1=========================== 2RS485 Serial Communications 3=========================== 4 51. Introduction 6=============== 7 8 EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the 9 electrical characteristics of drivers and receivers for use in balanced 10 digital multipoint systems. 11 This standard is widely used for communications in industrial automation 12 because it can be used effectively over long distances and in electrically 13 noisy environments. 14 152. Hardware-related Considerations 16================================== 17 18 Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in 19 half-duplex mode capable of automatically controlling line direction by 20 toggling RTS or DTR signals. That can be used to control external 21 half-duplex hardware like an RS485 transceiver or any RS232-connected 22 half-duplex devices like some modems. 23 24 For these microcontrollers, the Linux driver should be made capable of 25 working in both modes, and proper ioctls (see later) should be made 26 available at user-level to allow switching from one mode to the other, and 27 vice versa. 28 293. Data Structures Already Available in the Kernel 30================================================== 31 32 The Linux kernel provides the struct serial_rs485 to handle RS485 33 communications. This data structure is used to set and configure RS485 34 parameters in the platform data and in ioctls. 35 36 The device tree can also provide RS485 boot time parameters 37 [#DT-bindings]_. The serial core fills the struct serial_rs485 from the 38 values given by the device tree when the driver calls 39 uart_get_rs485_mode(). 40 41 Any driver for devices capable of working both as RS232 and RS485 should 42 implement the ``rs485_config`` callback and provide ``rs485_supported`` 43 in the ``struct uart_port``. The serial core calls ``rs485_config`` to do 44 the device specific part in response to TIOCSRS485 ioctl (see below). The 45 ``rs485_config`` callback receives a pointer to a sanitizated struct 46 serial_rs485. The struct serial_rs485 userspace provides is sanitized 47 before calling ``rs485_config`` using ``rs485_supported`` that indicates 48 what RS485 features the driver supports for the ``struct uart_port``. 49 TIOCGRS485 ioctl can be used to read back the struct serial_rs485 50 matching to the current configuration. 51 52.. kernel-doc:: include/uapi/linux/serial.h 53 :identifiers: serial_rs485 54 55.. kernel-doc:: drivers/tty/serial/serial_core.c 56 :identifiers: uart_get_rs485_mode 57 584. Usage from user-level 59======================== 60 61 From user-level, RS485 configuration can be get/set using the previous 62 ioctls. For instance, to set RS485 you can use the following code:: 63 64 #include <linux/serial.h> 65 66 /* Include definition for RS485 ioctls: TIOCGRS485 and TIOCSRS485 */ 67 #include <sys/ioctl.h> 68 69 /* Open your specific device (e.g., /dev/mydevice): */ 70 int fd = open ("/dev/mydevice", O_RDWR); 71 if (fd < 0) { 72 /* Error handling. See errno. */ 73 } 74 75 struct serial_rs485 rs485conf; 76 77 /* Enable RS485 mode: */ 78 rs485conf.flags |= SER_RS485_ENABLED; 79 80 /* Set logical level for RTS pin equal to 1 when sending: */ 81 rs485conf.flags |= SER_RS485_RTS_ON_SEND; 82 /* or, set logical level for RTS pin equal to 0 when sending: */ 83 rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND); 84 85 /* Set logical level for RTS pin equal to 1 after sending: */ 86 rs485conf.flags |= SER_RS485_RTS_AFTER_SEND; 87 /* or, set logical level for RTS pin equal to 0 after sending: */ 88 rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND); 89 90 /* Set rts delay before send, if needed: */ 91 rs485conf.delay_rts_before_send = ...; 92 93 /* Set rts delay after send, if needed: */ 94 rs485conf.delay_rts_after_send = ...; 95 96 /* Set this flag if you want to receive data even while sending data */ 97 rs485conf.flags |= SER_RS485_RX_DURING_TX; 98 99 if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) { 100 /* Error handling. See errno. */ 101 } 102 103 /* Use read() and write() syscalls here... */ 104 105 /* Close the device when finished: */ 106 if (close (fd) < 0) { 107 /* Error handling. See errno. */ 108 } 109 1105. Multipoint Addressing 111======================== 112 113 The Linux kernel provides addressing mode for multipoint RS-485 serial 114 communications line. The addressing mode is enabled with 115 ``SER_RS485_ADDRB`` flag in struct serial_rs485. The struct serial_rs485 116 has two additional flags and fields for enabling receive and destination 117 addresses. 118 119 Address mode flags: 120 - ``SER_RS485_ADDRB``: Enabled addressing mode (sets also ADDRB in termios). 121 - ``SER_RS485_ADDR_RECV``: Receive (filter) address enabled. 122 - ``SER_RS485_ADDR_DEST``: Set destination address. 123 124 Address fields (enabled with corresponding ``SER_RS485_ADDR_*`` flag): 125 - ``addr_recv``: Receive address. 126 - ``addr_dest``: Destination address. 127 128 Once a receive address is set, the communication can occur only with the 129 particular device and other peers are filtered out. It is left up to the 130 receiver side to enforce the filtering. Receive address will be cleared 131 if ``SER_RS485_ADDR_RECV`` is not set. 132 133 Note: not all devices supporting RS485 support multipoint addressing. 134 1356. References 136============= 137 138.. [#DT-bindings] Documentation/devicetree/bindings/serial/rs485.yaml 139