1 ===========================
2 RS485 Serial Communications
3 ===========================
4
5 1. Introduction
6 ===============
7
8 EIA-485, also known as TIA/EIA-485 or RS-48
9 electrical characteristics of drivers and r
10 digital multipoint systems.
11 This standard is widely used for communicat
12 because it can be used effectively over lon
13 noisy environments.
14
15 2. Hardware-related Considerations
16 ==================================
17
18 Some CPUs/UARTs (e.g., Atmel AT91 or 16C950
19 half-duplex mode capable of automatically c
20 toggling RTS or DTR signals. That can be us
21 half-duplex hardware like an RS485 transcei
22 half-duplex devices like some modems.
23
24 For these microcontrollers, the Linux drive
25 working in both modes, and proper ioctls (s
26 available at user-level to allow switching
27 vice versa.
28
29 3. Data Structures Already Available in the Ke
30 ==============================================
31
32 The Linux kernel provides the struct serial
33 communications. This data structure is used
34 parameters in the platform data and in ioct
35
36 The device tree can also provide RS485 boot
37 [#DT-bindings]_. The serial core fills the
38 values given by the device tree when the dr
39 uart_get_rs485_mode().
40
41 Any driver for devices capable of working b
42 implement the ``rs485_config`` callback and
43 in the ``struct uart_port``. The serial cor
44 the device specific part in response to TIO
45 ``rs485_config`` callback receives a pointe
46 serial_rs485. The struct serial_rs485 users
47 before calling ``rs485_config`` using ``rs4
48 what RS485 features the driver supports for
49 TIOCGRS485 ioctl can be used to read back t
50 matching to the current configuration.
51
52 .. kernel-doc:: include/uapi/linux/serial.h
53 :identifiers: serial_rs485 uart_get_rs485_m
54
55 4. Usage from user-level
56 ========================
57
58 From user-level, RS485 configuration can be
59 ioctls. For instance, to set RS485 you can
60
61 #include <linux/serial.h>
62
63 /* Include definition for RS485 ioctls
64 #include <sys/ioctl.h>
65
66 /* Open your specific device (e.g., /d
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
78 rs485conf.flags |= SER_RS485_RTS_ON_SE
79 /* or, set logical level for RTS pin e
80 rs485conf.flags &= ~(SER_RS485_RTS_ON_
81
82 /* Set logical level for RTS pin equal
83 rs485conf.flags |= SER_RS485_RTS_AFTER
84 /* or, set logical level for RTS pin e
85 rs485conf.flags &= ~(SER_RS485_RTS_AFT
86
87 /* Set rts delay before send, if neede
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 receiv
94 rs485conf.flags |= SER_RS485_RX_DURING
95
96 if (ioctl (fd, TIOCSRS485, &rs485conf)
97 /* Error handling. See errno.
98 }
99
100 /* Use read() and write() syscalls her
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 f
111 communications line. The addressing mode is
112 ``SER_RS485_ADDRB`` flag in struct serial_r
113 has two additional flags and fields for ena
114 addresses.
115
116 Address mode flags:
117 - ``SER_RS485_ADDRB``: Enabled address
118 - ``SER_RS485_ADDR_RECV``: Receive (fi
119 - ``SER_RS485_ADDR_DEST``: Set destina
120
121 Address fields (enabled with corresponding
122 - ``addr_recv``: Receive address.
123 - ``addr_dest``: Destination address.
124
125 Once a receive address is set, the communic
126 particular device and other peers are filte
127 receiver side to enforce the filtering. Rec
128 if ``SER_RS485_ADDR_RECV`` is not set.
129
130 Note: not all devices supporting RS485 supp
131
132 6. References
133 =============
134
135 .. [#DT-bindings] Documentation/devicetr
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