Instrument I/O assistant randomly returning empty strings

[AWS770]

Hi everyone,

I'm trying to interface with two 15 y/o Newport PM500-C motion controllers by using the instrument I/O assistant. The assistant is using VISA interface, and the motion controllers are connected to each other with GPIB cables while the computer is connected to the whole setup with an NI GPIB-USB-HS adapter. My VI is able to read and write to the controllers for the first one or two dozen minutes of my code running, but, eventually, the assistant will start returning only empty strings instead of the position indicator strings it was happily returning before. During this time, I will also be unable to write to the motion controllers using the VISA Write function, which returns error code -1073807339, but may be able to do so with the NI MAX 488.2 communicator, though even this sometimes returns an error stating that there is no device connected even though all physical GPIB connections are secure. Restarting the 488.2 communicator clears this message. When the empty strings are being returned, the NI Spy reveals that the computer is only sending "VISA Get Attribute" commands instead of the query and write commands it showed when correct strings were returned. Restarting my VI temporarily corrects the problem, but it always returns after awhile. I am wondering if this connectivity problem is caused by the obsolescence of the motion controllers, or if there is something I can try to prevent the I/O assistant from returning empty strings in the first place. Any ideas would be appreciated. Thanks!

[AWS770]

Update: The original error seems to always occur on VISA Write for the same motion controller and returns code BFFF0015.

[rsmayer]

Hello,

That is interesting behavior that you are experiencing. Are you using the Newport ppm500 controller driver to control your device in LabVIEW? Is there anything in particular occurring when the device gets the timeout message?

[AWS770]

No. I'm using the NI driver software that came with the adapter. When the device gets the timeout message, the code keeps running while returning blank strings. The error is logged, the data strings become empty, but everything keeps going. I just won't be able to communicate with the instrument after the error is thrown. I have created a band-aid which opens and closes motion controller VISA sessions in a while loop until the assistant begins returning the correct strings again, but I'd still like a less unbounded solution.

This issue may also be related to the motion controller returning incorrect values when queried (i.e. it will sometimes give Y position instead of the requested X position, among other quirks), but this at least conforms to a pattern and can be handled with a simple case structure. 

[rsmayer]

Hmm.. I'd say that points to an issue with the device itself or an issue with how we are making calls to the device via the driver. I'd try using the IVI driver above to see if you get similar behavior. Otherwise it might just be an issue with your device.