Serial communications is the most popular and cheaper way of communicating between two devices. The obvious advantage is lesser number of conductors used. There are lots of
information available in internet, so we'll focus on our experiment without discussing the in-depth details.
Serial Interface
For communication between two or more devices there should be some well defined interface. In the world of Serial Communication it is specified by RS232. RS232 is defined by Electronic Industries Association (EIA). It defines the conventions to be followed for serial communication. At first, it is used for connecting telephone lines to the computer. RS232-standard specifies the voltage levels for MARK (HIGH) and SPACE (LOW).
With these hardware background now we are going to dive into software part. I'm using Ubuntu Linux-10.10 ( Maverick Meerkat ). Linux already has serial port drivers. For now we'll see how to use those drivers. I'll discuss how serial drivers works in a separate article.
In Unix/Linux everything is file. Yes, the serial port is also treated as file. So we can write and read the data to/from serial port just like a file.
The file names are as follows:
| System | Port-1 | Port-2 |
| Digital UNIX | /dev/tty01 | /dev/tty02 |
| HP-UX | /dev/tty1p0 | /dev/tty2p0 |
| IRIX | /dev/ttyf1 | /dev/ttyf2 |
| Linux | /dev/ttyS0 | /dev/ttyS1 |
| Solaris | /dev/ttya | /devttyb |
Coding:
#include <stdio.h> // Standard input/output definitions
#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 definitions
// function to open the port
int open_port(void)
{
int port;
//open the port and store the file descriptor in 'port'
port = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
if (port == -1){
// Could not open the port
perror("open_port: Unable to open /dev/ttyS0 - ");
}else{
fcntl(port, F_SETFL, 0); //leave this
}
return (port);
}
int main()
{
int port,n;
char str[30];
//termios - structure contains options for port manipulation
struct termios specs; // for setting baud rate
//setup part
port = open_port();
tcgetattr(port, &specs);
//now the specs points to the opened port's specifications
specs.c_cflag = (CLOCAL | CREAD ); //control flags
//output flags
//CR3 - delay of 150ms after transmitting every line
specs.c_oflag = (OPOST | CR3);
//set Baud Rate to 50bps
cfsetospeed(&specs,B50);
//our custom specifications set to the port
//TCSANOW - constant that prompts the system to set
//specifications immediately.
tcsetattr(port,TCSANOW,&specs);
//execution part
printf("\nEnter the data:\t");
scanf("%s",str);
n = write(port,str,11); // n = no of bytes written
if (n<0) {
printf("\nError");
}
//close the port
close(port);
return(0);
}
Here is mine. It is no a good practice to solder individual wore. Set of six wire sleeves are available in shops. Use them.
Connect the the Serial Cable to your PC and execute the program. Happy Blinking!!!
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