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Take a look of this picture...shake your mind..and then start reading the whole article
Here is a circuit for using the printer port of a PC, for control application using software and some interface hardware. The interface circuit along with the given software can be used with the printer port of any PC for controlling up to eight equipment .
The interface circuit shown in the figure is drawn for only one device, being controlled by D0 bit at pin 2 of the 25-pin parallel port. Identical circuits for the remaining data bits D1 through D7 (available at pins 3 through 9) have to be similarly wired. The use of opto-coupler ensures complete isolation of the PC from the relay driver circuitry.
Lots of ways to control the hardware can be implemented using software. In C/C++ one can use the outportb(portno,value) function where portno is the parallel port address (usually 378hex for LPT1) and 'value' is the data that is to be sent to the port. For a value=0 all the outputs (D0-D7) are off. For value=1 D0 is ON, value=2 D1 is ON, value=4, D2 is ON and so on. eg. If value=29(decimal) = 00011101(binary) ->D0,D2,D3,D4 are ON and the rest are OFF.
here is a program
/*program to control devices using PC parallel port
The devices are controlled by pressing the keys 1-8
that corresponds to each of the 8 possible devices
*/
#include<dos.h>
#include<stdio.h>
#include<conio.h>
#define PORT 0x378 /* This is the parallel port address */
main()
{
char val=0,key=0;
char str1[]="ON ";
char str2[]="OFF";
char *str;
clrscr();
printf("Press the approriate number key to turn on/off devices:nn");
printf("Here Device1 is connected to D0 of parallel port and so onnn");
printf("Press "x" to quitnn");
gotoxy(1,8);
printf("Device1:OFF Device2:OFF Device3:OFF Device4:OFFn");
printf("Device5:OFF Device6:OFF Device7:OFF Device8:OFF");
while(key!='x' && key!='X')
{
gotoxy(1,12);
printf("Value in hex sent to the port:");
key=getch();
switch(key){
case '1':
gotoxy(9,8);
val=(val&0x01)?(val&(~0x01)):val|0x01;
str=(val&0x01)?str1:str2;
printf("%s",str);
outportb(PORT,val);
gotoxy(1,13);
printf("%x",val);
break;
case '2':
gotoxy(21,8);
val=(val&0x02)?(val&(~0x02)):val|0x02;
str=(val&0x02)?str1:str2;
printf("%s",str);
outportb(PORT,val);
gotoxy(1,13);
printf("%x",val);
break;
case '3':
gotoxy(33,8);
val=(val&0x04)?(val&(~0x04)):val|0x04;
str=(val&0x04)?str1:str2;
printf("%s",str);
outportb(PORT,val);
gotoxy(1,13);
printf("%x",val);
break;
case '4':
gotoxy(45,8);
val=(val&0x08)?(val&(~0x08)):val|0x08;
str=(val&0x08)?str1:str2;
printf("%s",str);
outportb(PORT,val);
gotoxy(1,13);
printf("%x",val);
break;
case '5':
gotoxy(9,9);
val=(val&0x10)?(val&(~0x10)):val|0x10;
str=(val&0x10)?str1:str2;
printf("%s",str);
outportb(PORT,val);
gotoxy(1,13);
printf("%x",val);
break;
case '6':
gotoxy(21,9);
val=(val&0x20)?(val&(~0x20)):val|0x20;
str=(val&0x20)?str1:str2;
printf("%s",str);
outportb(PORT,val);
gotoxy(1,13);
printf("%x",val);
break;
case '7':
gotoxy(33,9);
val=(val&0x40)?(val&(~0x40)):val|0x40;
str=(val&0x40)?str1:str2;
printf("%s",str);
outportb(PORT,val);
gotoxy(1,13);
printf("%x",val);
break;
case '8':
gotoxy(45,9);
val=(val&0x80)?(val&(~0x80)):val|0x80;
str=(val&0x80)?str1:str2;
printf("%s",str);
outportb(PORT,val);
gotoxy(1,13);
printf("%x",(unsigned char)val);
break;
}
}
}
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