Frequency measurement on PCI-6025E

[EI]

I am in process of migrating from Traditional to daqmx programming. I encounter a problem in measuring frequency using example "DigFreq-LowFreq1Ctr". I added in DAQmxSetChanAttribute(taskHandle, "",DAQmx_CI_Freq_Term,"/Dev1/PFI8"); just before Start code. and feed in train of pulses into PFI8. The reading was actually double what the freq of the pulse is (Expecting 2.2kHz but got 4.4kHz). I have no problem if I use PFI9, PFI3 or PFI4 instead. So I tried the counter "Dev1/ctr1" to read from PFI8 but still get exactly same double value reading.

[John_P1]

Hi EI,

Is the signal 50% duty cycle by any chance?

The way the Low Freq 1 Ctr method works is to take a period measurement and invert in the driver to compute frequency.  Double the frequency would correspond to half of the period, which leads me to believe that the counter is probably picking up noise during the falling edge transition as an extra rising edge. 

I'm not immediately sure why PFI8 would be uniquely showing this behavior, but it's possible that some of the PFI lines might be more noise-tolerant than others.  Are you required to use PFI 8 for your application?  Which terminal block are you using to make the connections?


Best Regards,

Message Edited by John P on 05-18-2010 11:44 AM

[EI]

Thank you for quick reply.

The signal is 50% duty cycle and is TTL. It come from Wavetex Synthesiser Generator.

About terminal, I make it here using materials from Farnell.The connector is farnell part no 109-9032 and protyping PCB.

I will change the card and cable today to see if I still will get same problem.

Kind Regards

Shane

[EI]

Hi

I have change the card and cable. the problem is gone. I change again to put back the original card and cable the problem is gone too and work on PFI8,9,3 & 4.

It must have been the connection.

Kind Regards

Shane