RS485 Serial Communications

RS-485 serial communication

이 문서는 RS-485 serial communication의 kernel·device-tree·userspace configuration interface를 설명합니다.

===========================
RS485 Serial Communications
===========================

Balanced multipoint 표준

EIA-485는 TIA/EIA-485 또는 RS-485라고도 하며 balanced digital multipoint system에 사용하는 driver와 receiver의 electrical characteristic을 정의합니다.

긴 거리와 electrical noise가 많은 환경에서도 효과적으로 사용할 수 있어 industrial automation 통신에 널리 쓰입니다.

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.

Half-duplex direction control

Atmel AT91 또는 16C950 UART 같은 일부 CPU/UART에는 RTS나 DTR signal을 toggle해 line direction을 자동 제어하는 built-in half-duplex mode가 있습니다.

이 mode는 RS-485 transceiver 같은 external half-duplex hardware나 일부 modem처럼 RS-232로 연결된 half-duplex device를 제어할 수 있습니다.

이런 microcontroller의 Linux driver는 normal mode와 half-duplex mode를 모두 지원하고 userspace가 ioctl로 두 mode를 양방향 전환할 수 있게 해야 합니다.

2. Hardware-related Considerations
==================================

   Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in
   half-duplex mode capable of automatically controlling line direction by
   toggling RTS or DTR signals. That can be used to control external
   half-duplex hardware like an RS485 transceiver or any RS232-connected
   half-duplex devices like some modems.

   For these microcontrollers, the Linux driver should be made capable of
   working in both modes, and proper ioctls (see later) should be made
   available at user-level to allow switching from one mode to the other, and
   vice versa.

serial_rs485 sanitization과 callback

Kernel은 platform data와 ioctl에서 RS-485 parameter를 설정하는 `struct serial_rs485`를 제공합니다. Device tree도 boot-time parameter를 제공할 수 있고, driver가 `uart_get_rs485_mode()`를 호출하면 serial core가 그 값으로 structure를 채웁니다.

RS-232와 RS-485를 모두 지원하는 driver는 `struct uart_port`에 `rs485_config` callback과 `rs485_supported` capability를 제공해야 합니다. Serial core는 `TIOCSRS485`에 응답해 device-specific 설정을 수행할 때 callback을 호출합니다.

Userspace가 제공한 `struct serial_rs485`는 callback 전에 `rs485_supported`가 나타내는 지원 feature로 sanitize됩니다. `TIOCGRS485`는 현재 configuration에 맞는 structure를 읽는 데 사용합니다.

Kernel-doc은 `include/uapi/linux/serial.h`의 `serial_rs485`와 `uart_get_rs485_mode` identifier를 가져옵니다.

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
   [#DT-bindings]_. 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.

.. kernel-doc:: include/uapi/linux/serial.h
   :identifiers: serial_rs485 uart_get_rs485_mode

Userspace RS-485 설정 예제

Userspace 예제는 target device를 열고 `struct serial_rs485`에 RS-485 enable, 송신 중·송신 후 RTS level, 송신 전·후 delay와 송신 중 수신 허용 여부를 설정합니다.

`SER_RS485_ENABLED`가 mode를 켜고 `SER_RS485_RTS_ON_SEND`와 `SER_RS485_RTS_AFTER_SEND`는 각 단계의 RTS logical level을 정합니다. `delay_rts_before_send`·`delay_rts_after_send`는 direction 전환 timing을, `SER_RS485_RX_DURING_TX`는 송신 중 수신을 허용합니다.

완성한 configuration은 `TIOCSRS485` ioctl로 적용하고 이후 `read()`·`write()`로 통신한 뒤 device를 닫습니다.

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. */
        }

Multipoint addressing과 filtering

Kernel은 multipoint RS-485 line을 위한 addressing mode를 제공합니다. `struct serial_rs485`의 `SER_RS485_ADDRB`로 mode를 켜며 receive address와 destination address를 위한 flag와 field가 추가로 있습니다.

`SER_RS485_ADDR_RECV`는 receive filtering address를, `SER_RS485_ADDR_DEST`는 destination address를 enable합니다. 대응 field는 `addr_recv`와 `addr_dest`입니다.

Receive address를 설정하면 지정 device와만 통신하고 다른 peer는 filtering됩니다. Filtering 시행은 receiver 측 책임이며 `SER_RS485_ADDR_RECV`가 해제되면 receive address도 지워집니다.

RS-485를 지원하는 모든 device가 multipoint addressing까지 지원하는 것은 아닙니다.

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.

RS-485 device-tree binding

RS-485 boot-time parameter의 device-tree binding reference는 `Documentation/devicetree/bindings/serial/rs485.txt`입니다.

6. References
=============

.. [#DT-bindings]        Documentation/devicetree/bindings/serial/rs485.txt