I am trying to assign a RS-485 option to the product I am designing.
However, the physical constraint restricts me to use only 2 terminal block ports. So, I am forced to use two wire RS-485 configuration with Noninverting line and Invertingline, without Signal Ground terminal available.
I made the whole RS 485 side floating by galavanic isolation and put bias resistor as below. MCU will interface with the RS-485 chip via optical isolation.
Will above configuration work as well as one with signal ground connected over long distance or over fast baud rate?
Best Answer
The resitors of 47k are big for this purpose, other literature suggests 390 ohm and 220 ohm terminating resitor between A and B. Since the RS-485 cable has characteristics impedance of 120 ohm, those termination resitors of 120 ohm at each end of transission line are adequate.
I suggest you to look some schematics around Profibus, it's a rs-485 industrial network. You will find many ciruit diagrams and correct termination and bias. Also the speed is reducing with the cable length and can't be more than 1200m @ 9.6kbps.
http://i.stack.imgur.com/r94L4.png
The cable is shileded twisted pair and you connect the floating GND to the shield.