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;
/* Set RS485 mode: */
rs485conf.flags |= SER_RS485_ENABLED;
/* Set rts delay before send, if needed: */
rs485conf.flags |= SER_RS485_RTS_BEFORE_SEND;
rs485conf.delay_rts_before_send = ...;
/* Set rts delay after send, if needed: */
rs485conf.flags |= SER_RS485_RTS_AFTER_SEND;
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
