I/O Error using RS485 to USB

[smk_praxis]

Hardware: NI USB-485/2

      Configured to run Transceiver: 2 wire auto

      Bias resistors disabled

Problem: I am attempting to run a loopback test between the two com ports of the USB-485/2 converter. I have settings per NI specs for a half-duplex (two-wire) wire configuration. Upon running the program (which was a downloaded example from NI) I get the following error:

"Error -1073807298 occurred at VISA Read in 485 Loopback Test 2 Wire Auto_LV80.vi"

"VISA:  (Hex 0xBFFF003E) Could not perform operation because of I/O error."

I have tried to use the "Set VISA buffer" to bypass the automatic flush every 500ms. This did not fix the problem. The odd part is, I can enable the bias resistors and the program will run. But, the program does not read anything at the port.

So I am at the point where I can either disable bias resistors and get an ERROR, or I can enable resistors and fail the loopback test. I have tested every contact point in the loopback circuit and there are no issues there.

Any ideas???

[Eric-E]

Hello,

Are you working with this example?

https://decibel.ni.com/content/docs/DOC-4169/version/3

You could post the problem on that page as well.

What version of NI-Visa do you have?

http://ae.natinst.com/public.nsf/web/searchinternal/99c343f86d399d31862572d500043582?OpenDocument

Thank you.

[smk_praxis]

I am working with the example code and circuit from DOC-4169.

I am running LabVIEW 2010 9Windows XP) on a PXI computer chassis.