8433/4 not communicating in 2-wire auto mode in MAX or labview

[djbenson23]

All,

We're using 8433/4 PXI card to communication to a micro motion 7828 density transmitter. The NI-Serial RT software installed on the PXI and laptop is 3.3.0. The transmitter requires a 2-wire 485 setup.

I was able to communicate with the transmitter in MAX using a USB serial port on my laptop. I used a serial to rs485 2-wire converter. When I attempt to communicate using the PXI 8433 card in MAX I get no response.

Do I need more up-to-date serial rt software? or is there something else I'm overlooking in my setup?

Thanks

DB

[K C]

Hi,

Just a stupid question: are you sure all parameters are set correctly ?

Can you try to setup communication between two ports of the PXI 8433 just for the test.

Did you connect terminators at each side (preferable an active terminator at one side and a passive one at the other side)

Did you connect the 0V connections with each other.

Kees 

[DavidBoyd]

A little late, but here's another question:

Did you bridge the Tx and Rx pins together in your wiring scheme?  You should be aware that setting an NI 485 port to 2-wire mode (via Windows control panel, or the VISA property value) does not do this for you.  You still need to tie Tx+ to Rx+, and Tx- to Rx-, which on the NI 9-pin Dsub connector pattern is easy to do (pins 4 and 8 bridged, pins 5 and 9 bridged; those pairs are adjacent).

(Edit: I neglected to note that you're using a PXI card version.  The jacks on it are RJ-50 style; my comment above applies to the DB9 end of the adapter cables they provide.)

Easily overlooked step, causes endless frustration.

Dave

[djbenson23]

Thanks for both your replys David and KC. Given other forum posts I've read on the subject, your questions are certainly worth checking up on and not stupid. All communication setups were the same and should have worked. We have a custom PCB that we've used extensively with this card for several years now that shorts the Tx and Rx pins and terminates the line with a 120ohm resistor, so there shouldn't be any issue on that end. As for a termination resistor on the transmitter end, I'd like to think MicroMotion would have a built in terminator. In either case I didn't think it would be an issue given that the baud rate was 9600 and the wire length is only about 100ft, so a reflection shouldn't be big enough to wreak havoc on the signals.

Fortunetly, after lots of head banging I accidently found that the issue was with MAX. The version of MAX I'm using apparently has issues with properly setting up 485 ports. I setup a loopback test for 485 2 wire and wasn't getting any communication between 2 485 ports in MAX or labview. I figured out that I needed to completely close MAX and setup the VISA port using only Labview and its VISA package; I couldn't get labview to work before because I had MAX setup the port with the VISA test panel first and then tried communicating with both platforms.

Thanks again for your responses!

Cheers,

DB

[DavidBoyd]

DB,

Can you describe, exactly, the MAX version, along with VISA, and NI Serial too? I'd love to be able to avoid this issue, as I'm sure others would. Is there a KB article or bug ID?

Edit: I see you did quote NI Serial version earlier, oops.

Thanks,
Dave

[JasonSmith]

There was an issue related to this that was fixed in NI-Serial 3.6. Here is the relevant excerpt from the Readme:

ID 201827: Incorrect RS-485 wire mode setting could be used when opening port

Changing the RS-485 wire mode for PCI, PXI, and PCIe devices in Measurement& Automation Explorer and Device Manager had no effect. This has been fixed.

With this issue, the wire mode setting in MAX didn't do anything. The various wire modes worked fine if you set them programmatically in your application (such as with a VISA Property Node in LabVIEW), but the default setting was ignored. I don't know how long the issue had been in the driver, but it is possible that it impacts your NI-Serial 3.3 installation.

I would suggest that you download the latest version of NI-Serial (currently 14.0), and see if that resolves the issues you are seeing.

[DavidBoyd]

Jason,

Thank you for posting this. We use NI 485 interfaces (generally in 2-wire mode) and while I *think* all my installations are past this issue, version-wise, it's good to have this info just in case. And it may well save someone else's day.

Best regards,

Dave