Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751335Ab0HOWVm (ORCPT ); Sun, 15 Aug 2010 18:21:42 -0400 Received: from caramon.arm.linux.org.uk ([78.32.30.218]:59971 "EHLO caramon.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751169Ab0HOWVl (ORCPT ); Sun, 15 Aug 2010 18:21:41 -0400 Date: Sun, 15 Aug 2010 23:19:06 +0100 From: Russell King - ARM Linux To: Randy Dunlap Cc: Claudio Scordino , Philippe De Muyter , Wolfram Sang , Linux Kernel , Nicolas Ferre , Alan Cox , hskinnemoen@atmel.com, linux-arm-kernel , Rick Bronson , John Nicholls , Sebastian Heutling , Ryan Mallon , Bernhard Roth , Konrad Mattheis , Elektrolot , Prchal =?utf-8?B?SmnFmcOt?= , gregkh@suse.de, Andrew Morton Subject: Re: [PATCH] Documentation about RS485 serial communications Message-ID: <20100815221906.GA24153@n2100.arm.linux.org.uk> References: <20100408111310.524f7354@lxorguk.ukuu.org.uk> <4BBDD71D.1010700@evidence.eu.com> <4BFD1FB6.8050405@atmel.com> <4BFE2F38.8050200@evidence.eu.com> <4BFE41FD.2080804@atmel.com> <20100527102917.GB31253@pengutronix.de> <4C626CBF.9020609@evidence.eu.com> <20100811083214.21d2919e.randy.dunlap@oracle.com> <4C669120.8080707@evidence.eu.com> <4C686411.3060500@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4C686411.3060500@oracle.com> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3254 Lines: 82 On Sun, Aug 15, 2010 at 03:02:57PM -0700, Randy Dunlap wrote: > On 08/14/10 05:50, Claudio Scordino wrote: > > diff --git a/Documentation/serial/serial-rs485.txt b/Documentation/serial/serial-rs485.txt > > new file mode 100644 > > index 0000000..93b029e > > --- /dev/null > > +++ b/Documentation/serial/serial-rs485.txt > > @@ -0,0 +1,126 @@ > > + RS485 SERIAL COMMUNICATIONS > > + > > +1. INTRODUCTION > > + > > + EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the > > + electrical characteristics of drivers and receivers for use in balanced > > + digital multipoint systems. > > + This standard is widely used for communications in industrial automation > > + because it can be used effectively over long distances and in electrically > > + noisy environments. > > + Even though the data is transmitted over a 2-wire twisted pair bus, all > > + EIA-485 transceivers interpret the voltage levels of the differential > > + signals with respect to a third common voltage. Without this common > > + reference, a set of transceivers may interpret the differential signals > > + incorrectly. > > + See [1] for more information. There are devices on the market which are fully isolating RS485 transceivers which just take the A/B connections, measuring the differential voltage, and provide you with the TXD/RXD TTL signals for your UART. Also note that [1] appears a little confused about the number of pins required for RS485 - it says two, and then lists three pins. I don't think you can use this as being supportive of the requirement for three connections - and as there are these fully isolating transceivers... > > +4. USAGE FROM USER-LEVEL > > + > > + From user-level, RS485 configuration can be get/set using the previous > > + ioctls. For instance, to set RS485 you can use the following code: > > + > > + #include > > + > > + /* Driver-specific ioctls: */ > > + #define TIOCGRS485 0x542E > > + #define TIOCSRS485 0x542F > > + > > + /* Open your specific device (e.g., /dev/mydevice): */ > > + int fd = open ("/dev/mydevice", O_RDWR); > > + struct serial_rs485 rs485conf; > > + > > + /* Set RS485 mode: */ > > + rs485conf.flags |= SER_RS485_ENABLED; > > + > > + /* Set rts delay before send, if needed: */ > > + rs485conf.flags |= SER_RS485_RTS_BEFORE_SEND; > > + rs485conf.delay_rts_before_send = ...; > > + > > + /* Set rts delay after send, if needed: */ > > + rs485conf.flags |= SER_RS485_RTS_AFTER_SEND; > > + rs485conf.delay_rts_after_send = ...; > > + > > + ioctl (fd, TIOCSRS485, &rs485conf); Example code really should do things right, such as check for error conditions - otherwise people will copy'n'paste this and assume that the ioctl never fails. > > + > > + /* Use read() and write() syscalls here... */ > > + > > + /* Close the device when finished: */ > > + close (fd); > > + > > +5. REFERENCES > > + > > + [1] http://en.wikipedia.org/wiki/Rs485 > > + [2] include/linux/serial.h -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/