On PXI-8433 how do I get access to the isolated RS485 common? Is it the GND

[Chris55]

I am not seeing any discussion in NI documentation on how to treat the isolated RS485 common from a PXI-8433.

I would like to join it to the common of my isolated target to provide a proper data signal return path.

This NI card has a 10 position RJ connector as seen here http://www.ni.com/pdf/manuals/371253c.pdf

Could it be that pin 10 labeled as "GND" is actually the isolated common of this cards RS485 port?

My production group is having intermittent data error problems with my target and I believe this is part of the problem.

My product which provides for the common talks to itself without issue and is very robust to EMI.

This data return topic is often ignored and can be the cause of communication errors.

As reference on this return path topic check out this application note http://www.robustdc.com/library/an005.pdf

Many devices use the shield on an isolated RS485 system as the return path which is a compromise between cable costs and EMI.

A few devices actually have a separate common connection (check out Phoenix Contact RS232/485 converter for example) and then use a second twisted pair cable to satisfy this common.

This four terminal interface (data A, data B, common and shield) gives the best EMI performance at the expense of the cable.

Biasing is another issue but I see from the documentation that the PXI-8433 has socketed bias resistors.

Are these normally shipped engaged?

Thanks

Chris

[John_M.]

Hi Chris,

Your expectation is correct, and the GND pin (pin 10) is the isolated common for each RS485 port. Also, according to the KnowledgeBase article Do NI Serial boards come with Termination and Biasing Resistors Built-in?, the bias resistors are shipped engaged for the PXI-8433.

Best Regards,

John M

[K C]

Hi Chris,

Yes, a RS-485 should have a ground connection And yes most RS-485 that I see do not have a ground connection.

This can be a problem especially when you have an isolated port.

I always say; ‘use that ground wire otherwise you mat get intermittent problems’. The argument not to use a ground wire is mostly: ‘we only have 1 or 2 pair cable’

Also seen is that the shield is use as a ground connection. There are situations that are better of without a ground connection and connect the shield were it must be connected: to earth.

However, when you connect to ground line it can happen that there flows a lot of current through this ground line depending on the devices connected.

The ground connection is important to get the A and B signals within the common mode range of the RS-485 transceivers.

Another very important issue are the terminators. The least you should have is a terminator at each end of the line. Value 100-120 Ohm.

Even better is a fail-safe terminator at one side and a single terminator at the other side.

 Kees

[DavidDyck]

Looks like the link to
  https://www.ni.com/pdf/manuals/371253c.pdf
is broken.  I could not find where it moved to