LEDs to test the use of the parallel port

LEDs to test the use of the parallel port:-

In this section I’ll detail the construction of a piece of hardware that can be used to visualize the state of the parallel port with some simple LEDs.

WARNING: Connecting devices to the parallel port can harm your computer. Make sure that you are properly earthed and your computer is turned off when connecting the device. Any problems that arise due to undertaking these experiments is your sole responsibility.

The circuit to build is shown in figure 3 You can also read “PC & Electronics: Connecting Your PC to the Outside World” by Zoller as reference.

In order to use it, you must first ensure that all hardware is correctly connected. Next, switch off the PC and connect the device to the parallel port. The PC can then be turned on and all device drivers related to the parallel port should be removed (for example, lp, parport, parport_pc, etc.). The hotplug module of the Debian Sarge distribution is particularly annoying and should be removed. If the file /dev/parlelport does not exist, it must be created as root with the command:

# mknod /dev/parlelport c 61 0

Then it needs to be made readable and writable by anybody with:

# chmod 666 /dev/parlelport

The module can now be installed, parlelport. You can check that it is effectively reserving the input/output port addresses 0x378 with the command:

$ cat /proc/ioports

To turn on the LEDs and check that the system is working, execute the command:

$ echo -n A >/dev/parlelport

This should turn on LED zero and six, leaving all of the others off.

You can check the state of the parallel port issuing the command:

$ cat /dev/parlelport

Figure 3: Electronic diagram of the LED matrix to monitor the parallel port

Final application: flashing lights

Finally, I’ll develop a pretty application which will make the LEDs flash in succession. To achieve this, a program in user space needs to be written with which only one bit at a time will be written to the /dev/parlelport device.

<lights.c> =

#include <stdio.h>

#include <unistd.h></p>

int main() {

  unsigned char byte,dummy;

  FILE * PARLELPORT;

                  

  /* Opening the device parlelport */

  PARLELPORT=fopen("/dev/parlelport","w");

  /* We remove the buffer from the file i/o */

  setvbuf(PARLELPORT,&dummy,_IONBF,1);

  /* Initializing the variable to one */

  byte=1;

  /* We make an infinite loop */

  while (1) {

    /* Writing to the parallel port */

    /* to turn on a LED */

    printf("Byte value is %d\n",byte);

    fwrite(&byte,1,1,PARLELPORT);

    sleep(1);

    /* Updating the byte value */

    byte<<=1;

    if (byte == 0) byte = 1;

  }

  fclose(PARLELPORT);

}

It can be compiled in the usual way:

$ gcc -o lights lights.c

and can be executed with the command:

$ lights

The lights will flash successively one after the other! The flashing LEDs and the Linux computer running this program are shown in figure 4.

Conclusion

Having followed this brief tutorial you should now be capable of writing your own complete device driver for simple hardware like a relay board (see Appendix C), or a minimal device driver for complex hardware. Learning to understand some of these simple concepts behind the Linux kernel allows you, in a quick and easy way, to get up to speed with respect to writing device drivers. And, this will bring you another step closer to becoming a true Linux kernel developer.

Figure 4: Flashing LEDs mounted on the circuit board and the computer running Linux. Two terminals are shown: one where the “parlelport” module is loaded and another one where the “lights” program is run. Tux is closely following what is going on

Bibliography

A. Rubini, J. Corbert. 2001. Linux device drivers (second edition). Ed. O’Reilly. This book is available for free on the internet.

Jonathan Corbet. 2003/2004. Porting device drivers to the 2.6 kernel. This is a very valuable resource for porting drivers to the new 2.6 Linux kernel and also for learning about Linux device drivers.

B. Zoller. 1998. PC & Electronics: Connecting Your PC to the Outside World (Productivity Series). Nowadays it is probably easier to surf the web for hardware projects like this one.

M. Waite, S. Prata. 1990. C Programming. Any other good book on C programming would suffice.