Development of serial port and industrial protocol under Linux

1. Serial Communication principle

Serial Communication definition

Serial communication: The serial mode of data transmission . Serial communication can be divided into synchronous communication and asynchronous communication .

Synchronous communication: According to the software to identify the synchronization characters to achieve data transmission and reception.

• Send many characters into a group of messages
• Requires the sending clock and the receiving clock to maintain strict synchronization
• Relatively high efficiency, high error on double clock

　　　

Asynchronous communication: A method of communication using the resynchronization technique of characters .

• Transfer by character one at a
• Each transmission of a character, using the starting bit to notify the other side, in order to re-check the transceiver synchronization
• High reliability, but low efficiency

　　

Synchronous communication is transmitted in frames, and asynchronous communication is transmitted by character. (Data link layer)

9-Pin serial header layout

　　

Important parameters of serial communication:

Baud rate, data bit, stop bit, parity

Test of serial communication:

Connect the COM1 and COM2 two serial ports, write the data "Hello" in the/DEV/TTYS0 device file, and then look at the/DEV/TTYS1 device files. Serial devices can be accessed directly using Read/write.

ttyS0 corresponds to com1,ttys1 corresponding to COM2.

2. Common API for serial programming

Serial Port parameter setting

Serial port programming needs to use the header file

1#include <stdio.h>/*standard input and output*/2#include <stdlib.h>/*Standard Function Library*/3#include <unistd.h>/*UNIX standard function definition*/4#include <sys/types.h>/*Defining data Types*/5#include <sys/stat.h>/*define state-related data types*/6#include <fcntl.h>/*file Control definition*/7#include <termios.h>/*POSIX terminal control definition*/8#include <errno.h>/*Error Number Definition*/9#include <string.h>/*String function function*/

Settings for baud rate

Cfsetispeed () Set the baud rate of the input port

Cfsetospeed () Setting the baud rate of the output port

Cfgetispeed () Gets the baud rate of the input port

Cfgetospeed () Gets the baud rate of the output port

struct Termios opt; Defines a pointer to the Termios structure opt

Termios Structural Body

#define NCCS 19struct termios{    tcflag_t c_iflag;     Input parameters    tcflag_t C_oflag;    Output parameters    tcflag_t C_cflag;    Control parameters    tcflag_t c_ispeed;  Input baud rate    tcflag_t c_ospeed;//output baud rate    cc_t c_line;           Line control    cc_t C_cc[nccs];  control characters};

Baud Rate Setting Instance

Common operation of Serial port

Fd=open ("/dev/ttys0", O_RDWR | O_noctty);

int read (int fd, *buffer, length);

int write (int fd, *buffer, length);

int close (int fd);

Serial Control Flow

• Set baud rate
• Settings for check bit and stop bit
• Open the serial port
• Send serial data
• Read serial data
• Close the serial port

3. Serial Programming Practice

4. Modbus Protocol Introduction

An application-level protocol for the control and communication of electronic controllers. Through this program, the controller between each other, the controller through the network and the device can communicate between its devices.

Two commonly used transmission modes of Modbus:

ASCLL mode: Each 8Bit byte in the message is sent as two ascll characters, and the character is sent at an interval of 1 seconds without generating an error.

RTU mode: At the same baud rate, more data can be transmitted than the ASCLL method. (more commonly used)

Comparison of the two modes

function code bit

Example: Modbus An example

Send instructions: F1 C3

<slave Address 1 byte><function 1 Byte><start address 2 bytes><numberofbytes 2 bytes>< Checksum 2 bytes>

Receive a reply : 0B 9A

<slave Address 1 byte><function 1 byte><numberofbytes 1 bytes><dataieee32 x Byte><Checksum 2 B Ytes>

Communication mode of Modbus communication protocol

5. Modbus Programming Practice

Development of serial port and industrial protocol under Linux