136 lines
5.0 KiB
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136 lines
5.0 KiB
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===========================
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RS485 Serial Communications
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===========================
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1. Introduction
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===============
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EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the
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electrical characteristics of drivers and receivers for use in balanced
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digital multipoint systems.
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This standard is widely used for communications in industrial automation
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because it can be used effectively over long distances and in electrically
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noisy environments.
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2. Hardware-related Considerations
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==================================
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Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in
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half-duplex mode capable of automatically controlling line direction by
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toggling RTS or DTR signals. That can be used to control external
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half-duplex hardware like an RS485 transceiver or any RS232-connected
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half-duplex devices like some modems.
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For these microcontrollers, the Linux driver should be made capable of
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working in both modes, and proper ioctls (see later) should be made
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available at user-level to allow switching from one mode to the other, and
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vice versa.
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3. Data Structures Already Available in the Kernel
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==================================================
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The Linux kernel provides the struct serial_rs485 to handle RS485
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communications. This data structure is used to set and configure RS485
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parameters in the platform data and in ioctls.
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The device tree can also provide RS485 boot time parameters
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[#DT-bindings]_. The serial core fills the struct serial_rs485 from the
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values given by the device tree when the driver calls
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uart_get_rs485_mode().
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Any driver for devices capable of working both as RS232 and RS485 should
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implement the ``rs485_config`` callback and provide ``rs485_supported``
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in the ``struct uart_port``. The serial core calls ``rs485_config`` to do
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the device specific part in response to TIOCSRS485 ioctl (see below). The
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``rs485_config`` callback receives a pointer to a sanitizated struct
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serial_rs485. The struct serial_rs485 userspace provides is sanitized
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before calling ``rs485_config`` using ``rs485_supported`` that indicates
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what RS485 features the driver supports for the ``struct uart_port``.
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TIOCGRS485 ioctl can be used to read back the struct serial_rs485
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matching to the current configuration.
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.. kernel-doc:: include/uapi/linux/serial.h
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:identifiers: serial_rs485 uart_get_rs485_mode
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4. Usage from user-level
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========================
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From user-level, RS485 configuration can be get/set using the previous
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ioctls. For instance, to set RS485 you can use the following code::
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#include <linux/serial.h>
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/* Include definition for RS485 ioctls: TIOCGRS485 and TIOCSRS485 */
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#include <sys/ioctl.h>
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/* Open your specific device (e.g., /dev/mydevice): */
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int fd = open ("/dev/mydevice", O_RDWR);
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if (fd < 0) {
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/* Error handling. See errno. */
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}
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struct serial_rs485 rs485conf;
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/* Enable RS485 mode: */
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rs485conf.flags |= SER_RS485_ENABLED;
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/* Set logical level for RTS pin equal to 1 when sending: */
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rs485conf.flags |= SER_RS485_RTS_ON_SEND;
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/* or, set logical level for RTS pin equal to 0 when sending: */
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rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);
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/* Set logical level for RTS pin equal to 1 after sending: */
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rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
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/* or, set logical level for RTS pin equal to 0 after sending: */
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rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);
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/* Set rts delay before send, if needed: */
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rs485conf.delay_rts_before_send = ...;
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/* Set rts delay after send, if needed: */
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rs485conf.delay_rts_after_send = ...;
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/* Set this flag if you want to receive data even while sending data */
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rs485conf.flags |= SER_RS485_RX_DURING_TX;
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if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
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/* Error handling. See errno. */
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}
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/* Use read() and write() syscalls here... */
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/* Close the device when finished: */
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if (close (fd) < 0) {
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/* Error handling. See errno. */
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}
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5. Multipoint Addressing
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========================
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The Linux kernel provides addressing mode for multipoint RS-485 serial
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communications line. The addressing mode is enabled with
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``SER_RS485_ADDRB`` flag in struct serial_rs485. The struct serial_rs485
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has two additional flags and fields for enabling receive and destination
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addresses.
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Address mode flags:
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- ``SER_RS485_ADDRB``: Enabled addressing mode (sets also ADDRB in termios).
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- ``SER_RS485_ADDR_RECV``: Receive (filter) address enabled.
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- ``SER_RS485_ADDR_DEST``: Set destination address.
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Address fields (enabled with corresponding ``SER_RS485_ADDR_*`` flag):
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- ``addr_recv``: Receive address.
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- ``addr_dest``: Destination address.
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Once a receive address is set, the communication can occur only with the
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particular device and other peers are filtered out. It is left up to the
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receiver side to enforce the filtering. Receive address will be cleared
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if ``SER_RS485_ADDR_RECV`` is not set.
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Note: not all devices supporting RS485 support multipoint addressing.
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6. References
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=============
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.. [#DT-bindings] Documentation/devicetree/bindings/serial/rs485.txt
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