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1===========================2RS485 Serial Communications3===========================4 51. Introduction6===============7 8 EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the9 electrical characteristics of drivers and receivers for use in balanced10 digital multipoint systems.11 This standard is widely used for communications in industrial automation12 because it can be used effectively over long distances and in electrically13 noisy environments.14 152. Hardware-related Considerations16==================================17 18 Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in19 half-duplex mode capable of automatically controlling line direction by20 toggling RTS or DTR signals. That can be used to control external21 half-duplex hardware like an RS485 transceiver or any RS232-connected22 half-duplex devices like some modems.23 24 For these microcontrollers, the Linux driver should be made capable of25 working in both modes, and proper ioctls (see later) should be made26 available at user-level to allow switching from one mode to the other, and27 vice versa.28 293. Data Structures Already Available in the Kernel30==================================================31 32 The Linux kernel provides the struct serial_rs485 to handle RS48533 communications. This data structure is used to set and configure RS48534 parameters in the platform data and in ioctls.35 36 The device tree can also provide RS485 boot time parameters37 [#DT-bindings]_. The serial core fills the struct serial_rs485 from the38 values given by the device tree when the driver calls39 uart_get_rs485_mode().40 41 Any driver for devices capable of working both as RS232 and RS485 should42 implement the ``rs485_config`` callback and provide ``rs485_supported``43 in the ``struct uart_port``. The serial core calls ``rs485_config`` to do44 the device specific part in response to TIOCSRS485 ioctl (see below). The45 ``rs485_config`` callback receives a pointer to a sanitizated struct46 serial_rs485. The struct serial_rs485 userspace provides is sanitized47 before calling ``rs485_config`` using ``rs485_supported`` that indicates48 what RS485 features the driver supports for the ``struct uart_port``.49 TIOCGRS485 ioctl can be used to read back the struct serial_rs48550 matching to the current configuration.51 52.. kernel-doc:: include/uapi/linux/serial.h53 :identifiers: serial_rs485 uart_get_rs485_mode54 554. Usage from user-level56========================57 58 From user-level, RS485 configuration can be get/set using the previous59 ioctls. For instance, to set RS485 you can use the following code::60 61 #include <linux/serial.h>62 63 /* Include definition for RS485 ioctls: TIOCGRS485 and TIOCSRS485 */64 #include <sys/ioctl.h>65 66 /* Open your specific device (e.g., /dev/mydevice): */67 int fd = open ("/dev/mydevice", O_RDWR);68 if (fd < 0) {69 /* Error handling. See errno. */70 }71 72 struct serial_rs485 rs485conf;73 74 /* Enable RS485 mode: */75 rs485conf.flags |= SER_RS485_ENABLED;76 77 /* Set logical level for RTS pin equal to 1 when sending: */78 rs485conf.flags |= SER_RS485_RTS_ON_SEND;79 /* or, set logical level for RTS pin equal to 0 when sending: */80 rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);81 82 /* Set logical level for RTS pin equal to 1 after sending: */83 rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;84 /* or, set logical level for RTS pin equal to 0 after sending: */85 rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);86 87 /* Set rts delay before send, if needed: */88 rs485conf.delay_rts_before_send = ...;89 90 /* Set rts delay after send, if needed: */91 rs485conf.delay_rts_after_send = ...;92 93 /* Set this flag if you want to receive data even while sending data */94 rs485conf.flags |= SER_RS485_RX_DURING_TX;95 96 if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {97 /* Error handling. See errno. */98 }99 100 /* Use read() and write() syscalls here... */101 102 /* Close the device when finished: */103 if (close (fd) < 0) {104 /* Error handling. See errno. */105 }106 1075. Multipoint Addressing108========================109 110 The Linux kernel provides addressing mode for multipoint RS-485 serial111 communications line. The addressing mode is enabled with112 ``SER_RS485_ADDRB`` flag in struct serial_rs485. The struct serial_rs485113 has two additional flags and fields for enabling receive and destination114 addresses.115 116 Address mode flags:117 - ``SER_RS485_ADDRB``: Enabled addressing mode (sets also ADDRB in termios).118 - ``SER_RS485_ADDR_RECV``: Receive (filter) address enabled.119 - ``SER_RS485_ADDR_DEST``: Set destination address.120 121 Address fields (enabled with corresponding ``SER_RS485_ADDR_*`` flag):122 - ``addr_recv``: Receive address.123 - ``addr_dest``: Destination address.124 125 Once a receive address is set, the communication can occur only with the126 particular device and other peers are filtered out. It is left up to the127 receiver side to enforce the filtering. Receive address will be cleared128 if ``SER_RS485_ADDR_RECV`` is not set.129 130 Note: not all devices supporting RS485 support multipoint addressing.131 1326. References133=============134 135.. [#DT-bindings] Documentation/devicetree/bindings/serial/rs485.txt136