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Claudio Scordino63295cb2010-11-11 11:22:36 +01001 RS485 SERIAL COMMUNICATIONS
2
31. 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.
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122. HARDWARE-RELATED CONSIDERATIONS
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Yegor Yefremovde6f86c2010-11-22 11:06:32 +010014 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.
Claudio Scordino63295cb2010-11-11 11:22:36 +010019
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
253. DATA STRUCTURES ALREADY AVAILABLE IN THE KERNEL
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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.
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31 Any driver for devices capable of working both as RS232 and RS485 should
32 provide at least the following ioctls:
33
34 - TIOCSRS485 (typically associated with number 0x542F). This ioctl is used
35 to enable/disable RS485 mode from user-space
36
37 - TIOCGRS485 (typically associated with number 0x542E). This ioctl is used
38 to get RS485 mode from kernel-space (i.e., driver) to user-space.
39
40 In other words, the serial driver should contain a code similar to the next
41 one:
42
43 static struct uart_ops atmel_pops = {
44 /* ... */
45 .ioctl = handle_ioctl,
46 };
47
48 static int handle_ioctl(struct uart_port *port,
49 unsigned int cmd,
50 unsigned long arg)
51 {
52 struct serial_rs485 rs485conf;
53
54 switch (cmd) {
55 case TIOCSRS485:
56 if (copy_from_user(&rs485conf,
57 (struct serial_rs485 *) arg,
58 sizeof(rs485conf)))
59 return -EFAULT;
60
61 /* ... */
62 break;
63
64 case TIOCGRS485:
65 if (copy_to_user((struct serial_rs485 *) arg,
66 ...,
67 sizeof(rs485conf)))
68 return -EFAULT;
69 /* ... */
70 break;
71
72 /* ... */
73 }
74 }
75
76
774. USAGE FROM USER-LEVEL
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79 From user-level, RS485 configuration can be get/set using the previous
80 ioctls. For instance, to set RS485 you can use the following code:
81
82 #include <linux/serial.h>
83
84 /* Driver-specific ioctls: */
85 #define TIOCGRS485 0x542E
86 #define TIOCSRS485 0x542F
87
88 /* Open your specific device (e.g., /dev/mydevice): */
89 int fd = open ("/dev/mydevice", O_RDWR);
90 if (fd < 0) {
91 /* Error handling. See errno. */
92 }
93
94 struct serial_rs485 rs485conf;
95
96 /* Set RS485 mode: */
97 rs485conf.flags |= SER_RS485_ENABLED;
98
99 /* Set rts delay before send, if needed: */
100 rs485conf.flags |= SER_RS485_RTS_BEFORE_SEND;
101 rs485conf.delay_rts_before_send = ...;
102
103 /* Set rts delay after send, if needed: */
104 rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
105 rs485conf.delay_rts_after_send = ...;
106
107 if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
108 /* Error handling. See errno. */
109 }
110
111 /* Use read() and write() syscalls here... */
112
113 /* Close the device when finished: */
114 if (close (fd) < 0) {
115 /* Error handling. See errno. */
116 }
117
1185. REFERENCES
119
120 [1] include/linux/serial.h