This article describes a simple way to interface the ADC0804 8-bit analog-to-digital converter IC to the parallel port of an IBM-compatible PC. It largely superceeds my old page on how to interface the (now obsolete) ZN448E.

The ADC0804 from National Semiconductor is a simple, cheap, easily-available 8-bit ADC. By default, the analog input range is 0 to 5V, but by using an external reference, and/or lifting the Vin- pin above GND, a wide range of input levels can be catered for. With a maximum sampling rate of 8kHz, low-quality audio work could be attempted.

See the manufacturers information and datasheet on the ADC0804 at www.national.com/pf/AD/ADC0804.html, and also my Introduction to parallel port interfacing.

Simplest ADC interface circuit

The IC draws only 1mA power, so the 5V supply can be taken from a 78L05 regulator and a PP3 battery or wall plug-in PSU. In fact, the IC still seems to work with no external power, owing to currents flowing down the signal lines from the PC (however this is not recommended, especially for accurate measurements)!

                       +5v --@--         ---                 parallel
                             |          |   |                 port
                             |          |   v GND             
                             |          |                    'D'          reg.
			--------------------------           pin  name    bit
		       |     20         1         |
		       |    Vcc        -CS        |
		       |                   -intr 5|--->----- 15  -Error   S3+
		       |                          |
		       |                    -RD  2|---<-----  1  -Strobe  C0-
      analog           |                    -WR  3|---<----- 16  -Init    C2+
      signal  ----->---|6 Vin+                    |    MSB
      0 - 5v           |                     D7 11|--->-----  9  D7       D7
		       |                     D6 12|--->-----  8  D6       D6
		   ----|7 Vin-               D5 13|--->-----  7  D5       D5
		  |    |                     D4 14|--->-----  6  D4       D4
		  |    |                     D3 15|--->-----  5  D3       D3
		  v    |        ADC0804      D2 16|--->-----  4  D2       D2
		 GND   |                     D1 17|--->-----  3  D1       D1
		       |                     D0 18|--->-----  2  D0       D0
		       |                          |    LSB
		NC ----|9 Vref/2                  |
		       |                          |
		       |   Clk   Clk    An   Dig  |
		       |    R    in     GND  GND  |      --- 18  GND      N/A
		       |   19     4      8    10  |     |    -25
			--------------------------      |
                            |     |      |    |         |
                        10k| |    |      |    |         v
			   | |    |      |    |        GND
			    |	  |      v    v
			     -----@     GND  GND
				  |
				  |
			    150pF---
				 ---
				  |
				  v
				 GND

// Program to collect data from a parallel port ADC,
// 8-bits at a time using simple bi-dir protocol.

// Reading data from ADC0804LCN
// (NB Circuit will self-power off port!!!)


// (C) W.A. Steer 1998


#include <iostream.h>
#include <dos.h>

// Some (symbolic) constants...
const c0=1, c1=2, c2=4, c3=8;
const s0=1, s1=2, s2=4, s3=8;

void main()
{
 // Set the port address variables etc.
 unsigned base, data, stat, ctrl, eppdata;

 base=0x278;  // 0x278 for LPT2
              // 0x378 for LPT1

 data=base; stat=base+1; ctrl=base+2; eppdata=base+4;

 unsigned char value;
 int lc;


 for (int sample=0; sample<3000; sample++)
 {
  // To read a value from the ADC...

  // Initialise ADC conversion (pulse C2+, 'D' pin 16, LOW momentarily)
  outportb(ctrl, 32);
  outportb(ctrl, 36);

  // wait until conversion complete (until S3+, 'D' pin 15, goes HIGH)
  lc=0; do { lc++; } while (((inportb(stat) & s3)==0) && (lc!=256));

  if (lc==256) cout << "Timed out. (Hardware not connected/powered/functioning?)" << endl ;
  else
  {
   outportb(ctrl, 37);  // set C0- (set 'D' pin 1 LOW), enabling ADC output
   value=inportb(data);
   outportb(ctrl, 36);  // unset C0- (set 'D' pin 1 HIGH), disabling ADC output

   cout << "ADC value: " << (unsigned int)value << endl;
  }

  // end of ADC-read procedure
 }

}