serial: add Documentation about RS485 serial communications

	- 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

                        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