NI VISA Read drops bytes read by the same USB device in CVI

[ljaeggi]

Hi all, 

I have a USB device with correct drivers installed that shows up as a COM port with NI VISA. I am having issues with the VISA read function. It appears to be dropping bytes. After thinking it was an issue with my application, I started using the NI basic serial read. The same set of bytes are being dropped. 

I know the bytes are there because I have a CVI application that reads the bytes from the same setup (not at the same time). This shows a specific set of bytes not being read by the VISA read function. I have confirmed the bytes are being sent by the device by using a device monitor. 

Has anybody had issues with NI VISA dropping bytes from the example program but being fine in CVI and other applications?

I have posted on the LabVIEW board because it appears to be LabVIEW causing the issue rather than CVI.

Many thanks for your help.

[billko]

LabVIEW has been known to have issues with some USB to Serial chipsets, but I can't remember the specifics.  😞

[crossrulz]

I have had issues with one USB-Serial adapter dropping a single byte every few hours or so.  It was enough to really throw off collected data.  Since that instrument just streamed data constantly, I just detected the missing byte and used the next message when there was an issue.

If memory serves correctly, the USB-Serial adapters that use the real FTDI chipset (not the counterfit) have data the most success with LabVIEW and VISA.  This is one of the reasons I plea for my customers to use a PCI(e) card for serial ports.

[ljaeggi]

Two questions follow on from your reply, 

Assuming this is a hardware issue, is there a safe list or know unsupported hardware list somewhere? My searching the forums have not appeared to have found anything

Does CVI use different different method to read serial devices when using VISA? If this is the case, can LabVIEW access that method some how? 

Thanks for taking the time to reply. The net connection where I am can be a bit iffy! 

[Dennis_Knutson]

A CVI application may not use VISA. There is a lower level serial function available. In my experience, it is seldom that VISA is the problem. It is usually a problem with the driver for the serial port. You have not said what driver is being used and that would be useful information.