1 =========================== 1 ===========================
2 RS485 Serial Communications 2 RS485 Serial Communications
3 =========================== 3 ===========================
4 4
5 1. Introduction 5 1. Introduction
6 =============== 6 ===============
7 7
8 EIA-485, also known as TIA/EIA-485 or RS-48 8 EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the
9 electrical characteristics of drivers and r 9 electrical characteristics of drivers and receivers for use in balanced
10 digital multipoint systems. 10 digital multipoint systems.
11 This standard is widely used for communicat 11 This standard is widely used for communications in industrial automation
12 because it can be used effectively over lon 12 because it can be used effectively over long distances and in electrically
13 noisy environments. 13 noisy environments.
14 14
15 2. Hardware-related Considerations 15 2. Hardware-related Considerations
16 ================================== 16 ==================================
17 17
18 Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 18 Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in
19 half-duplex mode capable of automatically c 19 half-duplex mode capable of automatically controlling line direction by
20 toggling RTS or DTR signals. That can be us 20 toggling RTS or DTR signals. That can be used to control external
21 half-duplex hardware like an RS485 transcei 21 half-duplex hardware like an RS485 transceiver or any RS232-connected
22 half-duplex devices like some modems. 22 half-duplex devices like some modems.
23 23
24 For these microcontrollers, the Linux drive 24 For these microcontrollers, the Linux driver should be made capable of
25 working in both modes, and proper ioctls (s 25 working in both modes, and proper ioctls (see later) should be made
26 available at user-level to allow switching 26 available at user-level to allow switching from one mode to the other, and
27 vice versa. 27 vice versa.
28 28
29 3. Data Structures Already Available in the Ke 29 3. Data Structures Already Available in the Kernel
30 ============================================== 30 ==================================================
31 31
32 The Linux kernel provides the struct serial !! 32 The Linux kernel provides the serial_rs485 structure (see [1]) to handle
33 communications. This data structure is used !! 33 RS485 communications. This data structure is used to set and configure RS485
34 parameters in the platform data and in ioct 34 parameters in the platform data and in ioctls.
35 35
36 The device tree can also provide RS485 boot !! 36 The device tree can also provide RS485 boot time parameters (see [2]
37 [#DT-bindings]_. The serial core fills the !! 37 for bindings). The driver is in charge of filling this data structure from
38 values given by the device tree when the dr !! 38 the values given by the device tree.
39 uart_get_rs485_mode(). <<
40 39
41 Any driver for devices capable of working b 40 Any driver for devices capable of working both as RS232 and RS485 should
42 implement the ``rs485_config`` callback and !! 41 implement the rs485_config callback in the uart_port structure. The
43 in the ``struct uart_port``. The serial cor !! 42 serial_core calls rs485_config to do the device specific part in response
44 the device specific part in response to TIO !! 43 to TIOCSRS485 and TIOCGRS485 ioctls (see below). The rs485_config callback
45 ``rs485_config`` callback receives a pointe !! 44 receives a pointer to struct serial_rs485.
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 45
55 4. Usage from user-level 46 4. Usage from user-level
56 ======================== 47 ========================
57 48
58 From user-level, RS485 configuration can be 49 From user-level, RS485 configuration can be get/set using the previous
59 ioctls. For instance, to set RS485 you can 50 ioctls. For instance, to set RS485 you can use the following code::
60 51
61 #include <linux/serial.h> 52 #include <linux/serial.h>
62 53
63 /* Include definition for RS485 ioctls 54 /* Include definition for RS485 ioctls: TIOCGRS485 and TIOCSRS485 */
64 #include <sys/ioctl.h> 55 #include <sys/ioctl.h>
65 56
66 /* Open your specific device (e.g., /d 57 /* Open your specific device (e.g., /dev/mydevice): */
67 int fd = open ("/dev/mydevice", O_RDWR 58 int fd = open ("/dev/mydevice", O_RDWR);
68 if (fd < 0) { 59 if (fd < 0) {
69 /* Error handling. See errno. 60 /* Error handling. See errno. */
70 } 61 }
71 62
72 struct serial_rs485 rs485conf; 63 struct serial_rs485 rs485conf;
73 64
74 /* Enable RS485 mode: */ 65 /* Enable RS485 mode: */
75 rs485conf.flags |= SER_RS485_ENABLED; 66 rs485conf.flags |= SER_RS485_ENABLED;
76 67
77 /* Set logical level for RTS pin equal 68 /* Set logical level for RTS pin equal to 1 when sending: */
78 rs485conf.flags |= SER_RS485_RTS_ON_SE 69 rs485conf.flags |= SER_RS485_RTS_ON_SEND;
79 /* or, set logical level for RTS pin e 70 /* or, set logical level for RTS pin equal to 0 when sending: */
80 rs485conf.flags &= ~(SER_RS485_RTS_ON_ 71 rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);
81 72
82 /* Set logical level for RTS pin equal 73 /* Set logical level for RTS pin equal to 1 after sending: */
83 rs485conf.flags |= SER_RS485_RTS_AFTER 74 rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
84 /* or, set logical level for RTS pin e 75 /* or, set logical level for RTS pin equal to 0 after sending: */
85 rs485conf.flags &= ~(SER_RS485_RTS_AFT 76 rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);
86 77
87 /* Set rts delay before send, if neede 78 /* Set rts delay before send, if needed: */
88 rs485conf.delay_rts_before_send = ...; 79 rs485conf.delay_rts_before_send = ...;
89 80
90 /* Set rts delay after send, if needed 81 /* Set rts delay after send, if needed: */
91 rs485conf.delay_rts_after_send = ...; 82 rs485conf.delay_rts_after_send = ...;
92 83
93 /* Set this flag if you want to receiv 84 /* Set this flag if you want to receive data even while sending data */
94 rs485conf.flags |= SER_RS485_RX_DURING 85 rs485conf.flags |= SER_RS485_RX_DURING_TX;
95 86
96 if (ioctl (fd, TIOCSRS485, &rs485conf) 87 if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
97 /* Error handling. See errno. 88 /* Error handling. See errno. */
98 } 89 }
99 90
100 /* Use read() and write() syscalls her 91 /* Use read() and write() syscalls here... */
101 92
102 /* Close the device when finished: */ 93 /* Close the device when finished: */
103 if (close (fd) < 0) { 94 if (close (fd) < 0) {
104 /* Error handling. See errno. 95 /* Error handling. See errno. */
105 } 96 }
106 97
107 5. Multipoint Addressing !! 98 5. References
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 ============= 99 =============
134 100
135 .. [#DT-bindings] Documentation/devicetr !! 101 [1] include/uapi/linux/serial.h
>> 102
>> 103 [2] Documentation/devicetree/bindings/serial/rs485.txt
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