I want to use the following MAX3485 chip: https://datasheet.lcsc.com/szlcsc/2006181334_UMW-Youtai-Semiconductor-Co-Ltd-MAX3485ESA_C668204.pdf

I do not speak any chinese, but my question is more in a general sense: does an MAX3485 chip need additional pull-up and pull-down resistors for lines A and B?

This and some other datasheets have figures about the topology where there are no pull-up and pull-down resistors. But at the end of this datasheet, and I have also seen that on the internet when I was searching for this, line A is pulled to VCC with a 10k resistor and B is pulled to ground with a 10k resistor (the 10k I have seen in the internet not in the datasheet).

What is the purpose of those resistors and when are they necessary?

  • 1
    \$\begingroup\$ Is there any reason you can't just open the english datasheet? \$\endgroup\$
    – Justme
    Mar 6, 2021 at 13:56
  • \$\begingroup\$ The datasheet is not the point, I have opened english datasheets, I also wrote that I have seen other datasheets as well :D I could have linked this: datasheets.maximintegrated.com/en/ds/MAX3483-MAX3491.pdf, still no pullup resistor in the images, the question still stands, what is the purpose of the resistors that I have seen in some circuits (maybe they were wrong and there is no purpose at all) \$\endgroup\$ Mar 6, 2021 at 14:00
  • \$\begingroup\$ Interesting driver VI curves \$\endgroup\$ Mar 6, 2021 at 14:24

1 Answer 1


The pull up/down on A/B lines are used for fail-safe bias, to passively set an idle voltage on a bus where all devices are listening and no transmitter is active.

So no, the MAX3485 itself should not need them, but the fail-safe bias can be used with any tranceiver chip if the system benefits from them.

  • \$\begingroup\$ National Semiconductor had a document 'ten ways to bulletproof RS485' Should be available from TI (who gobbled up natsemi). This will explain the purpose of termination and the bias resistors. Short answer - some systems demand bias and termination, others don't. \$\endgroup\$
    – Kartman
    Mar 6, 2021 at 14:42

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