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.
3. Data Structures Already Available in the Kernel¶
The Linux kernel provides the
struct serial_rs485
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 1. The serial core fills the
struct serial_rs485
from the values given by the device tree when the driver calls uart_get_rs485_mode().Any driver for devices capable of working both as RS232 and RS485 should implement the
rs485_config
callback and providers485_supported
in thestruct uart_port
. The serial core callsrs485_config
to do the device specific part in response to TIOCSRS485 ioctl (see below). Thers485_config
callback receives a pointer to a sanitizatedstruct serial_rs485
. Thestruct serial_rs485
userspace provides is sanitized before callingrs485_config
usingrs485_supported
that indicates what RS485 features the driver supports for thestruct uart_port
. TIOCGRS485 ioctl can be used to read back thestruct serial_rs485
matching to the current configuration.
-
struct serial_rs485¶
serial interface for controlling RS485 settings.
Definition:
struct serial_rs485 {
__u32 flags;
#define SER_RS485_ENABLED (1 << 0);
#define SER_RS485_RTS_ON_SEND (1 << 1);
#define SER_RS485_RTS_AFTER_SEND (1 << 2);
#define SER_RS485_RX_DURING_TX (1 << 4);
#define SER_RS485_TERMINATE_BUS (1 << 5);
#define SER_RS485_ADDRB (1 << 6);
#define SER_RS485_ADDR_RECV (1 << 7);
#define SER_RS485_ADDR_DEST (1 << 8);
__u32 delay_rts_before_send;
__u32 delay_rts_after_send;
union {
__u32 padding[5];
struct {
__u8 addr_recv;
__u8 addr_dest;
__u8 padding0[2];
__u32 padding1[4];
};
};
};
Members
flags
RS485 feature flags.
delay_rts_before_send
Delay before send (milliseconds).
delay_rts_after_send
Delay after send (milliseconds).
{unnamed_union}
anonymous
padding
Deprecated, use padding0 and padding1 instead. Do not use with addr_recv and addr_dest (due to overlap).
{unnamed_struct}
anonymous
addr_recv
Receive filter for RS485 addressing mode (used only when
SER_RS485_ADDR_RECV
is set).addr_dest
Destination address for RS485 addressing mode (used only when
SER_RS485_ADDR_DEST
is set).padding0
Padding (set to zero).
padding1
Padding (set to zero).
Description
Serial interface for controlling RS485 settings on chips with suitable support. Set with TIOCSRS485 and get with TIOCGRS485 if supported by your platform. The set function returns the new state, with any unsupported bits reverted appropriately.
The flag bits are:
SER_RS485_ENABLED
- RS485 enabled.SER_RS485_RTS_ON_SEND
- Logical level for RTS pin when sending.SER_RS485_RTS_AFTER_SEND
- Logical level for RTS pin after sent.SER_RS485_RX_DURING_TX
- Full-duplex RS485 line.SER_RS485_TERMINATE_BUS
- Enable bus termination (if supported).SER_RS485_ADDRB
- Enable RS485 addressing mode.SER_RS485_ADDR_RECV
- Receive address filter (enables addr_recv). RequiresSER_RS485_ADDRB
.SER_RS485_ADDR_DEST
- Destination address (enables addr_dest). RequiresSER_RS485_ADDRB
.
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. Multipoint Addressing¶
The Linux kernel provides addressing mode for multipoint RS-485 serial communications line. The addressing mode is enabled with
SER_RS485_ADDRB
flag instruct serial_rs485
. Thestruct serial_rs485
has two additional flags and fields for enabling receive and destination addresses.
- Address mode flags:
SER_RS485_ADDRB
: Enabled addressing mode (sets also ADDRB in termios).
SER_RS485_ADDR_RECV
: Receive (filter) address enabled.
SER_RS485_ADDR_DEST
: Set destination address.- Address fields (enabled with corresponding
SER_RS485_ADDR_*
flag):
addr_recv
: Receive address.
addr_dest
: Destination address.Once a receive address is set, the communication can occur only with the particular device and other peers are filtered out. It is left up to the receiver side to enforce the filtering. Receive address will be cleared if
SER_RS485_ADDR_RECV
is not set.Note: not all devices supporting RS485 support multipoint addressing.
6. References¶
- 1
Documentation/devicetree/bindings/serial/rs485.txt