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1 | =========================== |
2 | RS485 Serial Communications | |
3 | =========================== | |
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5 | 1. Introduction |
6 | =============== | |
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7 | |
8 | EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the | |
9 | electrical characteristics of drivers and receivers for use in balanced | |
10 | digital multipoint systems. | |
11 | This standard is widely used for communications in industrial automation | |
12 | because it can be used effectively over long distances and in electrically | |
13 | noisy environments. | |
14 | ||
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15 | 2. Hardware-related Considerations |
16 | ================================== | |
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18 | Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in |
19 | half-duplex mode capable of automatically controlling line direction by | |
20 | toggling RTS or DTR signals. That can be used to control external | |
21 | half-duplex hardware like an RS485 transceiver or any RS232-connected | |
22 | half-duplex devices like some modems. | |
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23 | |
24 | For these microcontrollers, the Linux driver should be made capable of | |
25 | working in both modes, and proper ioctls (see later) should be made | |
26 | available at user-level to allow switching from one mode to the other, and | |
27 | vice versa. | |
28 | ||
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29 | 3. Data Structures Already Available in the Kernel |
30 | ================================================== | |
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31 | |
32 | The Linux kernel provides the serial_rs485 structure (see [1]) to handle | |
33 | RS485 communications. This data structure is used to set and configure RS485 | |
34 | parameters in the platform data and in ioctls. | |
35 | ||
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36 | The device tree can also provide RS485 boot time parameters (see [2] |
37 | for bindings). The driver is in charge of filling this data structure from | |
38 | the values given by the device tree. | |
39 | ||
63295cb2 | 40 | Any driver for devices capable of working both as RS232 and RS485 should |
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41 | implement the rs485_config callback in the uart_port structure. The |
42 | serial_core calls rs485_config to do the device specific part in response | |
43 | to TIOCSRS485 and TIOCGRS485 ioctls (see below). The rs485_config callback | |
44 | receives a pointer to struct serial_rs485. | |
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46 | 4. Usage from user-level |
47 | ======================== | |
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48 | |
49 | From user-level, RS485 configuration can be get/set using the previous | |
f1374017 | 50 | ioctls. For instance, to set RS485 you can use the following code:: |
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51 | |
52 | #include <linux/serial.h> | |
53 | ||
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54 | /* Include definition for RS485 ioctls: TIOCGRS485 and TIOCSRS485 */ |
55 | #include <sys/ioctl.h> | |
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56 | |
57 | /* Open your specific device (e.g., /dev/mydevice): */ | |
58 | int fd = open ("/dev/mydevice", O_RDWR); | |
59 | if (fd < 0) { | |
60 | /* Error handling. See errno. */ | |
61 | } | |
62 | ||
63 | struct serial_rs485 rs485conf; | |
64 | ||
93f3350c | 65 | /* Enable RS485 mode: */ |
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66 | rs485conf.flags |= SER_RS485_ENABLED; |
67 | ||
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68 | /* Set logical level for RTS pin equal to 1 when sending: */ |
69 | rs485conf.flags |= SER_RS485_RTS_ON_SEND; | |
70 | /* or, set logical level for RTS pin equal to 0 when sending: */ | |
71 | rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND); | |
72 | ||
73 | /* Set logical level for RTS pin equal to 1 after sending: */ | |
74 | rs485conf.flags |= SER_RS485_RTS_AFTER_SEND; | |
75 | /* or, set logical level for RTS pin equal to 0 after sending: */ | |
76 | rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND); | |
77 | ||
63295cb2 | 78 | /* Set rts delay before send, if needed: */ |
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79 | rs485conf.delay_rts_before_send = ...; |
80 | ||
81 | /* Set rts delay after send, if needed: */ | |
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82 | rs485conf.delay_rts_after_send = ...; |
83 | ||
806654a9 | 84 | /* Set this flag if you want to receive data even while sending data */ |
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85 | rs485conf.flags |= SER_RS485_RX_DURING_TX; |
86 | ||
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87 | if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) { |
88 | /* Error handling. See errno. */ | |
89 | } | |
90 | ||
91 | /* Use read() and write() syscalls here... */ | |
92 | ||
93 | /* Close the device when finished: */ | |
94 | if (close (fd) < 0) { | |
95 | /* Error handling. See errno. */ | |
96 | } | |
97 | ||
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98 | 5. References |
99 | ============= | |
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8307959d | 101 | [1] include/uapi/linux/serial.h |
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0331bbf3 | 103 | [2] Documentation/devicetree/bindings/serial/rs485.txt |