Re: Enabling auto RS485 half-duplex control

[Matthias Fuchs <mfuchs@ma-fu.de>]

Hi Ted,

you missunderstood me. This special UART does all the required stuff
in hardware. You only have to enable this mode once by writing some
magic to a UART register. From that time on the UART will automatically
assert the RTS# line when transmitting.

I am not talking about a software implementation. Your arguments
deal with a software implementation where you have to do it all by hand,
which will not work safely as you mentioned.

Matthias


> On Wed, Dec 17, 2008 at 10:24:13PM +0100, Matthias Fuchs wrote:
> > Hi,
> >
> > some Exar UARTs like the XR16C2850 support a feature
> > called auto RS485 half-duplex control. In this mode the
> > RTS line has a special role when using the UART for
> > RS485 communication.
>
> So I haven't maintained the serial driver for a long time, but when I
> did, I refused to try to implement hardware half-duplex control
> because there was absolutely no standards in this space.  Which RS-232
> lines are involved wasn't standardized, and there was often very
> strange timing restrictions as well.  Some devices required there to
> be a minimum delay of XX milliseconds between when RTS is raised
> and when DTR is raised.  Other devices have a maximum time between
> when one side raises RTS and the others side raises DTR.
>
> So the challenge is designing an interface for half-duplex which is
> sufficiently general that it will actually work for all/most of the
> half-dozen people in the world who still care about half-ruplex RS-232
> communications.  :-)
>
> 						- Ted
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