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Message-ID: <20100811100212.GA4070@frolo.macqel>
Date: Wed, 11 Aug 2010 12:02:12 +0200
From: Philippe De Muyter <phdm@...qel.be>
To: Claudio Scordino <claudio@...dence.eu.com>
Cc: Wolfram Sang <w.sang@...gutronix.de>,
Linux Kernel <linux-kernel@...r.kernel.org>,
Nicolas Ferre <nicolas.ferre@...el.com>,
Alan Cox <alan@...rguk.ukuu.org.uk>, hskinnemoen@...el.com,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
Rick Bronson <rick@....org>, John Nicholls <john@...nlinx.com>,
Sebastian Heutling <Sebastian.Heutling@...-ing.de>,
Ryan Mallon <ryan@...ewatersys.com>, rmk@....linux.org.uk,
Bernhard Roth <br@...net.de>,
Konrad Mattheis <mattheis@...a.de>,
Elektrolot <elektrolot@...pl>,
Prchal Jiří <jiri.prchal@...ignal.cz>,
gregkh@...e.de, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Documentation about RS485 serial communications
On Wed, Aug 11, 2010 at 11:26:23AM +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.
>
> Here is the document (updated with the very last changes in the
> interface).
Thanks
>
> Best regards,
>
> Claudio
>
>
> Documentation about RS485 serial communications.
>
> Signed-off-by: Claudio Scordino <claudio@...dence.eu.com>
> ---
> Documentation/serial/00-INDEX | 2 +
> Documentation/serial/serial-rs485.txt | 123 +++++++++++++++++++++++++++++++++
> 2 files changed, 125 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..f594831
> --- /dev/null
> +++ b/Documentation/serial/serial-rs485.txt
> @@ -0,0 +1,123 @@
> + 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
> + able 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
> + viceversa.
> +
> + 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 <linux/serial.h>
> +
> + /* Driver-specific ioctls: */
> + #define TIOCGRS485 0x542E
> + #define TIOCSRS485 0x542F
Should those defines not come from a linux header file ?
> +
> + /* Open specific device: */
> + 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);
I surmise that all the real work, the read() and write() system calls,
must come here, before the close() system call.
> +
> + close (fd);
> +
> +5. REFERENCES
> +
> + [1] http://en.wikipedia.org/wiki/Rs485
> + [2] include/linux/serial.h
> --
> 1.6.0.4
>
--
Philippe De Muyter phdm at macqel dot be Tel +32 27029044
Macq Electronique SA rue de l'Aeronef 2 B-1140 Bruxelles Fax +32 27029077
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