Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756428Ab0HNMuw (ORCPT ); Sat, 14 Aug 2010 08:50:52 -0400 Received: from borg.asidev.net ([95.141.32.69]:51927 "EHLO borg.asidev.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756301Ab0HNMuv (ORCPT ); Sat, 14 Aug 2010 08:50:51 -0400 Message-ID: <4C669120.8080707@evidence.eu.com> Date: Sat, 14 Aug 2010 14:50:40 +0200 From: Claudio Scordino User-Agent: Thunderbird 2.0.0.23 (X11/20090817) MIME-Version: 1.0 To: Randy Dunlap , Philippe De Muyter CC: Wolfram Sang , Linux Kernel , Nicolas Ferre , Alan Cox , hskinnemoen@atmel.com, linux-arm-kernel , Rick Bronson , John Nicholls , Sebastian Heutling , Ryan Mallon , rmk@arm.linux.org.uk, Bernhard Roth , Konrad Mattheis , Elektrolot , =?UTF-8?B?UHJjaGFsIEppxZnDrQ==?= , gregkh@suse.de, Andrew Morton Subject: Re: [PATCH] Documentation about RS485 serial communications References: <4BB053C4.7050700@evidence.eu.com> <4BB10304.6090006@bluewatersys.com> <4BB1BF49.2000609@evidence.eu.com> <4BB252E3.3030703@bluewatersys.com> <4BBD8C9D.3070401@evidence.eu.com> <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> In-Reply-To: <20100811083214.21d2919e.randy.dunlap@oracle.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5923 Lines: 189 Randy Dunlap ha scritto: > On Wed, 11 Aug 2010 11:26:23 +0200 Claudio Scordino wrote: > >> Hi all, >> >> some time ago I've been asked (by both Wolfram and Philippe) to >> provide some minimal documentation about the usage of the RS485 >> interface. >> [...] > > Thanks for the addition. > Hi all, here is the document about RS485 with the requested additions. If OK, somebody please provide for merging. Best regards, Claudio Documentation about RS485 serial communications. Signed-off-by: Claudio Scordino --- Documentation/serial/00-INDEX | 2 + Documentation/serial/serial-rs485.txt | 126 +++++++++++++++++++++++++++++++++ 2 files changed, 128 insertions(+), 0 deletions(-) create mode 100644 Documentation/serial/serial-rs485.txt diff --git a/Documentation/serial/00-INDEX b/Documentation/serial/00-INDEX index 07dcdb0..e09468a 100644 --- a/Documentation/serial/00-INDEX +++ b/Documentation/serial/00-INDEX @@ -14,6 +14,8 @@ riscom8.txt - notes on using the RISCom/8 multi-port serial driver. rocket.txt - info on the Comtrol RocketPort multiport serial driver. +serial-rs485.txt + - info about RS485 structures and support in the kernel. specialix.txt - info on hardware/driver for specialix IO8+ multiport serial card. stallion.txt 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. + + +2. HARDWARE-RELATED CONSIDERATIONS + + Some CPUs (e.g., Atmel AT91) contain a transceiver capable of working both + as RS232 and RS485. For these microcontrollers, the Linux driver should be + made capable of working in both modes, and proper ioctls (see later) should + be made available at user-level to allow switching from one mode to the + other, and vice versa. + + On some other CPUs (e.g., Freescale imx25) the RS485 transceiver is not + integrated inside the microcontroller itself. Therefore, manufacturers who + use these microcontrollers to produce embedded boards need to connect an + external transceiver to some pin of the CPU. + On these architectures, therefore, no assumptions can be done at the + CPU-level about the presence of a RS485 transceiver, because the connection + (if any) is done outside the microcontroller. Moreover, even in case of + RS485 transceiver, the manufacturer is free to choose the CPU pin used for + the connection. + + +3. DATA STRUCTURES ALREADY AVAILABLE IN THE KERNEL + + The Linux kernel provides the serial_rs485 structure (see [2]) to handle + RS485 communications. This data structure is used to set and configure RS485 + parameters in the platform data and in ioctls. + + Any driver for devices capable of working both as RS232 and RS485 should + provide at least the following ioctls: + + - TIOCSRS485 (typically associated with number 0x542F). This ioctl is used + to enable/disable RS485 mode from user-space + + - TIOCGRS485 (typically associated with number 0x542E). This ioctl is used + to get RS485 mode from kernel-space (i.e., driver) to user-space. + + In other words, the serial driver should contain a code similar to the next + one: + + static struct uart_ops atmel_pops = { + /* ... */ + .ioctl = handle_ioctl, + }; + + static int handle_ioctl(struct uart_port *port, + unsigned int cmd, + unsigned long arg) + { + struct serial_rs485 rs485conf; + + switch (cmd) { + case TIOCSRS485: + if (copy_from_user(&rs485conf, + (struct serial_rs485 *) arg, + sizeof(rs485conf))) + return -EFAULT; + + /* ... */ + break; + + case TIOCGRS485: + if (copy_to_user((struct serial_rs485 *) arg, + ..., + sizeof(rs485conf))) + return -EFAULT; + /* ... */ + break; + + /* ... */ + } + } + + +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); + + /* 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 -- 1.6.0.4 -- 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/