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I know I should have a terminating resistor on RS 485.

Do I need biasing resistors on the +/- lines of an RS485 differential signal?

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  • \$\begingroup\$ So is the moral of the story here don't worry about it if the receiver has a fail-safe built into the IC? \$\endgroup\$
    – Pugz
    Commented Apr 19, 2014 at 23:45
  • \$\begingroup\$ AFAIK external biasing may be required, even if the transceiver integrates fail safe circuitry. It depends on the number of clients and number of termination resistors (depending on net topology), because these factors influence the bus voltage. It has been a while that I read papers about that, but there are pretty thorough application notes from the transceiver manufacturers out there. \$\endgroup\$
    – Rev
    Commented Apr 20, 2014 at 12:58
  • \$\begingroup\$ There should only ever be 2 termination resistors, one at each end of the bus. You should have, at most, 1 set of bias resistors. If you have more, it will load the bus unnecessarily and degrade your signal levels. Pay attention to the chip makers' app notes. \$\endgroup\$ Commented Apr 21, 2014 at 19:43

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The biasing resistors are meant to put the differential bus voltage into a defined state, even when the bus is not actively driven or is in a fail state.

In RS485 application notes, the fail states are described as something like

  • open circuit
  • short circuit
  • idle bus (well, this is not really a fail state)

However, most RS485 transceivers already have built in functionality to address this. For example one of the features in the data sheet from the SN65HVD485E from TI states "Bus-Open-Failsafe Receiver".

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  • \$\begingroup\$ How come it is not recommended in Controller Area network? \$\endgroup\$
    – warpi
    Commented Dec 13, 2023 at 9:38
  • \$\begingroup\$ @warpi I do not know, what you are referring to and why this would be recommended there. \$\endgroup\$
    – Rev
    Commented Dec 13, 2023 at 13:47
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If you need to have a guaranteed level when no transmitter is enabled then you need biasing resistors (assuming a receiver that does not have a built-in offset, as some Maxim parts have).

Not having bias resistors means that you could have false received data.

Typically you'd have one resistor from A to +5 and another resistor from B to GND and a third resistor from A to B. The values are chosen so that the voltage level such that the logic level is defined (200mV or greater) and the desired differential termination resistance is presented.

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Not normally. Maybe you've seen circuits that do put biasing resistors on? Were the signals somehow ac coupled either via a transformer or maybe capacitors? If so then bias resistors will be needed.

I've seen terminator resistors split into two resistors, one for each wire and these might give the impression they are biasing resistors but more likely they are trying to terminate the normal signal AND provide common mode termination.

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