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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?

Steve
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    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. – brhans Nov 03 '17 at 02:45
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    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. – AKR Nov 03 '17 at 03:30
  • @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. – Steve Nov 03 '17 at 05:18
  • @Steve When you say Power, shall I assume its both both Power line and GND line.? – AKR Nov 03 '17 at 07:07
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    Common-mode voltage will bite you without a ground connection. https://electronics.stackexchange.com/questions/232975/rs485-network-ground-pin-when-to-connect/232987 https://electronics.stackexchange.com/questions/128618/ground-connection-in-differential-signals/137371 – brhans Nov 04 '17 at 01:54
  • @AKr Yes. you can assume that. Good point. – Steve Nov 06 '17 at 09:39
  • @brhans Why it could still bite you even though everything is isolated? – Unknown123 Oct 21 '19 at 09:50
  • @Unknown123 - inductive & capacitive coupling, static pick, etc can induce voltages in isolated systems. If you don't connect your isolated grounds together across your RS485 link you have no idea what the voltage difference is between them and can easily exceed the allowable common-mode voltage of your RS485 tranceivers' data lines. – brhans Oct 21 '19 at 12:26
  • @brhans I don't see such practice on [ADM2485](https://www.analog.com/media/en/technical-documentation/data-sheets/ADM2485.pdf) datasheet, are you sure? – Unknown123 Oct 22 '19 at 01:12
  • @Unknown123 - that datasheet doesn't show any examples of 2 or more RS485 drivers connected together, so it's not surprising that you don't see what I'm describing there. Try Analog's [AN-960](https://www.analog.com/media/en/technical-documentation/application-notes/AN-960.pdf) or Maxim's [Tutorial 763](https://www.maximintegrated.com/en/design/technical-documents/tutorials/7/763.html) and [others](https://www.ccontrols.com/pdf/ExtV1N1.pdf), [Fig.10 in this TI App Note](http://www.grantronics.com.au/docs/TIA-EIA-485.pdf). – brhans Oct 22 '19 at 02:07
  • ... and just so we're clear - I'm not talking about connecting GND1 and GND2 on your ADM2485. – brhans Oct 22 '19 at 02:08
  • @brhans It's the ground between transceivers that you're talking about. I misunderstood, sorry. – Unknown123 Oct 22 '19 at 08:36

1 Answers1

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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.

schematic

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.

Marko Buršič
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