Subject: [PATCH] Documentation about RS485 serial communications

From: [email-masked] (Grant Edwards)
Date: Tue, 19 Oct 2010 17:22:32 +0000 (UTC)

On 2010-10-19, Alexander Stein <[email-masked]> wrote:
> On Tuesday 19 October 2010, 16:29:48 Grant Edwards wrote:
>> On 2010-10-19, Claudio Scordino <[email-masked]> wrote:
>> > +2. HARDWARE-RELATED CONSIDERATIONS
>> > +
>> > +   Some CPUs (e.g., Atmel AT91) contain a transceiver capable of working
>> > both +   as RS232 and RS485.
>> 
>> Pardon my curiosity, but I've looked at a few AT91 parts, and I've
>> never seen on with an internal transceiver -- nor does Google seem
>> able to find any.
>> 
>> Frankly, I'd be pretty surprised, since it would be very difficult
>> (fab-wise), to build a mircoprocessor with I/O cells capable of
>> withstanding the voltage levels specified in RS232.
>> 
>> Can you specify to which parts you're referring?
>
> I doubt the AT91 do have an internal transceiver but it has a
> dedicated pin feature (RTS or CTS or something) which can be used to
> switch between RS232 and RS485 by setting a bit in the USART provided
> the pin is connected properly.

Yes, the AT91 UARTs do have a built-in half-duplex mode that will
automatically control line direction by toggling RTS.

That can used to control external half-duplex hardware like an RS485
transceiver or any RS232-connected half-duplex device like some
modems.

Though the Atmel docs refer to it as "RS485 mode" there is no RS485
transceiver in the AT91 parts (AFAIK), and "RS485 mode" isn't specific
to RS485 - it can be used with any external half-duplex hardware.

-- 
Grant Edwards               grant.b.edwards        Yow! I Know A Joke!!
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