Half-duplex RS485: simple auto-direction solution

From	pozz <pozzugno@gmail.com>
Newsgroups	comp.arch.embedded
Subject	Half-duplex RS485: simple auto-direction solution
Date	2023-03-03 17:15 +0100
Organization	A noiseless patient Spider
Message-ID	<ttt6er$j2af$1@dont-email.me> (permalink)

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I think you already know this trick: connect DE/RE signals of RS485 
half-duplex transceiver to TX *negated* signal from UART and connect TX 
input of transceiver to GND.

If the bus is not driven by anyone, A signal goes high and B signal goes 
low thanks to the pull-up/down resistors.

The trick uses the fact that the undriven bus and transmitting 1 is the 
same thing: A is high and B is low. So, when transmitting 1 the driver 
is disabled (DE=0), when transmitting 0 the driver is enabled (DE=1). 
Because TX is set to 0, A goes low and B goes high.

Of course this works in ideal and good conditions. When should I worry 
of this trick? I suspect the edges of 0 to 1 transitions are slowed down.

I don't use mega-baudrate, in this application I'm using 38400bps.

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Half-duplex RS485: simple auto-direction solution pozz <pozzugno@gmail.com> - 2023-03-03 17:15 +0100
  Re: Half-duplex RS485: simple auto-direction solution Richard Damon <Richard@Damon-Family.org> - 2023-03-03 11:26 -0500
  Re: Half-duplex RS485: simple auto-direction solution David Brown <david.brown@hesbynett.no> - 2023-03-04 12:06 +0100
    Re: Half-duplex RS485: simple auto-direction solution pozz <pozzugno@gmail.com> - 2023-03-06 10:41 +0100
      Re: Half-duplex RS485: simple auto-direction solution David Brown <david.brown@hesbynett.no> - 2023-03-06 12:48 +0100
      Re: Half-duplex RS485: simple auto-direction solution Rick C <gnuarm.deletethisbit@gmail.com> - 2023-03-06 11:23 -0800
        Re: Half-duplex RS485: simple auto-direction solution pozz <pozzugno@gmail.com> - 2023-03-06 22:20 +0100
          Re: Half-duplex RS485: simple auto-direction solution Rick C <gnuarm.deletethisbit@gmail.com> - 2023-03-06 16:47 -0800
  Re: Half-duplex RS485: simple auto-direction solution Don Y <blockedofcourse@foo.invalid> - 2023-03-06 14:23 -0700

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