GPIB SPOLL Affects Reading Rate HP 3478A

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Hello all, 

Disclaimer: I am relatively new to LabVIEW so it's very possible that there's something easy and obvious that I am missing. That said, I thank you in advance for your help. 

I am writing a data acquisition program for an experimental setup featuring multiple instruments from which I need to record data asynchronously over a GPIB connection.

My plan, I thought, was pretty straightforward: I wish to use the GPIB I/O functions to wait for an instrument to interrupt the controller, serial poll to find out which instrument has data ready, read, then repeat. 

For the moment, I am testing my program with  an HP 3478A DMM. 

My problem is this: I noticed using NI I/O Trace that my program gets hung up for more than ten seconds whenever it needs to read an instrument output. However, I know I have read from this instrument at far greater speeds. I built a quick test VI, and noticed that if I simply read from the instrument without checking for SRQ or Data Ready then GPIB Read operates quickly (0.2 s/measurement). However, if I insert an SPOLL before the Read (not doing anything with it, just calling the function) then it hangs for around 17 seconds before continuing.

I have attached my simple test VI, an image of it, as well as screen-shots of the I/O Trace outputs for when the SPOLL is enabled vs disabled. 

Again, I am pretty new to LabVIEW; it may be that this is some simple error, or that it's something specific to the  3478A, or our GPIB set up.

Thank you again for your help!

[mikeporter]

A serial poll call should not take that long. As I recall this type of poll returns a bunch is status bits. It is not beyond the realm of possibility that there is a bug in the instrument.

Mike...

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Hi Mike, 

Thanks for replying.

To clarify, the serial poll call itself executed in around 2 ms. In my actual program I use the status byte to determine whether an instrument has data ready.

It was the subsequent call to GPIB Read which took a long time depending on whether I serial polled before it.

I appreciate the suggestion though, I will check it again with a different instrument.

[mikeporter]

Hmmm... Now that is even more bizarre. Remember that when you are considering whether or not you have an instrument problem, HP stands for "Highly Probable".

Mike...

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Haha I'll keep that in mind. 

Strangely, the error seems to have fixed itself overnight. I cannot begin to guess what changed, but I suppose maybe it has something to do with our extended gpib system rather than the individual instrument. 

Thank you for your help!