From mboxrd@z Thu Jan 1 00:00:00 1970 From: nicolas.ferre@atmel.com (Nicolas Ferre) Date: Wed, 20 Oct 2010 09:36:36 +0200 Subject: [PATCH] Documentation about RS485 serial communications In-Reply-To: References: <20100408111310.524f7354@lxorguk.ukuu.org.uk> <4CBD8EE4.5040808@evidence.eu.com> <201010191727.53314.alexander.stein@systec-electronic.com> Message-ID: <4CBE9C04.7000005@atmel.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Le 19/10/2010 19:22, Grant Edwards : > On 2010-10-19, Alexander Stein wrote: >> On Tuesday 19 October 2010, 16:29:48 Grant Edwards wrote: >>> On 2010-10-19, Claudio Scordino 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. 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