I am designing an RS-485 communication.

The communication interface is isolated. So, only the noninverting and inverting line will be connected outside to the cable.

I know that I have to include the signal ground for the RS-485 interface to be perfect. However, I am short on the number of terminals I can use.

Will it be OK if the interface is isolated and the noninverting and inverting line is pulled up and down with 47k Ohm resistor?

Will 47K Ohm pull-down resistor some how work level the ground level of the system to same level of the network even if the signal ground is not connected?

  • 1
    \$\begingroup\$ RS-485 is not a current loop. You must connect the "grounds" of the transceivers together - note this is the 'isolated' ground on each side. \$\endgroup\$ – brhans Nov 3 '17 at 2:45
  • 2
    \$\begingroup\$ Steve, would you mind sharing the transceiver with the scheme how you are connecting RS485 lines. since Galvanic isolation means, everything should be isolated Power, GND and RS485 lines. \$\endgroup\$ – AKR Nov 3 '17 at 3:30
  • \$\begingroup\$ @AKR The power is isolated through transformer from main power. the TX, RX, and DE lines from MCU are isolated through digital isolator (from Silicon Labs). Then the other side of digital isolator is connected to RS485 Transceiver. \$\endgroup\$ – Steve Nov 3 '17 at 5:18
  • \$\begingroup\$ @Steve When you say Power, shall I assume its both both Power line and GND line.? \$\endgroup\$ – AKR Nov 3 '17 at 7:07
  • 1
    \$\begingroup\$ Common-mode voltage will bite you without a ground connection. electronics.stackexchange.com/questions/232975/… electronics.stackexchange.com/questions/128618/… \$\endgroup\$ – brhans Nov 4 '17 at 1:54

Yes it will work. Perhaps 47k is little to high (it's a very big resistance). It depends on how many nodes you will connect. 47K is OK for each node, but then you can put additional lower resistance at the both ends.


simulate this circuit – Schematic created using CircuitLab

The Vcc and GND are floating power rails of each independent power source, the shield of the cable is connected to the earth. GNDs of nodes are not connected together as well the Vcc.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.