 2121 attached.

I'm attempting to get period measurements from a single PMT with a signal splitter and am feeding it into PFI's 39 and 31 (default inputs for the High Frequency method).

My first attempt was to take the example, "Meas Dig Periods-Buffered-Continuous-High Freq 2 Ctr.vi", and duplicate everything. Since the signal is split from the same PMT I know that the two columns of data being outputed should be the same. However, they are not.

Could anyone tell me what I'm not seeing here? Shouldn't it be this simple? <sigh>

Cheers,

CJ




 

[Kevin_Price]

At a quick glance, the main issue is that the two tasks are not synchronized to start at the exact same time.  If you look closely at your data, one of the counters starts later and its measurement # i should correspond to the other counter's measurement # (i+n), where n is the # of samples worth of offset due to different start times.

To sync the start times you'll need both counter tasks to use an "Arm Start" Trigger, which can be configured with options under the DAQmx Trigger Property Node.  The regular digital start trigger can only be used to sync counter outputs, the "arm start" trigger is needed to sync counter input measurements.

-Kevin P.

[CJKS]

Hi Kevin,

Thanks for the reply. I have followed your advice and added a Trigger Property Node to each counter task. Changing the "ArmStart.DigEdge.Src" to PFI 39 on both channels does indeed give me two columns of identical data. However, I am still unsure of what exactly this is doing. Am I now telling the second channel to collect data only when PFI39 starts? If so, why do I need a Trigger Property node for EACH channel? Why doesn't it work with just one property node on the second channel?

Thanks again for all your help,

CJ

[50ohmTerminator]

Hi CJ,

You need to arm both counters to ensure that they start on the same edge.

Have a good one

Malay D
NI