96 lines
3.4 KiB
Plaintext
96 lines
3.4 KiB
Plaintext
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
|
|
implement the rs485_config callback in the uart_port structure. The
|
|
serial_core calls rs485_config to do the device specific part in response
|
|
to TIOCSRS485 and TIOCGRS485 ioctls (see below). The rs485_config callback
|
|
receives a pointer to struct serial_rs485.
|
|
|
|
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>
|
|
|
|
/* Include definition for RS485 ioctls: TIOCGRS485 and TIOCSRS485 */
|
|
#include <sys/ioctl.h>
|
|
|
|
/* 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 while 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/uapi/linux/serial.h
|
|
[2] Documentation/devicetree/bindings/serial/rs485.txt
|