[PATCH] Documentation about RS485 serial communications

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Sun, Aug 15, 2010 at 03:02:57PM -0700, Randy Dunlap wrote:
> On 08/14/10 05:50, Claudio Scordino wrote:
> > 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.

There are devices on the market which are fully isolating RS485
transceivers which just take the A/B connections, measuring the
differential voltage, and provide you with the TXD/RXD TTL signals
for your UART.

Also note that [1] appears a little confused about the number of pins
required for RS485 - it says two, and then lists three pins.  I don't
think you can use this as being supportive of the requirement for three
connections - and as there are these fully isolating transceivers...

> > +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);
> > +	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);

Example code really should do things right, such as check for error
conditions - otherwise people will copy'n'paste this and assume that
the ioctl never fails.

> > +
> > +	/* 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