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.

3. Data Structures Already Available in the Kernel

The Linux kernel provides the struct serial_rs485 to handle RS485 communications. This data structure is used to set and configure RS485 parameters in the platform data and in ioctls.

The device tree can also provide RS485 boot time parameters [1]. The serial core fills the struct serial_rs485 from the values given by the device tree when the driver calls uart_get_rs485_mode().

Any driver for devices capable of working both as RS232 and RS485 should implement the rs485_config callback and provide rs485_supported in the struct uart_port. The serial core calls rs485_config to do the device specific part in response to TIOCSRS485 ioctl (see below). The rs485_config callback receives a pointer to a sanitizated struct serial_rs485. The struct serial_rs485 userspace provides is sanitized before calling rs485_config using rs485_supported that indicates what RS485 features the driver supports for the struct uart_port. TIOCGRS485 ioctl can be used to read back the struct serial_rs485 matching to the current configuration.

struct serial_rs485

serial interface for controlling RS485 settings.


struct serial_rs485 {
    __u32 flags;
#define SER_RS485_ENABLED               _BITUL(0);
#define SER_RS485_RTS_ON_SEND           _BITUL(1);
#define SER_RS485_RTS_AFTER_SEND        _BITUL(2);
#define SER_RS485_RX_DURING_TX          _BITUL(4);
#define SER_RS485_TERMINATE_BUS         _BITUL(5);
#define SER_RS485_ADDRB                 _BITUL(6);
#define SER_RS485_ADDR_RECV             _BITUL(7);
#define SER_RS485_ADDR_DEST             _BITUL(8);
#define SER_RS485_MODE_RS422            _BITUL(9);
    __u32 delay_rts_before_send;
    __u32 delay_rts_after_send;
    union {
        __u32 padding[5];
        struct {
            __u8 addr_recv;
            __u8 addr_dest;
            __u8 padding0[2];
            __u32 padding1[4];



RS485 feature flags.


Delay before send (milliseconds).


Delay after send (milliseconds).




Deprecated, use padding0 and padding1 instead. Do not use with addr_recv and addr_dest (due to overlap).




Receive filter for RS485 addressing mode (used only when SER_RS485_ADDR_RECV is set).


Destination address for RS485 addressing mode (used only when SER_RS485_ADDR_DEST is set).


Padding (set to zero).


Padding (set to zero).


Serial interface for controlling RS485 settings on chips with suitable support. Set with TIOCSRS485 and get with TIOCGRS485 if supported by your platform. The set function returns the new state, with any unsupported bits reverted appropriately.

The flag bits are:

  • SER_RS485_ENABLED - RS485 enabled.

  • SER_RS485_RTS_ON_SEND - Logical level for RTS pin when sending.

  • SER_RS485_RTS_AFTER_SEND - Logical level for RTS pin after sent.

  • SER_RS485_RX_DURING_TX - Full-duplex RS485 line.

  • SER_RS485_TERMINATE_BUS - Enable bus termination (if supported).

  • SER_RS485_ADDRB - Enable RS485 addressing mode.

  • SER_RS485_ADDR_RECV - Receive address filter (enables addr_recv). Requires SER_RS485_ADDRB.

  • SER_RS485_ADDR_DEST - Destination address (enables addr_dest). Requires SER_RS485_ADDRB.

  • SER_RS485_MODE_RS422 - Enable RS422. Requires SER_RS485_ENABLED.

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>

/* Include definition for RS485 ioctls: TIOCGRS485 and TIOCSRS485 */
#include <sys/ioctl.h>

/* 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;

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

/* Set logical level for RTS pin equal to 1 when sending: */
rs485conf.flags |= SER_RS485_RTS_ON_SEND;
/* or, set logical level for RTS pin equal to 0 when sending: */
rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);

/* Set logical level for RTS pin equal to 1 after sending: */
rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
/* or, set logical level for RTS pin equal to 0 after sending: */
rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);

/* Set rts delay before send, if needed: */
rs485conf.delay_rts_before_send = ...;

/* Set rts delay after send, if needed: */
rs485conf.delay_rts_after_send = ...;

/* Set this flag if you want to receive data even while sending data */
rs485conf.flags |= SER_RS485_RX_DURING_TX;

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. Multipoint Addressing

The Linux kernel provides addressing mode for multipoint RS-485 serial communications line. The addressing mode is enabled with SER_RS485_ADDRB flag in struct serial_rs485. The struct serial_rs485 has two additional flags and fields for enabling receive and destination addresses.

Address mode flags:
  • SER_RS485_ADDRB: Enabled addressing mode (sets also ADDRB in termios).

  • SER_RS485_ADDR_RECV: Receive (filter) address enabled.

  • SER_RS485_ADDR_DEST: Set destination address.

Address fields (enabled with corresponding SER_RS485_ADDR_* flag):
  • addr_recv: Receive address.

  • addr_dest: Destination address.

Once a receive address is set, the communication can occur only with the particular device and other peers are filtered out. It is left up to the receiver side to enforce the filtering. Receive address will be cleared if SER_RS485_ADDR_RECV is not set.

Note: not all devices supporting RS485 support multipoint addressing.

6. References