CD32 Floppy Disk Drive Interface  V1.1

                         By Bruce Abbott



                            Description

 This circuit adds either an internal or external floppy disk drive to
 your CD32. It is wired directly onto the CD32 motherboard.


                             Features

 - Supports 1 internal drive, and 3 external drives.
 - Uses standard electronic parts (no custom chips).
 - Low cost.
 - Internal fitting.
 - Compatible with FMV cartridge.
 - Printed Circuit Board design provided.



                           How It Works

 The circuit is designed to emulate the disk drive controls of CIAA and
 CIAB. This requires an output port at address $bfd100, and an input port
 at $bfe001. Since The AKIKO chip already has an input port at $bfe001,
 we must disable AKIKO at this address, and take over all functions that
 this port monitors. This includes the the existing /FIRE0, /FIRE1, /LED
 and /MUTE. To these we add the required disk drive status signals  /RDY,
 /TRK0, /WPRO, and /CHNG. This is done with a 74LS541 8 bit tristate
 buffer (U1).

 To disable AKIKO we need to pull down the PUNT line (using diode D1).
 When we do this AKIKO will stay off the bus. When the /DS (data strobe)
 goes low we assert /DSACK1 (with U4 & U7), which tells the CPU that data
 is ready for reading.

 The output port at $bfd100 is also present in AKIKO, but only as an
 internal register. We need to supply an external register (U2) in
 parallel with this, which will generate the disk drive control signals.
 Since the register will power up in an unknown state, it is necessary
 to disable the outputs until the first time the processor accesses the
 port. This is done by using a flip-flop (2nd half of U6) connected to
 the output enable line.

 The rest of the circuit is quite simple. U5, U3 and part of U4 select the
 correct addresses for the ports. A disk drive motor latch and ID circuit
 is formed from U6, U4 and U7 (this is required for a bare internal drive
 only. External drives already have this circuitry). Finally, the write
 enable signal must be inverted.


                             Limitations

 Suppporting the INDEX pulse would require a complex circuit to generate
 an interrupt. Luckily, this signal is ignored by the system when using
 Amiga disks, so we can get get away with not providing it. Unfortunatey
 this does not apply to MSDOS formatted disks, which need the INDEX to
 determine the start of a track. Some hardware banging games also use it
 (eg. Cool Spot), and therefore will not work with my interface.



                             Installation

 I have designed a printed circuit board that fits completely inside the
 CD32, connecting to the motherboard via the Kickstart ROM. A few extra
 wires also have to be soldered onto the motherboard. It is possible to
 mount a disk drive inside the CD32 case, in the space reserved for an
 FMV cartridge.



                           Legal Mumbo Jumbo

 I am placing this project into the public domain, so you are free to do
with it whatever you see fit.




                           Bruce Abbott
                           604 Buller Street
                           Hastings 4201
                           New Zealand

                    email: bhabbott@inhb.co.nz