[PATCH] Documentation about RS485 serial communications

[Nicolas Ferre]

Le 24/10/2010 13:29, Claudio Scordino :

[..]

> Hi all,
> 
> 	sorry for the misuse of the term "transceiver", and thank you
> for your corrections.
> 
> Hopefully, this new version of the document is slightly better.

Hi all,

This documentation is floating around for some time now. Claudio has
enhanced it and it is maybe time to merge it in mainline...

So, can serial people include it in their tree (Greg?) or should it go
through another path?

Thanks to all of you.

Best regards,

> Many thanks,
> 
> 	Claudio	
> 
> 
> Documentation about RS485 serial communications
> 
> Signed-off-by: Claudio Scordino <claudio at evidence.eu.com>
> ---
>  Documentation/serial/00-INDEX         |    2 +
>  Documentation/serial/serial-rs485.txt |  119 +++++++++++++++++++++++++++++++++
>  2 files changed, 121 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..40f09c6
> --- /dev/null
> +++ b/Documentation/serial/serial-rs485.txt
> @@ -0,0 +1,119 @@
> +                        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.
> +
> +2. HARDWARE-RELATED CONSIDERATIONS
> +
> +   Some CPUs (e.g., Atmel AT91) contain a built-in half-duplex mode capable of
> +   automatically controlling 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.
> +
> +   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.
> +
> +3. DATA STRUCTURES ALREADY AVAILABLE IN THE KERNEL
> +
> +   The Linux kernel provides the serial_rs485 structure (see [1]) 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
> +
> +	/* Open your specific device (e.g., /dev/mydevice): */
> +	int fd = open ("/dev/mydevice", O_RDWR);
> +	if (fd < 0) {
> +		/* Error handling. See errno. */
> +	}
> +
> +	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 = ...;
> +
> +	if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
> +		/* Error handling. See errno. */
> +	}
> +
> +	/* Use read() and write() syscalls here... */
> +
> +	/* Close the device when finished: */
> +	if (close (fd) < 0) {
> +		/* Error handling. See errno. */
> +	}
> +
> +5. REFERENCES
> +
> + [1]	include/linux/serial.h


-- 
Nicolas Ferre