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. enter image description here

Will above configuration work as well as one with signal ground connected over long distance or over fast baud rate?

  • \$\begingroup\$ I think you'd be better off using a small isolation transformer on the output rather than isolating the MAX485 chip. Take a look how it's done in Ethernet transceivers. \$\endgroup\$ – Alexxx Aug 3 '15 at 11:24
  • \$\begingroup\$ What do you mean by "safe"? Safe for equipment or safe for people to touch? .... both apply here but safe for people to touch is often defined by the safety regulation you are working with.... \$\endgroup\$ – Spoon Aug 3 '15 at 12:15
  • \$\begingroup\$ I like your solution but you may be able to get away with just a terminating resistor unless you need to have a bias on the pair in the case of no transmitters on the line. Galvanicly isolated differential signalling is actually a pretty robust system that can go a few kilometres at modest speeds (9600 bps). Good cable is required and a overall screen connected at the other end will help with some electric interference. \$\endgroup\$ – KalleMP Aug 3 '15 at 21:28

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.


The cable is shileded twisted pair and you connect the floating GND to the shield.

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