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34461A - Sync trigger of multiple DMMs

Question asked by sebe on Dec 11, 2014
Latest reply on Dec 28, 2014 by tayezisk

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Hi, i am using several 34461A DMMs with Labview -> SCPI ->VISA -> LAN -> 34461A.

I want to use the internal memory up to 10000 readings in each DMM to save a long term measurement. I want to set up each DMM to trigger in sync with all DMMs to get readings that were taken at the same time.

When I use the trigger delay and set 10s and use the sample count to 10000 per trigger and send a trigger signal from Labview I can see that my measurements drift away from each other after a few minutes/hours.
I think this is because 1 DMM measures temperature, 1 measures Current and 1 measures Voltage. All DMMs are set to 10 PLC so each measurement has the same length but I think the internal processing is different.

Is there a way other than sending a single trigger from Labview every 10 seconds?
I wanted to unplug and shut down my Labview computer during the measurement....

Greets,

Bernd

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jdwindrvr

Dec 16, 2014 9:25 AM

Hi Bernd,

You are correct that internal processing of voltage, current, and temperature measurements may cause the measurements to drift (become unsynchronized) from each other as the ends of the sample counts are reached. To minimize the variation in measurement time between different functions you can try turning off the display, using fixed ranges, and if auto-zero is on (volts, ohms), turn it off.

The trigger delay as you know is between the receipt of the trigger and the *first* reading in the sample count. There is no way to set a delay to pace individual readings.

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sebe @ jdwindrvr on Dec 16, 2014 9:43 AM

Hi jdwindrvr ,

thanks for your reply. I will try this and maybe there will be fewer differences.

Another option I found is to connect the DMMs with each other by hardware. One DMM as master from its "VM Comp" BNC output to all other DMMs as slaves to their "Ext Trigger" BNC inputs.

Of course you must configure only one DMM trigger from Labview to the master DMM and configure all slaves to the external trigger Input.

This way all DMMs are in sync over a long time, but if you look on one single trigger/measurement there is a Little time difference because the "VM comp" Output is toggled AFTER a measurent is taken, so master differs from slaves.

Regards,
Bernd

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tayezisk @ sebe on Dec 28, 2014 3:17 PM

For relatively slow DAQ (at or below about 1 Hz) I have been running several 34410A, 34461A remotely on a Wifi bridge (Linksys WES610N 4-Port Dual-Band N Entertainment Bridge). (LabVIEW 2014)

Each time around the LabVIEW vi main while loop, I have a cascade of software triggers for each DMM forced into order by the error wire path and sequence structures (one per DMM, could each be a vi too).

Only after leaving the cascade of software triggers (a very fast sequence), I read each DMM. The data logging is performed by adding each value to an array (plotting each one to a graph vs. time) and sequencing by each DMM data loop using it's own shift register for its waveform wire. Each time around the while loop, I add the new value to each waveform record using waveform to waveform component (the double array) and build array to add each new value into the double array. Out of the build array, the latest array goes to the waveform graph, then back into a build waveform function to the shift register.

Inside each loop, each new DMM value gets written along a new line into a .txt file for the data logging record.

Each time before each software trigger, I run through four vi, not sure if this is necessary. trigger configure, configure multipoint, then initiate measurement, and send software trigger. A one time set up of the trigger with just the two initiate measurement, and send software trigger around the loop does not appear to work. It seems you have send something before each initiate measurement and send software trigger.

To run two 34410A and one 34461A in a one second loop, I use an additional loop delay of 732 ms (just a wasted time delay to set my data logging rate of about one second, perhaps +/- about 5 ms, the remaining part of a second what it takes to run the entire loop including trigger, measure, read, graph, and write with 5 1/2 digit precision. (for long apertures, say 100 plc (1.667s), 1 second loops are of course, not possible.

I suppose more accurate timing might be had by finishing the loop in the 268 ms, then waiting for a time on the clock.

Because each DMM is triggered in fast sequence each time around the loop, there is no synchronization drift. It might be possible to software trigger faster in parallel, not sure. (It is only ms in series sequence, but I have not timed the spread).

Also, the OP mentioned wanting to turn the LabView computer off, so in contrast, this approach of course needs the LV computer to be running.

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jdwindrvr Dec 16, 2014 9:25 AM
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Hi Bernd,

You are correct that internal processing of voltage, current, and temperature measurements may cause the measurements to drift (become unsynchronized) from each other as the ends of the sample counts are reached. To minimize the variation in measurement time between different functions you can try turning off the display, using fixed ranges, and if auto-zero is on (volts, ohms), turn it off.

The trigger delay as you know is between the receipt of the trigger and the *first* reading in the sample count. There is no way to set a delay to pace individual readings.
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- sebe @ jdwindrvr on Dec 16, 2014 9:43 AM
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  Hi jdwindrvr ,
  
  thanks for your reply. I will try this and maybe there will be fewer differences.
  
  Another option I found is to connect the DMMs with each other by hardware. One DMM as master from its "VM Comp" BNC output to all other DMMs as slaves to their "Ext Trigger" BNC inputs.
  
  Of course you must configure only one DMM trigger from Labview to the master DMM and configure all slaves to the external trigger Input.
  
  This way all DMMs are in sync over a long time, but if you look on one single trigger/measurement there is a Little time difference because the "VM comp" Output is toggled AFTER a measurent is taken, so master differs from slaves.
  
  Regards,
  Bernd
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  - tayezisk @ sebe on Dec 28, 2014 3:17 PM
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    For relatively slow DAQ (at or below about 1 Hz) I have been running several 34410A, 34461A remotely on a Wifi bridge (Linksys WES610N 4-Port Dual-Band N Entertainment Bridge). (LabVIEW 2014)
    
    Each time around the LabVIEW vi main while loop, I have a cascade of software triggers for each DMM forced into order by the error wire path and sequence structures (one per DMM, could each be a vi too).
    
    Only after leaving the cascade of software triggers (a very fast sequence), I read each DMM. The data logging is performed by adding each value to an array (plotting each one to a graph vs. time) and sequencing by each DMM data loop using it's own shift register for its waveform wire. Each time around the while loop, I add the new value to each waveform record using waveform to waveform component (the double array) and build array to add each new value into the double array. Out of the build array, the latest array goes to the waveform graph, then back into a build waveform function to the shift register.
    
    Inside each loop, each new DMM value gets written along a new line into a .txt file for the data logging record.
    
    Each time before each software trigger, I run through four vi, not sure if this is necessary. trigger configure, configure multipoint, then initiate measurement, and send software trigger. A one time set up of the trigger with just the two initiate measurement, and send software trigger around the loop does not appear to work. It seems you have send something before each initiate measurement and send software trigger.
    
    To run two 34410A and one 34461A in a one second loop, I use an additional loop delay of 732 ms (just a wasted time delay to set my data logging rate of about one second, perhaps +/- about 5 ms, the remaining part of a second what it takes to run the entire loop including trigger, measure, read, graph, and write with 5 1/2 digit precision. (for long apertures, say 100 plc (1.667s), 1 second loops are of course, not possible.
    
    I suppose more accurate timing might be had by finishing the loop in the 268 ms, then waiting for a time on the clock.
    
    Because each DMM is triggered in fast sequence each time around the loop, there is no synchronization drift. It might be possible to software trigger faster in parallel, not sure. (It is only ms in series sequence, but I have not timed the spread).
    
    Also, the OP mentioned wanting to turn the LabView computer off, so in contrast, this approach of course needs the LV computer to be running.
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34461A - Sync trigger of multiple DMMs

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