1                        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
 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
 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
 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
 814. 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
1335. REFERENCES
134
135 [1]	include/linux/serial.h
136 [2]	Documentation/devicetree/bindings/serial/rs485.txt