This document describes three hardware hacks necessary to make the Printer
Interface working. 

Some minor modifications are necessary if you want to connect the interface 
without these hacks, as there are:

1) You need an external +5V DC filtered and stabilized power supply, as the
parallel expansion port of the 800XL series does not provide +5V.

2) An additional address decoder is needed, to map the I/O chip into the
address space. A binary comparator like the 7485 or 74677 will suffer here.
The software expects the chip to appear in the range $d600 to $d6ff.

3) The printer driver software expects control of the MathPack-Disable signal
by software, thru an unused pin of the Atari-PIA. The software must be changed
to run without this hack, for example by assembling it with the TURBO flag
set to 1. The resulting driver will, however, be less stable and tolerant to
software since it is now very easy to overwrite it.


To apply the hardware hacks to your computer:

-Connect pin 47 of the parallel port (unused) directly with the incomming 
positive power supply. Don't connect it to the Vcc pin of some internal
chip.

-Connect pin 48 of the parallel port (unused) with the pin 9 of the chip
U2, a 74 LS 138. It is located in the roughly middle of the board. This chip
generates the address selection signals for all custom chips, pin 9 is set
low if an address in the range $d600 to $d6ff is accessed.

-Connect pin 43 of the parallel port (MathPack Disable) with pin 16 (PB6) of 
the internal PIA (6520A) chip, unit U23. This will disable the math package
if bit 6 of $d301 is set to low. Make sure that you don't connect any
hardware to the external port that disables the math pack itself, as this
might result in a short cut! This hack is only needed by the software, the
hardware will work without it.