Visa Close is Freezing

[PR_Kim]

I use an USB based - Virtual Serial port - communication unit in my application. It uses the supply to send a signal to my DUT. Voltage one way and Current the other way. It is plugged directly into the PC, not via an USB hub

For every DUT the Serial port is opened and closed again after communication is done.

The problem is that once in a while the VISA Close is FREEZING. I see it maybe once for 1000-5000 units which is about 2-3 months of production. I have seen it on 3 different LABVIEW/TestStand versions with different VISA driver versions. Each are running on different computers

Could it be the VISA Driver?

Could It be the HW in the communication unit?

Could it be the Driver of the communication unit?

I have only seen the VISA Close to freeze in relation to this Hardware.

If it is related to the Hardware, why is the VISA Close freezing instead of returning an error?

The workaround is to unplug and re-plug the communication unit

Any suggestions are welcome

[cstorey]

Sounds like USB selective suspend power settings causing issues.  What OS, what PC are you using?

See links in - https://forums.ni.com/t5/Instrument-Control-GPIB-Serial/VISA-Limitations-Number-of-Instruments-Type-...

[PR_Kim]

Thanks for link cstorey

I will change the USB power setting to check if it helps. I am using Win10 and Win8

Does anyone else has a suggestion to my problem or should I assume that USB by default is unstable?

[Albert.Geven]

Hi

A long time ago there were visa problems after n (big) times of opening and closing visa sessions. But you have to open and close visa sessions anyhow because you are switching devices. So if that is the case you only can minimize the open and close as long as the device is the same.

[paw510]

Apparently there are many flaky USB-serial converters out there.  For example similar problems were reported when using simultaneous readings with several boards:

https://lavag.org/topic/18562-visa-write-read-to-usb-instruments-in-parallel-crashes-labview

and were apparently tracked down to buggy virtual Com port drivers.

I would try another brand for your usb-serial dongle.