External Timing Source with Timed Loop not working as expected

[KeA]

I would like to quickly describe what I am trying to accomplish and afterwards post the actual problem. I would like to use the PXI 10MHz reference clock together with the counters on a PXI-6220 card for precise delay measurements and (delay-)timing of timed loops. I started with an external 10MHz reflclock generated by a function generator however, because I am not sure how to connect the PXI refclock to the counter internally.

In order to test if the concept works, I made a small VI (see attachment). The attached VI runs as expected when used with the built-in 1MHz timing source: It generates a rising edge and ~9.8µs later a 500Hz clock. If I connect the external timing source as shown in the picture however, the general behavior stays the same (delay and generation of a clock afterwards), but the timings are completely wrong (18ms period, 235ms delay). I suspected that the quality of the signal on the input terminal may not be sufficient, but a frequency measurement done with the same counter and terminal was perfectly fine and showed 10MHz, and after playing around with the setup of the external timing source VI and the timed loop, I keep getting wrong results consistently, so I would like to ask if somebody knows what I am doing wrong here?

[KeA]

I just saw that the config in the picture of the Timed Loop says "ms", even though an external timing source is connected. This seems only to be a display bug, and in the code I am running I double checked for the loop saying "ticks".

[ReginaW]

HI,

Did you have a look at this forum post?

http://forums.ni.com/t5/Counter-Timer/Using-an-external-reference-clock-for-counter-measurements/m-p...

I think they clearly describe how to realise this application. I wonder where you have got the VI "Create Timing Source" because in my case it does not have the same connectors as yours.

I think you need the appropriate property node to rout the external clock to your counter card.

Which Version of LabVIEW are you using?

I hope this helps

Best Regards

Regina

National Instruments

AE Germany

[KeA]

Thanks Regina for your answer! I already stumbled upon the thread you mentioned, but as far as I understood the problem described there, it is about using an external 10MHz clock as a reference clock for measurements. For me, I would only like to actually count the 10MHz clock on the PXI backplane, as it were any other signal. I am pretty sure that I would need to route the signal for this, but unfortunately this is not my main issue 😞

The VI I use is the standard "DAQmx Create Timing Source", which is a polymorphic VI. I am using the "Digital Edge using Counter" instance (turned off the display of the instance selection field, sorry for that). I am pretty sure that the counter sees the right signal, because the loop does not even execute if I turn off the external clock (as I mentioned, I am using an external function generator to generate a square wave on the input terminal of the counter).

I am using LabVIEW (RT) 10.0 with DAQmx 9.1.5.

[ReginaW]

Hi,

I now had time for a closer look at your VI. First of all I want to mention that generally a while loop around a timed loop never makes sense. Then I want to mention that you should wire out all error cluster wires.

despite all these things: I saw that in your timed loop there is shown "ms" this indicates for me that you configured the timed loop to use the internal timing.

When you double click the timed loop, then you will see that you can make additional configurations. There you can select the checkbox:"Use connection for timing source" in LabVIEW 2009.

According to your last post I guess you are working with LabVIEW 2010. There you don't have this checkbox. But if you connect the clock to the right connection (Source name) then the label "ticks" should appear. This is not the case in your code, this is something which looks strange for me.

Could you please post the VI so that I can have a look at this VI?

Best Regards

Regina

National Instruments

AE Germany

[Ledang]

Hi Regina,

I would appreciate if you can assists me with my problem. I constructed a simple Vi to controlled timed loop using external source (pulse train) from PFI3. During execution there is error comes from the Daqmx create timing source. I would like to ask what I am doing wrong here?  

Thank you

Arelone