How to synchronize DMM (PXI-4071) with DAQmx (PXIe-6124) in LabView

[Pewas]

Hallo,

for some power measurements I would like to measure the current with an DMM PXI-4071 and the voltage with a DAQmx PXIe-6124 in LabView 2009. The chassis is PXIe-1062Q. It is important that the two values (voltage and current) are acquired simultaneousely.

Unfortunately I hava a problem to synchronize the two devices. In some topics here I found the information that it is not possible for the DMM to provide a trigger. So in my opinion the trigger signal has to be provided by the DAQ-device on one of the 8 trigger lines of the trigger bus.

Because I was not able to manage it in a functioning way, I hope someone here can help me.Perhaps there is also a different way to synchronize the two devices?

Best regards,
Peter

[Marco_Brauner]

Hello Peter,

I have attached an example how to receive a trigger at a PFI on a DAQ-card and route the trigger to RTSI_0 (PXI-Trigger-Bus)

and how to configure the DMM to take this trigger. Hope this is what you´re looking for.

Marco Brauner NIG

[Pewas]

Thank you very much, for the fast response! 🙂

If I have understood it correctly, I need an external clock to "feed" the trigger line through  PFI 0. Can I use the TTL Signal from the function generator on the DAQ board or the PXI_CLK10 from the chassis?

Or is there also another possibility without using the PFI 0?

Best regards,
Peter

[Marco_Brauner]

Hi again,

you´ve got two choices: internal or external triggering.

Internal means that you trigger off a internally generated edge, such as "aistarttrigger" which is 

asserted every time you call "DAQmx Start". External means that your trigger signal is externally 

connected to a PFI-Line. As a trigger, you can use any TTL-Signal, but I guess what you´re looking for is 

what I mentioned first, right? So I think you should try "/DevX/ai/StartTrigger" as the trigger source.

Marco Brauner NIG

[Pewas]

Thank you again for the fast and satisfactory answer!

This is what I was looking for.

Best regards,
Peter

[Pewas]

Hi,

unfortunately I still have a problem with the synchronisation between the DAQ and the DMM. Generally, triggering the DMM via TTL0 from the DAQ/ai/StartTrigger seems to work. But when I'm using the DAQ-function generator as signal source for both, the DMM and the DAQ, the resulting waveforms are not on top of each other. The DMM-trigger delay is set to zero.

You can take look at the my vi. Perhaps it is too complicated for the simple task, which is the acquiring the samples of DAQ and DMM simultaneously ?

[Marco_Brauner]

Hello,

I have attached your VI with some little changes. Put the Initiate- or Start-Functions outside the loop and only the read/Fetch-Vis inside the loop. Otherwise you

get unnecessary overhead in your program. Furthermore the two cards are sampling with their own sample clocks which aren´t tied together. So you won´t get the

samples at exactly the same time. Please try and tell me if there´s any improvement.

Marco Brauner NIG

[Pewas]

Thank you!

I have put the initiate function inside the loop, because otherwise (in the case of the DMM initial block) I get the error message "The data is not available.  This can be caused by calling Fetch or FetchMultiPoint before calling Initiate or after calling Abort."
But the bigger problem for me lies in the different sample clocks. Do you think, there is a possibility of an exact simultaneous measurement (DMM and DAQ), anyway? Perhaps by synchronizig the sample clocks?

[Pewas]

Hi,

I found out, that the measured signal asynchronism changes during variation of the source signal frequency.
It becomes minimal (synchronous) at exactly 1kHz and all multiples of 1kHz. By increasing the frequency of the source measured signal at DMM moves left and in this way seems to be "faster" than the signal received on the DAQ.

I was using one source signal (sinus) for both devices (DMM and DAQ).