Cannot get RS-485 2-wire auto to work

[John_M]

I'm using a PXI-8433/2, trying to use 2 wite auto mode. I have performed a 4wire loop-back on both ports and that is successful. However trying a 2 wire auto loop-back between the two ports does not work. I followed the example in the knowledge base and used the vi contained there. I also just used hyperterminal connecting the two ports and that didn't work. Looking at the signal levels during the 2 wire loop-back, they are half (~2V) of the unconnected voltage for both ports. I have also used another RS-485 to USB converter in a loop-back setup and I can transmit from the PXI to the USB adapter but I cannot receive on the PXI when the USB adapter outputs.

Could this be a hardware issue? I've updated the NI-Serial and everything else I can think of, so I should hope it's not a software issue.

Any help would be greatly appreciated!

[EEsRULEtheWORLD]

Hi John_M,

What specific KnowledgeBase are you following for the 2-wire loopback test?  Have you tried going through the process explained in this community example?  The author of this community example found that his method was more successful than the method described in this KnowledgeBase. 

[John_M]

Sara,

I did follow the first community example that you listed, including running the attached vi. 

[K C]

Hi John,

A 4-wire loop I can understand but a 2-wire loop not. You cannot do a loopback test with RS-485 with 2-wires.

Further do you have error number or message (do you use labview ?)

[John_M]

KC,

I cannot do a loopback with the two wire. I have not tried to do a two wire that is not in auto mode, I have only tried the auto mode. I do not get any error messages. I just don't get anything when I try and receive. 

I'm using labview 2009.  

[K C]

OK, that's clear.

Do you know for sure that your device is receiving the Tx data from your computer ?

[John_M]

I can communicate with the UUT using a different RS485 adapter so it isn't the UUT. Looking at the waveforms on the scope when using the NI 485 I see the voltage levels are about half of what the levels are using the adapter. 

[K C]

How long is your cable.

Normally in a test situation I don't care for the terminator resistors, but for longer lines it might be a problem.

Depending on the value of the terminate resistors and if they are active or passive you should measure a resistance of 50 - 70 ohm. (with power off)

To much termination will kill your signal. With no termination at all it should work on short (<5 mtr I guess) lines.

I assume you set the auto 2-wire mode in MAX ?

Message Edited by K C on 04-26-2010 07:24 PM

[John_M]

I do not have terminator resistors, I did try added 120ohm terminators but that made the signal levels drop too low and killed the rx that was working. I think the total length is around 2m. There is a run from the pxi chassis to the test box, then through a pxi-2527 then back to the pxi chassis. I am still using the 2527 so I'm fairly certain the problem is not there. I did set it in MAX and checked it in the windows system hardware menu.

[K C]

Terminators of 120ohms at both ends is a normal value, so that should not kill your signal.

Can you measure the resistance to be sure that it is not too low ?

Probably no problem but are both ends at the same 0V level. RS485 connection are always (99.99%) made without a ground connection. This works when the difference between the 0V levels are within the common mode range of the transceivers.

The last thing I can come up with is a defective board, but I am sure that is something you already thought of.

I will think it over...............