RS-485

Informations and HOWTOs for ArmadeuS RS-485 support.

RS-485 mostly used in half duplex operation mode, and uses a differential balanced line over twisted pair. RS-485 drivers need to be put in transmit mode explicitly by asserting a signal to the driver. This allows RS-485 to implement linear topologies using only two wires, and it can span relatively large distances (up to 1,200m @ 100kbit/s).

Before the RS-485 related parts, please have a look on: Serial ports usage on Linux

Author: Users:JanosA

Supported Board and Software versions

• Use the latest Armadeus GIT/SF repository or the latest stable release.
• Currently tested only on the APF28 (i.MX28) board with Linux kernel 2.6.35 (patch 462) and 4.9 (patch 703). If device tree is used, you need to activate rts/cts on UART used for RS-485.
• Busybox stty command requires a patch included in Armadeus BSP, to handle specifics RS-485 parameters handling.

Changing port parameters

• Setup APF28 serial port for RS-485 communication:

# export SERIAL_DEV=/dev/ttySP0
# stty -F $SERIAL_DEV 9600
# stty -F $SERIAL_DEV raw
# stty -F $SERIAL_DEV -echo -echoe -echok
# stty -F $SERIAL_DEV rs485

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

# stty -F $SERIAL_DEV rs485delaybefore 100
# stty -F $SERIAL_DEV rs485delayafter 100

• To get current RS-485 parameters: (if the serial port supports the RS-485 options, you will see previous rs485 parameters values)

# 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/i.MX28 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 RS-485 mode is turned on, it will override (turn off) the RTS/CTS hardware flow settings of the UART, whatever is configured.

How to use it from C code

• The RS-485 mode can also 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 RS-485 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 RS-485 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 RS-485 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 RS-485 transceiver ICs. On kernel with device tree, this can be done with rs485-rts-active-high option when declaring UART config.
• When U-Boot runs and the board start to boot, the RTS pin works as an input, so please add a pull-up resistor to this pin, before the inverter, to prevent RS-485 bus lockups.

Links

• Some C example code to use Modbus RTU through RS485: https://github.com/artemysfr/embedded-tools
• De-asserting RTS fastest as possible is a well known problem on every platforms:
  • https://blog.savoirfairelinux.com/en-ca/2013/adding-rs-485-support-to-the-beaglebone-with-short-turnaround-delay/