Le 19/10/2010 19:22, Grant Edwards :

On 2010-10-19, Alexander Stein [off-list ref] wrote:

quoted

On Tuesday 19 October 2010, 16:29:48 Grant Edwards wrote:

quoted

On 2010-10-19, Claudio Scordino [off-list ref] wrote:

quoted

+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.

Yes, that is a vocabulary misuse to talk about "transceiver" but the
idea of Claudio's documentation is that the USART can work in both full
and half duplex and then can comply with RS-232 and RS-485 modes like
you describe precisely in your post.

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.

Yes, for sure you will have to use an external transceiver.

Thanks for clarifications.

Best regards,
-- 
Nicolas Ferre