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