Messages in this thread | | | Date | Sun, 15 Aug 2010 23:19:06 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH] Documentation about RS485 serial communications |
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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
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