1 ===========================
2 RS485 Serial Communications
3 ===========================
4
5 1. 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
15 2. 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
29 3. 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 uart_get_rs485_mode
54
55 4. Usage from user-level
56 ========================
57
58 From user-level, RS485 configuration can be get/set using the previous
59 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
107 5. Multipoint Addressing
108 ========================
109
110 The Linux kernel provides addressing mode for multipoint RS-485 serial
111 communications line. The addressing mode is enabled with
112 ``SER_RS485_ADDRB`` flag in struct serial_rs485. The struct serial_rs485
113 has two additional flags and fields for enabling receive and destination
114 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 the
126 particular device and other peers are filtered out. It is left up to the
127 receiver side to enforce the filtering. Receive address will be cleared
128 if ``SER_RS485_ADDR_RECV`` is not set.
129
130 Note: not all devices supporting RS485 support multipoint addressing.
131
132 6. References
133 =============
134
135 .. [#DT-bindings] Documentation/devicetree/bindings/serial/rs485.txt
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