Messages in this thread Patch in this message | | | Date | Fri, 28 May 2010 17:05:26 +0200 | From | Claudio Scordino <> | Subject | Re: [PATCH] atmel_serial: Atmel RS485 support v2 |
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Wolfram Sang ha scritto: > Hi Claudio, > > kudos for getting a RS485-implementation into the kernel! Very useful.
Many thanks!
I hope other people will find it useful too.
> >>> the latest version of the patch has been already merged in the >>> latest Linus' git tree through Russell King's patchsystem > > Nitpick: Wasn't there some documentation promised after the final patch ;)
Here is the first attempt.
Best regards,
Claudio
Documentation about RS485
Signed-off-by: Claudio Scordino <claudio@evidence.eu.com> --- Documentation/serial/00-INDEX | 2 + Documentation/serial/serial-rs485.txt | 118 +++++++++++++++++++++++++++++++++ 2 files changed, 120 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..dac98c3 --- /dev/null +++ b/Documentation/serial/serial-rs485.txt @@ -0,0 +1,118 @@ + 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 (inside + include/linux/serial.h) 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 interfaces 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 + + /* Open specific device: */ + int fd = open ("/dev/mydevice", O_RDWR); + struct serial_rs485 rs485conf; + + /* Set RS485 mode: */ + rs485conf.flags |= SER_RS485_ENABLED; + + /* Set delay: */ + rs485conf.delay_rts_before_send = 0x00000004; + ioctl (fd, TIOCSRS485, &rs485conf); + + close (fd); + +5. REFERENCES + + [1] http://en.wikipedia.org/wiki/Rs485 + + -- 1.6.0.4
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