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Diffstat (limited to 'kernel/Documentation/serial/serial-rs485.txt')
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diff --git a/kernel/Documentation/serial/serial-rs485.txt b/kernel/Documentation/serial/serial-rs485.txt new file mode 100644 index 000000000..39dac9542 --- /dev/null +++ b/kernel/Documentation/serial/serial-rs485.txt @@ -0,0 +1,136 @@ + 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 serial_rs485 structure (see [1]) 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 (see [2] + for bindings). The driver is in charge of filling this data structure from + the values given by the device tree. + + Any driver for devices capable of working both as RS232 and RS485 should + provide at least the following ioctls: + + - TIOCSRS485 (typically associated with number 0x542F). This ioctl is used + to enable/disable RS485 mode from user-space + + - TIOCGRS485 (typically associated with number 0x542E). This ioctl is used + to get RS485 mode from kernel-space (i.e., driver) to user-space. + + In other words, the serial driver should contain a code similar to the next + one: + + static struct uart_ops atmel_pops = { + /* ... */ + .ioctl = handle_ioctl, + }; + + static int handle_ioctl(struct uart_port *port, + unsigned int cmd, + unsigned long arg) + { + struct serial_rs485 rs485conf; + + switch (cmd) { + case TIOCSRS485: + if (copy_from_user(&rs485conf, + (struct serial_rs485 *) arg, + sizeof(rs485conf))) + return -EFAULT; + + /* ... */ + break; + + case TIOCGRS485: + if (copy_to_user((struct serial_rs485 *) arg, + ..., + sizeof(rs485conf))) + return -EFAULT; + /* ... */ + break; + + /* ... */ + } + } + + +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> + + /* Driver-specific ioctls: */ + #define TIOCGRS485 0x542E + #define TIOCSRS485 0x542F + + /* 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 whilst 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. REFERENCES + + [1] include/uapi/linux/serial.h + [2] Documentation/devicetree/bindings/serial/rs485.txt |