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Using a 10MHz reference for DAQ timing measurements.

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 due to the tolerance of the DAQ on board reference clocks. The measurement giving me problems is a 8,192kHz clocked output from a processor which is derived from an accuate 32,768kHz crystal. I need to measure to within a Hz accuracy and i am not able to achieve this between different DAQ-6221 cards.

The chassis' have a 10MHz GPS calibrated reference fed into them via the reference input. 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 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 10MHz reference as the base clock to take a period or frequency measurement would also be very much appreciated as i've not been able to find anything similar in the examples or on the forums.

Regards,
Dan
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Hi Danny,

I've been trying to get my head round your forum post, so I want to check if my understanding is completely correct. U have several (how many?) PXI chassis and want to synchronise the 6221 DAQ cards within each chassis and you wish to syncrhonise these within 1Hz of each other by using a 10MHz GPS signal. Currently you are getting this signal in by the controller reference point yes?

I think the easiest method for you to get this up and running is to wire in the 10Mhz signal in a slightly different way. The easiest would be to override the backplane clock source with your 10MHz signal. to do this, wire in your gps signal to the BNC connector on the back of each chassis. this will then override the backplane source, and them simply use this clock reference in your program.

If your chassis does not have this connector on the back (some of the lower end models do no), we can try using a PFI line on the actual card itself. this would make synchronisation alot easier than using the controller trigger input which is not very suitable for this kind of application.

Let me know how you get on

Best wishes.




Rob L

NI Applications Engineer

UK & Ireland


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Hi Rob,

Thanks for the reply. I decided this query was in the wrong thread so i moved it to the hardware counters thread instead. I have since actually managed to solve this issue now and i have posted my solution to my query in the other thread.

This can be found at:
http://forums.ni.com/ni/board/message?board.id=40&thread.id=6013

Best regards,

Dan.

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