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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 | ||
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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. | |
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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 | ||
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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 | ||
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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 | ||
93f3350c | 100 | /* Enable RS485 mode: */ |
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101 | rs485conf.flags |= SER_RS485_ENABLED; |
102 | ||
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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 | ||
63295cb2 | 113 | /* Set rts delay before send, if needed: */ |
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114 | rs485conf.delay_rts_before_send = ...; |
115 | ||
116 | /* Set rts delay after send, if needed: */ | |
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117 | rs485conf.delay_rts_after_send = ...; |
118 | ||
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119 | /* Set this flag if you want to receive data even whilst sending data */ |
120 | rs485conf.flags |= SER_RS485_RX_DURING_TX; | |
121 | ||
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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 | |
0331bbf3 | 136 | [2] Documentation/devicetree/bindings/serial/rs485.txt |