#include <stdio.h>
#include <fcntl.h> /* File Control Definitions */
#include <termios.h> /* POSIX Terminal Control Definitions */
#include <unistd.h> /* UNIX Standard Definitions */
#include <errno.h> /* ERROR Number Definitions */
#include <sys/ioctl.h> /* ioctl() */
void main(void)
{
int fd;/*File Descriptor*/
printf("\n +----------------------------------+");
printf("\n | USB To RS485 Write |");
printf("\n +----------------------------------+");
/*------------------------------- Opening the Serial Port -------------------------------*/
/* Change /dev/ttyUSB0 to the one corresponding to your system */
fd = open("/dev/ttyUSB0",O_RDWR | O_NOCTTY); /* ttyUSB0 is the FT232 based USB2SERIAL Converter */
/* O_RDWR Read/Write access to serial port */
/* O_NOCTTY - No terminal will control the process */
if(fd == -1) /* Error Checking */
printf("\n Error! in Opening ttyUSB0 ");
else
printf("\n ttyUSB0 Opened Successfully ");
/*---------- Setting the Attributes of the serial port using termios structure --------- */
struct termios SerialPortSettings; /* Create the structure */
tcgetattr(fd, &SerialPortSettings); /* Get the current attributes of the Serial port */
cfsetispeed(&SerialPortSettings,B4800); /* Set Read Speed as 9600 */
cfsetospeed(&SerialPortSettings,B4800); /* Set Write Speed as 9600 */
SerialPortSettings.c_cflag &= PARENB; /* Disables the Parity Enable bit(PARENB),So No Parity */
SerialPortSettings.c_cflag &= ~CSTOPB; /* CSTOPB = 2 Stop bits,here it is cleared so 1 Stop bit */
SerialPortSettings.c_cflag &= ~CSIZE; /* Clears the mask for setting the data size */
SerialPortSettings.c_cflag |= CS7; /* Set the data bits = 8 */
SerialPortSettings.c_cflag &= ~CRTSCTS; /* No Hardware flow Control */
SerialPortSettings.c_cflag |= CREAD | CLOCAL; /* Enable receiver,Ignore Modem Control lines */
SerialPortSettings.c_iflag &= ~(IXON | IXOFF | IXANY); /* Disable XON/XOFF flow control both i/p and o/p */
SerialPortSettings.c_iflag &= ~(ICANON | ECHO | ECHOE | ISIG); /* Non Cannonical mode */
SerialPortSettings.c_oflag &= ~OPOST;/*No Output Processing*/
/* Setting Time outs */
SerialPortSettings.c_cc[VMIN] = 1; /* Read at least 10 character */
SerialPortSettings.c_cc[VTIME] = 0; /* Wait indefinetly */
if((tcsetattr(fd,TCSANOW,&SerialPortSettings)) != 0) /* Set the attributes to the termios structure*/
printf("\n ERROR ! in Setting attributes");
else
printf("\n BaudRate = 9600 \n StopBits = 1 \n Parity = none");
/*---------------------------------------- Controlling RTS and DTR Pins --------------------------------------*/
/* ~RTS(USB2SERIAL) ---> ~RE(MAX485) */
/* ~DTR(USB2SERIAL) ---> DE(MAX485) */
/*------------------------- Putting MAX485 chip in USB2SERIAL in Transmit Mode ---------------------------*/
// //
// ----+ +-----------+ H +-----------+ //
// | | ~RTS| --------------> |~RE | //
// PC |==========>| FT232 | | MAX485 +(A,B)~~~~~~~~~~~~~~~>Data out(RS485) //
// | USB | ~DTR| --------------> | DE | Twisted Pair //
// ----+ +-----------+ H +-----------+ //
// //
//--------------------------------------------------------------------------------------------------------//
//TxMode - DE->High,~RE -> High
int RTS_flag,DTR_flag;
RTS_flag = TIOCM_RTS; /* Modem Constant for RTS pin */
DTR_flag = TIOCM_DTR; /* Modem Constant for DTR pin */
ioctl(fd,TIOCMBIC,&RTS_flag);/* ~RTS = 1,So ~RE pin of MAX485 is HIGH */
ioctl(fd,TIOCMBIC,&DTR_flag);/* ~DTR = 1,So DE pin of MAX485 is HIGH,Transmit Mode enabled */
/*------------------------------- Write data to serial port -----------------------------*/
tcflush(fd, TCIFLUSH); /* Discards old data in the rx buffer */
char write_buffer[] = "///?01!"; /* Buffer containing characters to write into port */
char write_buffer1[] = {'\r','\n'};
int bytes_written = 0; /* Value for storing the number of bytes written to the port */
bytes_written = write(fd,write_buffer,7);/* use write() to send data to port */
/* "fd" - file descriptor pointing to the opened serial port */
/* "write_buffer" - address of the buffer containing data */
/* "sizeof(write_buffer)" - No of bytes to write */
printf("\n %s written to ttyUSB0",write_buffer);
bytes_written = write(fd,write_buffer1,2);/* use write() to send data to port */
/* "fd" - file descriptor pointing to the opened serial port */
/* "write_buffer" - address of the buffer containing data */
/* "sizeof(write_buffer)" - No of bytes to write */
printf("\n %s written to ttyUSB0",write_buffer);
printf("\n %d Bytes written to ttyUSB0", bytes_written);
printf("\n +----------------------------------+\n\n");
char buf[20000];
sleep(2);
while(1)
{
printf("tring to read port\n");
int res = read(fd,buf,1);
sleep(1);
printf("%c :: %d\n",buf[0],res);
}
printf("\n +----------------------------------+\n\n\n");
close(fd); /* Close the serial port */
}
--------------------------------------------------------
设置运行代码：BAUD RATE为4800数据位为7，奇偶校验为EVEN，停止位为1。
如果我编译并运行代码，它将无法正常工作。在运行代码之前，如果我使用BAUD B4800和7E1打开和关闭小型通信，则我的代码有效。 任何人都可以提出问题所在，为什么它不起作用。