Andrew Greensted (Modified: 17 June 2008)
Electronics > PC Parallel Port

PC Parallel Port

Information on the PC Parallel port

• Parallel Port Hardware

The diagram below shows the pinout of the parallel port connector, when looking at the computer's connector.

The parallel port uses three registers, their bit assignment is shown in the diagram below.

Descriptions of the different bits are listed in the table below.

Pin	Reg Bit	Description	Direction
1	/C0	Strobe	Output
2	D0	Data Bit 0	Output
3	D1	Data Bit 1	Output
4	D2	Data Bit 2	Output
5	D3	Data Bit 3	Output
6	D4	Data Bit 4	Output
7	D5	Data Bit 5	Output
8	D6	Data Bit 6	Output
9	D7	Data Bit 7	Output
10	S6	Acknowledge	Input
11	/S7	Busy	Input
12	S5	Paper End	Input
13	S4	Select In	Input

Pin	Reg Bit	Description	Direction
14	/C1	Auto Feed	Output
15	S3	Error	Intput
16	C2	Initialise	Output
17	/C3	Select	Output
18	-	Ground	-
19	-	Ground	-
20	-	Ground	-
21	-	Ground	-
22	-	Ground	-
23	-	Ground	-
24	-	Ground	-
25	-	Ground	-

• Linux Parallel Port Code

Below are listed two options for controlling the parallel port under linux. The first option writes directly to the parallel port memory address. It's simple and works well, but because of the low level acess, you'll need to have root privileges. The second option, uses the ppdev module for user-space access, that means you don't need root privileges.

Compile either source file using the following command line.

> gcc cableTest.c -o cableTest

• File: cableTest.c

#include <stdio.h>
#include <inttypes.h>
#include <sys/io.h>
#include <signal.h>

#define BASE 0x378

int running = 1;

void signalHandler(int sig)
{
   running = 0;
}

int main(void)
{
   printf("Parallel Port Interface Test (Base: 0x%x)\n", BASE);

   signal(SIGINT, signalHandler);

   // Set permission bits of 4 ports starting from BASE
   int result = ioperm(BASE, 4, 1);

   // If setting permission failed, return
   if (result != 0)
   {
      printf("ERROR: Could not set permissions on ports");
      return result;
   }

   // This loop will keep running until ctrl-c is pressed
   while (running)
   {
      outb(0xFF, BASE);
      outb(0x00, BASE);
   }

   // Clear permission bits of 4 ports starting from BASE
   result = ioperm(BASE, 4, 0);

   if (result != 0)
   {
      printf("ERROR: Could not clear permissions on ports");
      return result;
   }

   return 0;
}

User-Mode Access

The code below is the user-mode version. It accesses the parallel port via the ppdev driver. You'll need to make sure that the driver is compiled into your kernel, or compiled as a module then loaded. The first command below can be used to check if the module is loaded, the second will load it.

> lsmod | grep ppdev
> modprobe ppdev

The ppdev kernel driver can be found in the location shown below.

-> Device Drivers
  -> Character Devices
    -> <M> Support for user-space parallel port device drivers

NOTE: Arguments to the ioctl function are passed by pointer, so be careful when using the functions. This is why mode, dir, dataH and dataL are defined first, then their addresses are passed to ioctl.

• File: cableTest2.c

#include <stdio.h>
#include <sys/ioctl.h>
#include <linux/parport.h>
#include <linux/ppdev.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>

int running = 1;

void signalHandler(int sig)
{
   running = 0;
}

int main(void)
{
   int fd;
   int result;

   printf("Parallel Port Interface Test\n");

   // Set ctrl-c handler
   signal(SIGINT, signalHandler);

   // Open the parallel port for reading and writing
   fd = open("/dev/parport0", O_RDWR);

   if (fd == -1)
   {
      perror("Could not open parallel port");
      return 1;
   }

   // Try to claim port
   if (ioctl(fd, PPCLAIM, NULL))
   {
      perror("Could not claim parallel port");
      close(fd);
      return 1;
   }

   // Set the Mode
   int mode = IEEE1284_MODE_BYTE;
   if (ioctl(fd, PPSETMODE, &mode))
   {
      perror("Could not set mode");
      ioctl(fd, PPRELEASE);
      close(fd);
      return 1;
   }

   // Set data pins to output
   int dir = 0x00;
   if (ioctl(fd, PPDATADIR, &dir))
   {
      perror("Could not set parallel port direction");
      ioctl(fd, PPRELEASE);
      close(fd);
      return 1;
   }

   char dataH = 0xFF;
   char dataL = 0x00;

   // This loop will keep running until ctrl-c is pressed
   while(running)
   {
      // Output some data
      // Note that data is passed by a pointer
      ioctl(fd, PPWDATA, &dataH);
      ioctl(fd, PPWDATA, &dataL);
   }

   printf("Shutting down\n");

   // Release and close the parallel port
   ioctl(fd, PPRELEASE);
   close(fd);

   return 0;
}

• Signal Termination

It is important that the data lines from the parallel port are properly terminated. The circuit below shows one of the many forms of cable termination. The resistor value, R, should match the characteristic impedance of the cable. The capacitor value is most easily found with trial and error.

The figures below show the difference between the unterminated and terminated signal. The component values used are those shown in the figure above.

Unterminated

Terminated