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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.
11
122. HARDWARE-RELATED CONSIDERATIONS
13
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
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
Nicolas Ferre0331bbf2011-10-12 18:06:56 +020031 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
Claudio Scordino63295cb2010-11-11 11:22:36 +010035 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
814. USAGE FROM USER-LEVEL
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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
Claudio Scordino93f33502011-11-09 15:51:49 +0100100 /* Enable RS485 mode: */
Claudio Scordino63295cb2010-11-11 11:22:36 +0100101 rs485conf.flags |= SER_RS485_ENABLED;
102
Claudio Scordino93f33502011-11-09 15:51:49 +0100103 /* 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
Claudio Scordino63295cb2010-11-11 11:22:36 +0100113 /* Set rts delay before send, if needed: */
Claudio Scordino63295cb2010-11-11 11:22:36 +0100114 rs485conf.delay_rts_before_send = ...;
115
116 /* Set rts delay after send, if needed: */
Claudio Scordino63295cb2010-11-11 11:22:36 +0100117 rs485conf.delay_rts_after_send = ...;
118
Bernhard Roth83cac9f2011-08-24 09:48:23 +0200119 /* Set this flag if you want to receive data even whilst sending data */
120 rs485conf.flags |= SER_RS485_RX_DURING_TX;
121
Claudio Scordino63295cb2010-11-11 11:22:36 +0100122 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
1335. REFERENCES
134
Yegor Yefremov83079592014-08-13 15:54:48 +0200135 [1] include/uapi/linux/serial.h
Nicolas Ferre0331bbf2011-10-12 18:06:56 +0200136 [2] Documentation/devicetree/bindings/serial/rs485.txt