NI 9871 Rs485 transceiver settings

[Lightwave-Computing]

I have a compactRIO NI-9871 RS485 module that connects to a tested custom RS-485 DAQ device.  I have done a loopback test on the 9871  using scan mode and it echos characters no problem.  I have tested the custom RS-485 device using an RS-485 to USB with LabVIEW in Windows without problem. When I connect my custom RS-485 device (4 wire mode) to the 9871 in scan mode it echos garbage back.  The device is only 2 metres away and not a noisy environment so I don't think I should need termination resistors.  My question is, how do I know that the 9871 port is setup as RS-485 4 wire?  In scan mode the tranceiver mode is not shown in properties, and it is also missing from the MAX COM port settings.  I am thinking maybe the 9871 thinks it is in 2 wire or RS422 mode.  Any ideas?

Thanks,

Rob Taylor

[K C]

Hi,

I assume, that you are sure that all setting are OK.

Normally when I make a short (test) connection with RS-485 I don't use any terminators and no 0V connection, but when it does not function like it should you have to think what could be the cause.

Terminator are maybe not necessary for the short line but is can be for some transceiver. I have also seen that a fail-safe terminator was needed to get it working.

So just try a terminator or add a resistor (~120Ohm) between the B-line and 0V just to see what it does.

If the ports are isolated and/or powered from different sources you can try to connect the 0V between the two. I know many will say 'you don't need 0V with a RS-485 connection' but some IC manufacturers tell you must connect the 0V between devices.

Hope this can help a bit.

Kees