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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

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