I am using the NI USB-485/2 box with CVI and was successful in configuring the two 485 ports in 4-wire mode, connecting the ports for loopback testing and tested by sending data out one port and receiving the same data in the second. Now I want to change the configuration to 2-wire because I need to interface to equipment using 2-wire so I have read about all the documentation I can find and still have confustion about 2-wire port configuration, transceiver control and loopback connections. I see three 2-wire port configuration options 1) DTR controlled with echo, 2) DTR controlled without echo, 3) Auto TXRDY controlled. I see the "RS-485 Transceiver Control" table in the manual and Quick Reference but do not understand what it is trying to tell me at all. I have studied it for a long time. There is no explaination of these three options. So my questions is what are the difference in these three and for each of these options how does the port need to be configured (any handshaking enabled?) and how would I wire the 2 ports together to run a loopback test?
I have setup port configurations through VISA attributes for 2-wire Auto with baud, num bits, stop bit, etc, and it seems to be returning successful responses. (I used VISA only because I did not find the ComSetEscape call before I completed the above). I connected portA TX- to portB TX- and portA TX+ to portB TX+ for loopback testing and could not get it to work in the short time I tested. It looks like the data transmits but never gets received. I am using the VISA write and read functions.
