C++ Serial Port Only Responding Once Using Write()

Question

All the code below works. My device responds, C,7 is a reset. When I run this the second time it doesn't respond. If I manually turn my device off and on, then run this script again it works. But not if I press the button to run the script the second time.

RS232: 57600,8,N,1

Any ideas?? Is there any more information needed to solve this?

*Also when I get this working I'm going to have to use the read() function to get the devices responses. Does anyone know the correct format I need to use, based on the below code? Sorry I'm new to C++...I'm more of a PHP guy.

*I also don't know if 1024 is right, but it seems to work so eh...

Thanks so much!

#include <termios.h>

int fd;
struct termios options;
fd=open("/dev/tty.KeySerial1", O_RDWR | O_NOCTTY | O_NDELAY);
fcntl(fd, F_SETFL, 0);
tcgetattr(fd,&options);
options.c_ispeed=57600;
options.c_ospeed=57600;
options.c_cflag |= (CLOCAL | CREAD);
options.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
options.c_cflag &= ~CSTOPB;
options.c_lflag &= ~ECHO;
options.c_oflag &= ~ECHO;
options.c_oflag &= ~OPOST;
options.c_cflag |= CS8;
options.c_cflag |= CRTSCTS;
options.c_cc[VMIN] = 0;
options.c_cc[VTIME] =10;
tcflush(fd, TCIFLUSH);
tcsetattr(fd,TCSANOW,&options);

write(fd, "C,7\r\n", 1024);

close(fd);

Answer 1

The 1024 may in fact be your problem. The third paramter to the write() function indicates the number of bytes to be written:

ssize_t write(int fildes, const void *buf, size_t nbyte);

See the man page for write() for details.

In your case, the number should be 5, since you are sending 5 characters ('C' ',' '7' '\r' and '\n').

By providing a value of 1024, you are actually sending another 1019 garbage characters over the serial channel.

update:

The read() function has almost the same arguments:

ssize_t read(int fildes, void *buf, size_t nbyte);

Note that you must provide a writable buffer as the second parameter. For example, to read 12 bytes you would use:

char someData[12];
read(fd, someData, 12);

I'm not quite sure how to determine the number of characters you need to read, but the ssize_t number returned by the function will tell you how many were actually read.