Documentation/serial/serial-rs485.txt

   1                         RS485 SERIAL COMMUNICATIONS
   2
   3 1. INTRODUCTION
   4
   5    EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the
   6    electrical characteristics of drivers and receivers for use in balanced
   7    digital multipoint systems.
   8    This standard is widely used for communications in industrial automation
   9    because it can be used effectively over long distances and in electrically
  10    noisy environments.
  11
  12 2. HARDWARE-RELATED CONSIDERATIONS
  13
  14    Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in
  15    half-duplex mode capable of automatically controlling line direction by
  16    toggling RTS or DTR signals. That can be used to control external
  17    half-duplex hardware like an RS485 transceiver or any RS232-connected
  18    half-duplex devices like some modems.
  19
  20    For these microcontrollers, the Linux driver should be made capable of
  21    working in both modes, and proper ioctls (see later) should be made
  22    available at user-level to allow switching from one mode to the other, and
  23    vice versa.
  24
  25 3. DATA STRUCTURES ALREADY AVAILABLE IN THE KERNEL
  26
  27    The Linux kernel provides the serial_rs485 structure (see [1]) to handle
  28    RS485 communications. This data structure is used to set and configure RS485
  29    parameters in the platform data and in ioctls.
  30
  31    The device tree can also provide RS485 boot time parameters (see [2]
  32    for bindings). The driver is in charge of filling this data structure from
  33    the values given by the device tree.
  34
  35    Any driver for devices capable of working both as RS232 and RS485 should
  36    provide at least the following ioctls:
  37
  38     - TIOCSRS485 (typically associated with number 0x542F). This ioctl is used
  39       to enable/disable RS485 mode from user-space
  40
  41     - TIOCGRS485 (typically associated with number 0x542E). This ioctl is used
  42       to get RS485 mode from kernel-space (i.e., driver) to user-space.
  43
  44    In other words, the serial driver should contain a code similar to the next
  45    one:
  46
  47         static struct uart_ops atmel_pops = {
  48                 /* ... */
  49                 .ioctl          = handle_ioctl,
  50         };
  51
  52         static int handle_ioctl(struct uart_port *port,
  53                 unsigned int cmd,
  54                 unsigned long arg)
  55         {
  56                 struct serial_rs485 rs485conf;
  57
  58                 switch (cmd) {
  59                 case TIOCSRS485:
  60                         if (copy_from_user(&rs485conf,
  61                                 (struct serial_rs485 *) arg,
  62                                 sizeof(rs485conf)))
  63                                         return -EFAULT;
  64
  65                         /* ... */
  66                         break;
  67
  68                 case TIOCGRS485:
  69                         if (copy_to_user((struct serial_rs485 *) arg,
  70                                 ...,
  71                                 sizeof(rs485conf)))
  72                                         return -EFAULT;
  73                         /* ... */
  74                         break;
  75
  76                 /* ... */
  77                 }
  78         }
  79
  80
  81 4. USAGE FROM USER-LEVEL
  82
  83    From user-level, RS485 configuration can be get/set using the previous
  84    ioctls. For instance, to set RS485 you can use the following code:
  85
  86         #include <linux/serial.h>
  87
  88         /* Driver-specific ioctls: */
  89         #define TIOCGRS485      0x542E
  90         #define TIOCSRS485      0x542F
  91
  92         /* Open your specific device (e.g., /dev/mydevice): */
  93         int fd = open ("/dev/mydevice", O_RDWR);
  94         if (fd < 0) {
  95                 /* Error handling. See errno. */
  96         }
  97
  98         struct serial_rs485 rs485conf;
  99
 100         /* Enable RS485 mode: */
 101         rs485conf.flags |= SER_RS485_ENABLED;
 102
 103         /* Set logical level for RTS pin equal to 1 when sending: */
 104         rs485conf.flags |= SER_RS485_RTS_ON_SEND;
 105         /* or, set logical level for RTS pin equal to 0 when sending: */
 106         rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);
 107
 108         /* Set logical level for RTS pin equal to 1 after sending: */
 109         rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
 110         /* or, set logical level for RTS pin equal to 0 after sending: */
 111         rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);
 112
 113         /* Set rts delay before send, if needed: */
 114         rs485conf.delay_rts_before_send = ...;
 115
 116         /* Set rts delay after send, if needed: */
 117         rs485conf.delay_rts_after_send = ...;
 118
 119         /* Set this flag if you want to receive data even whilst sending data */
 120         rs485conf.flags |= SER_RS485_RX_DURING_TX;
 121
 122         if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
 123                 /* Error handling. See errno. */
 124         }
 125
 126         /* Use read() and write() syscalls here... */
 127
 128         /* Close the device when finished: */
 129         if (close (fd) < 0) {
 130                 /* Error handling. See errno. */
 131         }
 132
 133 5. REFERENCES
 134
 135  [1]    include/linux/serial.h
 136  [2]    Documentation/devicetree/bindings/serial/rs485.txt