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TIDA-00540: Common RS232 / RS485 design and internal PULL-DOWN resistors

Mon2
Prodigy 220 points
Part Number: TIDA-00540

Hi. Working on a common pinout RS232 / RS422 / RS485 design where the respective transceivers will be disabled as required. Came across TIDA-00540 and concerned if this will function as noted due to the internal 5k pull-down resistors that are present on each of the RS232 inputs. Or do these internal 5k pull-down resistors also get disabled when the RS232 transceiver is disabled / powered down?

On this same note, do all TI RS232 transceivers disable this 5k internal pull-down during power down / disabled mode?

Concern is that the 5k pull-down resistors present inside the RS232 transceiver will affect the signal integrity / biasing of the shared RS422/RS485 transceivers.

Please advise.

Thanks and happy holidays!!

  • 0
    TI__Guru 62920 points
    Hi Kumar,

    The concern is valid - this is one of the challenges associated with combining RS-232 and RS-485 interfaces over a common line. The 5-kOhm pull-down resistance is always present on the RS-232 DIN pins (at least speaking for the current TI portfolio), so it can present a load on the RS-485 bus. In most case this can be tolerated on the RS-485 bus since the driver circuits are generally designed to work with buses including low-impedance termination resistances. If the bus has a large number of nodes or uses receivers with low input impedance already, though, then the additional loading could present issues.

    One other thing to consider is that many RS-485 buses uses biasing resistances on the A/B signal lines to establish a differential high level as a default idle state (i.e., one that is present even when all driver circuits are disabled). The effects of the pull-down loading of the RS-232 transceiver on the idle-state bias voltage should be considered. Usual solutions are to ensure that any external biasing resistances are low enough in value to compensate for the 5-kOhm pull-down or to use a connector pinout that only multiplexes RS-232 "DIN" ports on the inverting "B" input of an RS-485 transceiver.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 220 points
    Thank you Max for the very prompt reply. We anticipated that this dual xcvr design would be impacted by the # of nodes that can be supported on the RS485 interface. Please take these issues into consideration to offer perhaps a single chip RS232 xcvr where if the part is DISABLED, the 5k PDs are also disabled to allow for pin sharing. Then this would ease our mating to other RS485 transceiver variants. BTW - We are using a mix of your parts including the digital isolators and RS485 xcvrs with excellent results. Thanks again and happy holidays!