Revision as of 18:23, 13 May 2013

RS485

Informations and mini HOWTOs for the APF28/iMX28 mxs-auart and busybox stty patches.

Author: JanosA

Linux stty

• Setup APF28 serial port for RS485 communication:

# stty -F /dev/ttySP0 raw
# stty -F /dev/ttySP0 -echo -echoe -echok
# stty -F /dev/ttySP0 rs485

• Set extra delays for TXEN/RTS signaling before and after the transmitted packet (in microseconds):

# stty -F /dev/ttySP0 rs485delaybefore 100
# stty -F /dev/ttySP0 rs485delayafter 100

• To get current RS485 parameters. (If the port device supports the RS485 options you will see the last two lines)

# stty -F /dev/ttySP0
speed 9600 baud; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R;
werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-brkint -imaxbel
rs485 -rs485rtsonsend -rs485rtsaftersend -rs485rxduringtx
rs485delaybefore = 100; rs485delayafter = 100; rs485delaylastchartx = 1250;

Please note:

• You can't set the "rs485delaylastchartx" parameter, it's calculated automatically from the baudrate.

Special notes for the APF28/iMX28 mxs-auart driver:

• All delays are in microseconds.
• The "rs485delaylastchartx" parameter is not used in DMA mode.
• The parameters: "rs485rtsonsend" "rs485rtsaftersend" "rs485rxduringtx" are not used. (Only used by atmel_serial.c and crisv10.c drivers)
• If the RS485 mode is turned on, it will override (turn off) the RTS/CTS hardware flow settings of the uart, don't care what is configured.

C code

• The RS485 mode can be turned on with ioctl calls from C code, here is the example:

#include <stdio.h>
#include <fcntl.h>
#include <linux/ioctl.h>
#include <linux/serial.h>
#include <asm-generic/ioctls.h> /* TIOCGRS485 + TIOCSRS485 ioctl definitions */

int main(void) {
         struct serial_rs485 rs485conf;

         int fd = open ("/dev/ttySP0", O_RDWR);
         if (fd < 0) {
                 printf("Error: Can't open: /dev/ttySP0\n");
         }


         /* Don't forget to read first the current state of the RS485 options with ioctl.
            If You don't do this, You will destroy the rs485conf.delay_rts_last_char_tx
            parameter which is automatically calculated by the driver when You opens the
            port device. */
         if (ioctl (fd, TIOCGRS485, &rs485conf) < 0) {
                 printf("Error: TIOCGRS485 ioctl not supported.\n");
         }

         /* Enable RS485 mode: */
         rs485conf.flags |= SER_RS485_ENABLED;

         /* Set rts/txen delay before send, if needed: (in microseconds) */
         rs485conf.delay_rts_before_send = 0;

         /* Set rts/txen delay after send, if needed: (in microseconds) */
         rs485conf.delay_rts_after_send = 0;

         if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
                 printf("Error: TIOCSRS485 ioctl not supported.\n");
         }

         fcntl(fd, F_SETFL, 0);
         int n = write(fd, "ABC\r\n", 5);
         if (n < 0) {
                 /* Error handling */
         }

         if (close (fd) < 0) {
                 printf("Error: Can't close: /dev/ttySP0\n");
         }
}

Hardware Notes

• The RS485 Transmit Enable (TXEN) output will be the RTS pin of the uart.
• The TXEN/RTS output is _active low_, it must be inverted to use it with the popular RS485 transceiver ICs.
• When the u-boot runs and the board start to boot, the RTS pin works as input, please add a pullup resistor to this pin before the inverter, to prevent RS485 bus lockups.