I have been unable to get the NI USB485 cable to communicate with my instuments. Could someone provide the required pin connections to go from the NI-USB485 DB9 connector to a 2 wire RS485 instument?
Looks like the link to
is broken. I could not find where it moved to 😞
All the RS-485 devices I have ever used were connected A to A and B to B (2 wire) also A' to A' and B' to B' (4 wire) on the RS-485 side of the interface adaptor.
As alluded to by @RTSLVU, you must wire the TxD and RxD pairs yourself when using an NI 485 interface in two-wire mode. Setting the mode in the VISA property sets up the driver to automatically gate the transmitter on and off, and also suppresses readback of one's own transmitted data, but it doesn't internally bridge the connections for you. So bridge Tx+ and RX+, and let's call that "plus", and bridge Tx- and Rx-, and let's call that "minus"; now wire "plus" and "minus" to whatever the far-end device calls its pin pair... plus/minus, or A/B, or blue and yellow (I'm joking now).
And if it doesn't work, and you're sure about character formatting and baud rate, swap polarity and try once more. In my experience, polarity of 2-wire RS-485 is pretty arbitrary with some device manufacturers.
All good suggestion to troubleshoot your RS-485 connection.
If it is a short connection the cable type, network layout and Terminators are not that important. But with longer connections this could cause problems.
Two other items from my practice:
Some manufacturers swap the RS-485 connections (One told me 'just like RS-232' !?!?) Saw naming like A- and B+
Try to swap the two data lines.
If the connection share the same 0V level than you don't need a 0V connection. But if the devices have galvanic isolation it could be that you need the 0V connection. btw. I always recommend to add the 0V connection.
Definitely make sure to have a (protected) ground connection. And in industrial environments you should check the ground potential of both sides in respect to earth. There can sometimes be huge differences that when connected together can cause significant currents to flow. With a ground connection, the worst thing that can happen is your cable goes crazy. Without, your RS-422/485 transceivers will go into smoke and if you are unlucky much more than that. When the ground potential is not the same and/or there is significant chance for external influences like lightning you should really consider a galvanic isolator between the two sides (and then you really need to have a ground connection from both sides to the isolator.