137 lines
4.0 KiB
Plaintext
137 lines
4.0 KiB
Plaintext
RS485 SERIAL COMMUNICATIONS
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1. INTRODUCTION
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EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the
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electrical characteristics of drivers and receivers for use in balanced
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digital multipoint systems.
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This standard is widely used for communications in industrial automation
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because it can be used effectively over long distances and in electrically
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noisy environments.
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2. HARDWARE-RELATED CONSIDERATIONS
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Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in
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half-duplex mode capable of automatically controlling line direction by
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toggling RTS or DTR signals. That can be used to control external
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half-duplex hardware like an RS485 transceiver or any RS232-connected
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half-duplex devices like some modems.
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For these microcontrollers, the Linux driver should be made capable of
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working in both modes, and proper ioctls (see later) should be made
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available at user-level to allow switching from one mode to the other, and
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vice versa.
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3. DATA STRUCTURES ALREADY AVAILABLE IN THE KERNEL
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The Linux kernel provides the serial_rs485 structure (see [1]) to handle
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RS485 communications. This data structure is used to set and configure RS485
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parameters in the platform data and in ioctls.
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The device tree can also provide RS485 boot time parameters (see [2]
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for bindings). The driver is in charge of filling this data structure from
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the values given by the device tree.
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Any driver for devices capable of working both as RS232 and RS485 should
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provide at least the following ioctls:
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- TIOCSRS485 (typically associated with number 0x542F). This ioctl is used
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to enable/disable RS485 mode from user-space
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- TIOCGRS485 (typically associated with number 0x542E). This ioctl is used
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to get RS485 mode from kernel-space (i.e., driver) to user-space.
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In other words, the serial driver should contain a code similar to the next
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one:
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static struct uart_ops atmel_pops = {
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/* ... */
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.ioctl = handle_ioctl,
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};
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static int handle_ioctl(struct uart_port *port,
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unsigned int cmd,
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unsigned long arg)
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{
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struct serial_rs485 rs485conf;
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switch (cmd) {
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case TIOCSRS485:
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if (copy_from_user(&rs485conf,
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(struct serial_rs485 *) arg,
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sizeof(rs485conf)))
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return -EFAULT;
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/* ... */
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break;
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case TIOCGRS485:
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if (copy_to_user((struct serial_rs485 *) arg,
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...,
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sizeof(rs485conf)))
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return -EFAULT;
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/* ... */
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break;
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/* ... */
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}
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}
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4. USAGE FROM USER-LEVEL
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From user-level, RS485 configuration can be get/set using the previous
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ioctls. For instance, to set RS485 you can use the following code:
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#include <linux/serial.h>
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/* Driver-specific ioctls: */
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#define TIOCGRS485 0x542E
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#define TIOCSRS485 0x542F
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/* Open your specific device (e.g., /dev/mydevice): */
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int fd = open ("/dev/mydevice", O_RDWR);
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if (fd < 0) {
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/* Error handling. See errno. */
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}
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struct serial_rs485 rs485conf;
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/* Enable RS485 mode: */
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rs485conf.flags |= SER_RS485_ENABLED;
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/* Set logical level for RTS pin equal to 1 when sending: */
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rs485conf.flags |= SER_RS485_RTS_ON_SEND;
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/* or, set logical level for RTS pin equal to 0 when sending: */
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rs485conf.flags &= ~(SER_RS485_RTS_ON_SEND);
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/* Set logical level for RTS pin equal to 1 after sending: */
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rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
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/* or, set logical level for RTS pin equal to 0 after sending: */
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rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);
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/* Set rts delay before send, if needed: */
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rs485conf.delay_rts_before_send = ...;
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/* Set rts delay after send, if needed: */
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rs485conf.delay_rts_after_send = ...;
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/* Set this flag if you want to receive data even whilst sending data */
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rs485conf.flags |= SER_RS485_RX_DURING_TX;
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if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
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/* Error handling. See errno. */
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}
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/* Use read() and write() syscalls here... */
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/* Close the device when finished: */
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if (close (fd) < 0) {
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/* Error handling. See errno. */
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}
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5. REFERENCES
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[1] include/uapi/linux/serial.h
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[2] Documentation/devicetree/bindings/serial/rs485.txt
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