Re: Enabling auto RS485 half-duplex control

[Grant Edwards <grante@visi.com>]

On 2008-12-18, Theodore Tso <tytso@mit.edu> wrote:
> 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,

Yes there are.  The standard is that RTS is asserted when one
whishes to transmit (and de-asserted when one has finished
transmitting). Optionally, one waits for CTS to be asserted
before transmitting.  Isn't that the definition of RTS and CTS
that's in the standard?

> 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.

I've been doing half-duplex stuff for 30 years, and I've never
seen DTR used in half-duplex flow control.

> 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.  :-)

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