Hello,
I'm trying to install an additional device to a NI 9871 (4 port RS-485) module (in 2-wire configuration) to its 2nd port with no luck. The module communicates fine with one other RS-485 device over modbus in my network that is connected to the first port of the module. The device in question (a WattNode power meter - http://www.ccontrolsys.com/w/WattNode_Modbus) has 3 terminals: "A-" (which the manual states is RxD-,TxD- and "B+" (RxD+,TxD+ in manual) and a Common - the X terminal is there but isn't used.
I tried twisting together wires from pin 4 (RxD+) and 8 (TxD+) and landed on terminal A- of the device, while tying together pin 5 (RxD-) and 9 (TxD-) and landed on terminal B+ of the device, with the Common terminal to pin 1 (the pin configuration of the standard DB9 that comes with the module, theother end being RJ50). This seemed like the correct orientation but I do not get a COM light on the device. I tried reversing the wire pairs landed on A and B to no avail as well.
Some other notes:
1) The baud rate, parity, etc is all correct because I was able to get the device to an RS-485 to RS-232 convertor and communicate fine using the on board cRIO serial port.
2) It is a short run (about 10 feet) so did not use resistors at first. But then tried a 125 ohm resistor (what I had on hand) that didn't solve the problem.
3) I configured the previous (working) device under the Project Folder and have double checked that I created the Modbus I/O server in a similar fashion but I'm also not sure if I need both these Modbus I/O servers under the same library or some other intricacy here.
Thanks in advance for any responders!