HIGH DENCITY DRIVES ON THE ST

This document outlines the steps necessary to connect a PC High
Dencity drive to the ST.  It will allow somebody with a basic
understanding and experience of electronics and the ST to go ahead
and do the mods but it is not a step by step set of instructions.

The upgrade is not difficult but you will need some knowledge of
electronics and the inside of your ST.  Think about it carefully
because you're on your own if it goes wrong.

CONNECTING THE DRIVE

Connecting a standard 1.44 MB drive to your ST is not a problem.
You will simply need to buy an HD drive and put it in the place of
your internal drive.  The problem?  Well it will only be able to
read/write 720K disks.

MODIFYING YOUR ST

The floppy disk drive controller chip for the ST is the WD1772 chip.
This chip has a clock input on pin-8 of 8 MHz to provide the correct
stepping pulse for DD floppies.  This will also work with DD
floppies in HD drives.  However, high dencity disks require double
the stepping pulse and therefore the clock input needs to be changed
to 16 MHz in order to read and write them.  When an HD drive is
using a DD disk though, it still needs 8 MHz clock for the
controller chip.

What needs to be done it to provide a means of detecting whether the
disk HD.  If it is then the standard clock input must be doubled to
16 MHz.

High dencity drives detect whether the disk is HD by looking for the
extra hole.  If the disk is high dencity then the HD detect line
connected to the drive is set low (high for DD).  This HD detect
line is pin-2 on the 34 way drive connector cable.

This means that if we have a 16 MHz clock generator we can use the
HD detect line to select either our new 16 MHz clock and apply it to
pin-8 of the floppy disk controller or use the existing 8 MHz clock.
To do this we need a couple of CMOS (must be CMOS to operate at
16MHz) logic chips and a multivibrator.  Pin 8 of the disk
controller must be carefully de-soldered and lifted off the track.
A jump wire is then attached to the track to get our 8 MHz input.
The lifted pin should then be hooked up to the output of our
circuit.  A suggested logic schematic is represented below.

 16 MHz    -----|--------|
                |  NAND  |-----------------|
 HD detect -----|--------|                 |
            |                              |---|-------|
            |---|--------|                     |  XOR  |--- Pin 8
            |   |  NAND  |--|              |---|-------|  (WD1172)
            |---|--------|  |--|--------|  |
                               |  NAND  |--|
 8 MHz   ----------------------|--------|


NOTES AND WARNINGS

The WD1172 chip is only designed for 8 MHz operation and so,
inevitably, some will fail at 16 MHz.  There is no way of telling if
yours will work beforehand.  If the chip does fail you will have to
order an STE Ajax chip from Atari (tel. 0753 533344).  This new chip
does work at 16 MHz and will replace your old failed chip.

If you have two floppy drives then, unless you make other
alterations, they must both be HD as DD disks will not work reliably
with 16 MHz clock.

Lifting the pin on the disk controller chip is a delecate job and
there is a high risk of damace to the pin, chip or track.  You
should drfinitely use a heat sink clipped onto the pin to prevent
heat damage to the chip.  Perhaps it would be easier to de-solder
the whole chip and insert a doctored socket to plug it back into.