This article describes how to connect a Commodore 64 compatible printer to
an IBM PC compatible computer.  The information originally appeared in an
article that I wrote for the Tech Forum section of the March 1995 issue of
Nuts & Volts magazine.  This information may be used and distributed so
long as proper credit is given to the author and Nuts & Volts magazine.
Copyright (C) 1995 by John Keyerleber and Nuts & Volts Magazine.

The problem with Commodore 64 compatible printers is that these devices use
the Commodore synchronous serial interface to communicate with the C64.  An
IBM PC compatible printer port is a parallel interface, however it is
possible to use this parallel port to emulate the C64 serial interface by
constructing a special cable and writing a little bit of software.

The cable is used to connect the PC DB25 pin parallel printer port to the 6
pin DIN serial port on the C64 compatible printer.  The parts needed are
one DB25 male connector (Radio Shack #276-1429), one 6 pin DIN plug (RS
#274-020), and some six conductor cable (RS #278-874 or equivalent).  Note
that the cable length should be kept to less than six feet.  The wiring
connections and signal descriptions for this cable are given as follows:

Commodore 64 DIN plug pin:    Parallel printer DB25 connector pin:
     1 (SRQ IN)                    2 (DATA0)
     2 (GND)                  20 (GND)
     3 (SER ATN)                   3 (DATA1)
     4 (SER CLK)                   4 (DATA2)
     5 (SER DAT)                   5 (DATA3) and 11 (BUSY)
     6 (RST)                  6 (DATA4)

The software uses the PC parallel port to emulate a C64 serial port.  The
example program, C64.C (Listing 1), is written in Borland's Turbo C.  It
reads input and sends each character to the printer.  It can be used as a
filter in an MS-DOS command line pipe.  For example, DOS commands such as:
dir | c64 would print a directory listing, or type a_file | c64 would print
the contents of a file.

I have also written an MS-DOS device driver which can be used to allow any
PC application to print to a C64 compatible printer, and it supports
multiple printers connected to a single PC parallel port.  The listing for
this software is too lengthy to print, however I will make this software
and assembled cables available to Commodore 64 enthusiasts for a nominal
charge; just write me for details.  With these C64 compatible printers
often selling for less than $25 at garage sales and flea markets, it's easy
to add a couple of additional "scratch" printers to any PC system.

John Keyerleber
26300 Chardonview Drive
Cleveland, OH  44143
(216) 261-9676
E-mail:  jkeyerleber@bailey.com

Listing 1:

#include <stdio.h>
#include <ctype.h>
#include <dos.h>

#define   PORTADR   0x378     // LPT1=0x378, LPT2 = 0x278
#define   PRINTADR  0x04      // C64 compatible printer address
#define   SRQ       0x01      // C64 SRQ IN signal
#define   ATN       0x02      // C64 ATN signal
#define   CLK       0x04      // C64 CLK signal
#define   DATOUT    0x08      // C64 DAT signal (output)
#define   DATIN     0x80      // C64 DAT signal (input)
#define   RST       0x10      // C64 RST signal

// function prototypes
void initc64(void);
void putc64(unsigned char);
void puts64(char *);
void putbyte(unsigned char, unsigned char);

// function main() loops to read input characters and send them to the printer
void main(void)
{
     char buff[128], *ptr;

     initc64();
     while(fgets(buff, 80, stdin))
     {
          for(ptr = buff; *ptr; putc64(*ptr++));
     }
}

// function init64() initializes the driver and the C64 compatible printer
void initc64(void)
{
     // set initial port output value to all bits high
     outportb(PORTADR, RST | SRQ | ATN | CLK | DATOUT);

     // set interface to output
     outportb(PORTADR + 2, 0x00);

     // initialize printer: RST=0, then RST=1
     outportb(PORTADR, SRQ | ATN | CLK | DATOUT);
     delay(250);
     outportb(PORTADR, RST | SRQ | ATN | CLK | DATOUT);
     delay(2250);

     // Set printer to listener: ATN and CLK low, printer will ACK with DAT low
     outportb(PORTADR, RST | SRQ | DATOUT);
     while(!(inportb(PORTADR + 1) & DATIN));

     // Send printer address (currently set for address 4)
     putbyte(0x20 | PRINTADR, 0);

     // Send printer mode
     putbyte(0x67, 0);
}

// function putc64() sends a byte of data (byteval) to the C64 compatible printer
void putc64(unsigned char byteval)
{
     // Check for upper/lower case conversion
     if (isalpha(byteval))
     {
          byteval = (isupper(byteval)) ? tolower(byteval) : toupper(byteval);
     }
     // Send character
     putbyte(byteval, ATN);
}

// function putbyte clocks the actual byte to the C64 compatible printer bit by bit
void putbyte(unsigned char byteval, unsigned char mode)
{
     register int bitloop;

     // Set CLK high, printer will ACK with DAT high
     outportb(PORTADR, RST | SRQ | CLK | DATOUT | mode);
     while(inportb(PORTADR + 1) & DATIN);

     // Loop to output bits of data
     delay(1);
     for (bitloop=0; bitloop<8; bitloop++)
     {
          outportb(PORTADR, RST | SRQ | ((byteval & 1) ? DATOUT : 0) | mode);
          delay(1);
          outportb(PORTADR, RST | SRQ | CLK | ((byteval & 1) ? DATOUT : 0) | mode);
          delay(1);
          byteval >>= 1;
     }

     // Set CLK low and DAT high for end of byte, printer will ACK with DAT low
     outportb(PORTADR, RST | SRQ | DATOUT | mode);
     while(!(inportb(PORTADR + 1) & DATIN));
}