USB-RS485 comm problem

[richb]

I had an application running that queried the set point and PV on an Omega temperature controller using Modbus and the NI USB-RS485 controller.  Now I can send a command to the Omega but cannot get a proper response back.  If I turn the Omega off or on all I get back in a read is a copy of what I sent.  Since I get the echo back even when the Omega is off I have to assume that the USB-RS485 is doing it.

Any ideas?

Rich

[pBerg]

Howdy richb,

The first thing I'd check is the Transceiver mode for your USB-485 interface. KnowledgeBase 2N8IN3YF: How Do I Set the Serial RS-485 Transceiver (Wire) Modes Programmatically Wit... explains how. It sounds like you have 2-Wire DTR Controlled With Echo selected.

Curious, too--do you still see an echo if you physically disconnect your Omega temp controller?

[richb]

pBerg

The USB-485 can't be set programatically but I checked in Max and it is set to no flow control.  I get the same echo back if the Omega is off so it must be from the 485 interface, because of the way NI made the device I had to screw it in from the inside of my box so it isn't easy to disconnect.

RichB

[pBerg]

Howdy richb,

How do you communicate with the Omega instrument? Do you use Hyperterminal, VISA Interactive Control, LabVIEW, or another means?

In any case, physically disconnect the Omega instrument from the USB-485 interface. Then test communication and let us know if you still receive an echo.

[richb]

Okay, disconnecting causes an error, I assume that rs-485 self powers.  I am using LabVIEW to talk to it and it did work at one point.

Rich

[Wayne.C]

Rich,

What version of NI-VISA and NI-Serial are you running?  Later versions of NI-Serial support programatically configuring the wire mode on a USB-485.  I'm running NI-VISA 4.1 and NI-Serial 3.1.0.

[richb]

I ahve Visa 4.1 and serial 3.2.  I got the 'did not work' from NI's site.  In my software I always set wire mode to '2 wire/Auto.

Rich

[Wayne.C]

Was this from the same doc that pBerg linked to in this thread?

[richb]

yup

[JasonSmith]

That document is out of date, programmatic control of USB-485 wire modes has been supported for quite some time. I have requested that we update the Knowledgebase.

If you are getting the echo back it certainly sounds like you are configured for 2-Wire DTR With echo. Did you confirm that this is set properly through the Advanced tab in MAX or the device manager?

-Jason S.


Message Edited by JasonS on 06-03-2008 05:16 PM