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34461A Periodic Fetch Latency

Question asked by Scott_A on Jan 19, 2017

Last year I wrote LabVIEW code to collect data from several 34411A DMMs in parallel at nominally 60 Hz.  This year I was provided 34461A DMMs and asked to collect more data.  I have encountered a communication timing problem with the 34461A devices that I did not have with the 34411A variants.


I have attached a sample LabVIEW program that demonstrates the issue.  The program communicates with one DMM and generates a log file summarizing a series of "R?" fetches.  The log file lists the time stamp the data was received and the number of samples in the returned data packet.  I have run this program against two different 34461A modules and consistently observe that every 150 ms there is a 50 ms delay in the response to an "R?" fetch.  This temporal gap then results in 3-4 samples being returned together in a subsequent "R?" fetch.


Normally the "R?" fetch executes in approximately 1 ms, meaning that every 16 fetches, 1 data point is returned.  My application is designed around streaming data point-by-point, but the 50 ms communication delay with the 34461A DMM prevents me from doing so.


When I worked with the 34411A models, I used the "VOLT:APER 0.01667" command to set the integration window and executed the "R?" fetch until a data point was returned.  However the 34461A model does not have this aperture setting, so I have refactored the code to use the "VOLT:NPLC 1" method instead.  I am unsure if this detail is significant, but it is the only change in my code between working with the 34411A and 34461A models.


Please advise on how I can collect data from the 34461A DMMs at 60 Hz, 1 data point at a time.


Thank you for your time.