Serial communication

[ffusion]

Hello! I'm not sure if this goes here because is my first thread. I have a problem with some program that tries to send and receive info through serial port (virtual). I need to send 2 commands and I can do it properly but then I need to read info buf the program gets blocked...

The code is here

#include <stdio.h> // standard input / output functions
#include <string.h> // string function definitions
#include <unistd.h> // UNIX standard function definitions
#include <fcntl.h> // File control definitions
#include <errno.h> // Error number definitions
#include <termios.h> // POSIX terminal control definitionss
#include <time.h>   // time calls

int open_port(void)
{
	int fd; // file description for the serial port
	fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY);
	return(fd);
}

int configure_port(int fd)      // configure the port
{
	struct termios port_settings;      // structure to store the port settings in

	cfsetispeed(&port_settings, B19200);    // set baud rates
	cfsetospeed(&port_settings, B19200);

	port_settings.c_cflag &= ~PARENB;    // set no parity
	port_settings.c_cflag &= CSTOPB;     // 2 stop bytes
	port_settings.c_cflag &= CREAD; 
	port_settings.c_cflag &= CLOCAL;
	port_settings.c_cflag &= ~CSIZE;     //8 bites of data 
	port_settings.c_cflag |= CS8;      

	tcsetattr(fd, TCSANOW, &port_settings);    // apply the settings to the port NOW
	return(fd);

}

int read_info(int fd)   // query modem with an AT command
{
        char datos[10];
	char command = 0x53, address=0x52,size=0x07;
	sprintf(datos,"%c%c%c", command,address,0x57);
	write(fd,datos,3); 
	sleep(1);
	printf("Frist write issued");
	command = 0x54, address=0x53;
	sprintf(datos,"%c%c%c", command,address,size);
	write(fd,datos,3); 
	printf("Second write issued");
	int rd = read(fd,datos,7);//Here is the trouble!
	int i;
	for(i=0;i<10;i++) {printf("Datos[%d]: %c\n",i,datos[i]);}

}
int main(void)
{
	int fd = open_port();
	configure_port(fd);
	printf("PORT PREPARED!\n");
	read_info(fd);
	return(0);
}

I hope somebody can tell me what I'm doing wrong... and I have to say that the writing was okey so the parameters seem to be right too.

[JohnGraham]

You mean it blocks indefinitely? For a non-blocking file descriptor to a terminal in canonical mode (which is what you have) it will block until it reads a complete line. Is the end you're reading from sending a

[ffusion]

Yes, it blocks forever. Maybe I'm using something wrong so I'll explain what the program is doing...

The frist command says "I'm going to check your status", the second says "Adress 0x53 read 7 bytes" and then the device fills its buffer with the info I need to get, so I read it this way. Probably the problem is that, as soon as it's not a modem, it dosen't send carriage return, could be that?

[JohnGraham]

Possibly - like I said, unless you tell the device otherwise, it will wait for a complete line of text before passing you the data.

On the other hand, I also notice you don't check the return values of any of the functions you call for errors - this is a programming error. For all you know, you may simply not have permission to open the device. Also, your comments say you're using AT commands, but all the AT commands I've used need carriage return/line feed after them, but you don't seem to be sending them.

If all else fails, have you tried using minicom to send the commands directly?

[ffusion]

Well, you are right, I don't check errors properly but I know the device recieves them because a led blinks when you do it,by the way I will fix that later. The device is a USB to I2c converter that works as a virtual COM port so I use commands especific to this device, not AT. By this reason I don't need to send carriage return to end a command, only to follow the proper format.

So the problem seems to be that the read function is expecting carriage return to end transimision and unblock, do you know how can we solve it?

Thx for your help.

[ffusion]

Any suggestions? Please!

[JohnGraham]

Have you tried disabling canonical mode? To do this, clear the ICANON flag in c_lflag before setting the port settings. Other than that, get a serial comms program like minicom and see what it shows you about what's happening on the wire.