RS485 network biasing question

User ColmR explained in other thread that RS485 specification is not dependent on the supply voltage of the tranceivers. Is that still valid if we consider a RS485 implementation with two nodes, one operating at 3V3 and other at 5V, with a biasing circuit (3 resistors scheme; pull-up tied do either 3V3 or 5V) in a given node of this network?

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    •  Analog Employees 
    on Feb 3, 2012 2:42 PM

    Excellent point!

    If you have a 4V VOD on a 5V transceiver, for example, & there's an external resistor biasing network on the other side to 3.3V, this could present a current path into the supply.  This path would need to be blocked alright.  But the external biasing presents a problem also for 3.3V transceiver to 3.3V transcevier at a higher common mode point.  e.g. 2V VOD on a 3.3V transceiver but with +5V common-mode offset (RS-485 spec allows +/-7V).  Depends on the application whether you need to care about this.

    The internal biasing schemes in the RS-485 receivers prevent the current flow into the supply for both larger VOD's and over hte CMR +12V/ /-7V typically.  So using the internal biasing of the ADM3485E (http://www.analog.com/ADM3485E) (open & floating failsafe) or ADM3078E (http://www.analog.com/ADM3078E) (short, open, floating failsafe) for  will allow driving 3.3Vreceiver with a 5V transceiver (e.g. http://www.analog.com/ADM1491E).

    The RS-485 Circuit Implementation guide has some more details on the failsafe receivers.

    http://www.analog.com/static/imported-files/application_notes/AN-960.pdf

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    •  Analog Employees 
    on Feb 3, 2012 2:42 PM

    Excellent point!

    If you have a 4V VOD on a 5V transceiver, for example, & there's an external resistor biasing network on the other side to 3.3V, this could present a current path into the supply.  This path would need to be blocked alright.  But the external biasing presents a problem also for 3.3V transceiver to 3.3V transcevier at a higher common mode point.  e.g. 2V VOD on a 3.3V transceiver but with +5V common-mode offset (RS-485 spec allows +/-7V).  Depends on the application whether you need to care about this.

    The internal biasing schemes in the RS-485 receivers prevent the current flow into the supply for both larger VOD's and over hte CMR +12V/ /-7V typically.  So using the internal biasing of the ADM3485E (http://www.analog.com/ADM3485E) (open & floating failsafe) or ADM3078E (http://www.analog.com/ADM3078E) (short, open, floating failsafe) for  will allow driving 3.3Vreceiver with a 5V transceiver (e.g. http://www.analog.com/ADM1491E).

    The RS-485 Circuit Implementation guide has some more details on the failsafe receivers.

    http://www.analog.com/static/imported-files/application_notes/AN-960.pdf

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