Documentation/driver-api/serial/serial-rs485.rst

Source file repositories/reference/linux-study-clean/Documentation/driver-api/serial/serial-rs485.rst

File Facts

System
Linux kernel
Corpus path
Documentation/driver-api/serial/serial-rs485.rst
Extension
.rst
Size
5142 bytes
Lines
136
Domain
Support Tooling And Documentation
Bucket
Documentation
Inferred role
Support Tooling And Documentation: documentation
Status
atlas-only

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Dependency Surface

Detected Declarations

Annotated Snippet

===========================
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.

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.

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

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. */
	}

Annotation

Implementation Notes