Using an external reference clock for counter measurements

[DannyV]

Hi all,

I have several PXI chassis configured for testing, each of which uses an NI-DAQ-6221 for conducting period and frequency measurements among other things. However each of my setups give me a slightly different reading when conducting identical measurements due to the tolerance of the DAQ on board reference clocks.

The chassis' have a 10MHz GPS calibrated reference fed into them via the reference input and this is connected to the backplane. I have tried and failed to try to route and use this as my reference base for my counter measurements so that each of my cards will give me the same reading every time. I have tried the routing method using the DAQ connect terminals vi but could not get this to work. I have looked through the labview examples and cant find anything thta shows me what i need to do to take frequency and period measurments using my external reference as my base clock. There are some examples showing the use of the DAQmx timing vi set to sample clock mode which allows a clock source to be selected. However these examples are only for counting digital events and when i apply the same vi to a frequency or period measurement an error is thrown saying 'invalid timing type for this channel', 'you can select: implicit, on demand'.

I do not understand the working of the DAQ hardware very well so was hoping someone could help explain to me how to do this if it is even possible or give me any guidance as to how else to overcome my problem. Any vis showing how to use my external 10MHz reference as the base clock for these kind of measurements would be greatly appreciated.

Regards,
Dan

[DanBrowne]

Hi,

With regard to you using the Connect Terminals VI, what exactly did you see when you tried to use it to route the terminals together? Was there an error message? Also did you verify that the source and destination terminals could actually be routed together by using MAX to view to routing table for the 6221?

Regards,

Dan - NIUK

[DannyV]

Hi Dan,

Thanks for the reply. When trying to use the connect terminals i just tried connecting the PXI10 to the onboard clock in the hope that this would configure it to use the PXI10 signal to synchronise the onboard clocks via the PLL. Doing this didn't throw any errors as i recall, it just gave me a reading of infinity as it couldn't measure the signal.

I have actually just managed to successfully do what i wanted thanks to help from Rob at NI tech support UK. There is a DAQmx timing property node which allows you to choose a reference clock source. By using the PXI_Clk10 signal this makes the DAQ synchronise its reference clocks to this signal. I have included an image of the code i used to achieve this to take period measurements.

Here is a very useful link which Rob sent me which may be of help to anyone else with similar problems

http://zone.ni.com/devzone/cda/tut/p/id/3615

Best regards,
Dan.