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The Linux Wacom Project manages the drivers, libraries, and documentation for configuring and running Wacom tablets under the Linux operating system. Our drivers are pre-installed in many major distributions and provide out-of-the box support for a wide variety of Wacom tablets and TabletPCs. Drivers: Phantom Device Drivers v 4.3-2. or above.To find kernel version type 'uname -r' at the terminal. For additional questions, check the online documents at the Developer Support Center or post questions to the developer forums.

1. Introduction¶

EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining theelectrical characteristics of drivers and receivers for use in balanceddigital multipoint systems.This standard is widely used for communications in industrial automationbecause it can be used effectively over long distances and in electricallynoisy environments.

2. Hardware-related Considerations¶

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

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

3. Data Structures Already Available in the Kernel¶

The Linux kernel provides the serial_rs485 structure (see [1]) to handleRS485 communications. This data structure is used to set and configure RS485parameters in the platform data and in ioctls.

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The device tree can also provide RS485 boot time parameters (see [2]for bindings). The driver is in charge of filling this data structure fromthe values given by the device tree.

Any driver for devices capable of working both as RS232 and RS485 shouldimplement the rs485_config callback in the uart_port structure. Theserial_core calls rs485_config to do the device specific part in responseto TIOCSRS485 and TIOCGRS485 ioctls (see below). The rs485_config callbackreceives a pointer to struct serial_rs485.

4. Usage from user-level¶

From user-level, RS485 configuration can be get/set using the previousioctls. For instance, to set RS485 you can use the following code:

5. References¶

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[1] include/uapi/linux/serial.h

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[2] Documentation/devicetree/bindings/serial/rs485.txt