serial-rs485.rst (3626B)
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 serial_rs485 structure (see [1]) to handle 33 RS485 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 (see [2] 37 for bindings). The driver is in charge of filling this data structure from 38 the values given by the device tree. 39 40 Any driver for devices capable of working both as RS232 and RS485 should 41 implement the rs485_config callback in the uart_port structure. The 42 serial_core calls rs485_config to do the device specific part in response 43 to TIOCSRS485 and TIOCGRS485 ioctls (see below). The rs485_config callback 44 receives a pointer to struct serial_rs485. 45 464. Usage from user-level 47======================== 48 49 From user-level, RS485 configuration can be get/set using the previous 50 ioctls. For instance, to set RS485 you can use the following code:: 51 52 #include <linux/serial.h> 53 54 /* Include definition for RS485 ioctls: TIOCGRS485 and TIOCSRS485 */ 55 #include <sys/ioctl.h> 56 57 /* Open your specific device (e.g., /dev/mydevice): */ 58 int fd = open ("/dev/mydevice", O_RDWR); 59 if (fd < 0) { 60 /* Error handling. See errno. */ 61 } 62 63 struct serial_rs485 rs485conf; 64 65 /* Enable RS485 mode: */ 66 rs485conf.flags |= SER_RS485_ENABLED; 67 68 /* Set logical level for RTS pin equal to 1 when sending: */ 69 rs485conf.flags |= SER_RS485_RTS_ON_SEND; 70 /* or, set logical level for RTS pin equal to 0 when sending: */ 71 rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND); 72 73 /* Set logical level for RTS pin equal to 1 after sending: */ 74 rs485conf.flags |= SER_RS485_RTS_AFTER_SEND; 75 /* or, set logical level for RTS pin equal to 0 after sending: */ 76 rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND); 77 78 /* Set rts delay before send, if needed: */ 79 rs485conf.delay_rts_before_send = ...; 80 81 /* Set rts delay after send, if needed: */ 82 rs485conf.delay_rts_after_send = ...; 83 84 /* Set this flag if you want to receive data even while sending data */ 85 rs485conf.flags |= SER_RS485_RX_DURING_TX; 86 87 if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) { 88 /* Error handling. See errno. */ 89 } 90 91 /* Use read() and write() syscalls here... */ 92 93 /* Close the device when finished: */ 94 if (close (fd) < 0) { 95 /* Error handling. See errno. */ 96 } 97 985. References 99============= 100 101 [1] include/uapi/linux/serial.h 102 103 [2] Documentation/devicetree/bindings/serial/rs485.txt