About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / serial / serial-rs485.txt




Custom Search

Based on kernel version 4.3. Page generated on 2015-11-02 12:51 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	   implement the rs485_config callback in the uart_port structure. The
37	   serial_core calls rs485_config to do the device specific part in response
38	   to TIOCSRS485 and TIOCGRS485 ioctls (see below). The rs485_config callback
39	   receives a pointer to struct serial_rs485.
40	
41	4. USAGE FROM USER-LEVEL
42	
43	   From user-level, RS485 configuration can be get/set using the previous
44	   ioctls. For instance, to set RS485 you can use the following code:
45	
46		#include <linux/serial.h>
47	
48		/* RS485 ioctls: */
49		#define TIOCGRS485      0x542E
50		#define TIOCSRS485      0x542F
51	
52		/* Open your specific device (e.g., /dev/mydevice): */
53		int fd = open ("/dev/mydevice", O_RDWR);
54		if (fd < 0) {
55			/* Error handling. See errno. */
56		}
57	
58		struct serial_rs485 rs485conf;
59	
60		/* Enable RS485 mode: */
61		rs485conf.flags |= SER_RS485_ENABLED;
62	
63		/* Set logical level for RTS pin equal to 1 when sending: */
64		rs485conf.flags |= SER_RS485_RTS_ON_SEND;
65		/* or, set logical level for RTS pin equal to 0 when sending: */
66		rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);
67	
68		/* Set logical level for RTS pin equal to 1 after sending: */
69		rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
70		/* or, set logical level for RTS pin equal to 0 after sending: */
71		rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);
72	
73		/* Set rts delay before send, if needed: */
74		rs485conf.delay_rts_before_send = ...;
75	
76		/* Set rts delay after send, if needed: */
77		rs485conf.delay_rts_after_send = ...;
78	
79		/* Set this flag if you want to receive data even whilst sending data */
80		rs485conf.flags |= SER_RS485_RX_DURING_TX;
81	
82		if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
83			/* Error handling. See errno. */
84		}
85	
86		/* Use read() and write() syscalls here... */
87	
88		/* Close the device when finished: */
89		if (close (fd) < 0) {
90			/* Error handling. See errno. */
91		}
92	
93	5. REFERENCES
94	
95	 [1]	include/uapi/linux/serial.h
96	 [2]	Documentation/devicetree/bindings/serial/rs485.txt
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.