Yet Another FDD Swapper for Amiga 500

This is my project of Amiga FDD swapper, based on ATtiny 404 MCU.

What it does?

It allows for swapping internal DF0: drive and DF1: external drive (by crossing SEL0 and SEL1 signals from MOS 8520 EVEN CIA). Holding Ctrl - Amiga - Amiga keys for at least 2 seconds switches between normal / swapped configurations. The last used configuration is automatically restored on power up (unless configured not to do so, see below). Switching is confirmed with 1 (for normal) or 2 (for swapped) beeps.

The project consists of 2 separate PCBs:

• a bigger one that goes under the EVEN CIA chip - responsible for SEL0 / SEL1 swapping.
• a smaller one that which goes between Amiga motherboard and floppy ribbon cable. This PCB allows for internal FDD to be detected as DF1 (it's optional).

This is how the boards look like assembled in my Amiga 500 Rev 6A.

Why another one?

Well, there are many FDD swapper projects already, all of these mods basically swap SEL0 and SEL1 of the Even CIA, using anything from simple mechanical switch to 74LS153 multiplexer (or even 4066 CMOS analog switch) driven by ATtiny MCU. Please note, that swapping SEL0/1 signals is not enough for internal drive to work as DF1:, additional circuitry is needed to drive ~RDY signal during the drive enumeration at boot time.

Looking for a solution, I've found Amiga Floppy Swap project, which is almost exactly what I need. However, I decided to make it slightly simpler:

• my project uses only a single chip, ATtiny 404 microcontroller, which costs below 0.50 EUR (at least at the time of this writing),
• the smaller board (which allows for internal drive being enumerated as DF1:) doesn't need to be connected with the bigger board. Actually it contains only 2 MOSFETs and 2 resistors and doesn't need any extra connections, beside the floppy flat cable (not even +5V supply!).
• it is configurable - one can chose if it beeps on every start (or only when changing the configuration) and which configuration (last used vs always swapped vs always straight) it should use upon powering up (see pcb_cia description for details).
• the smaller board is plugged into the mainboard (as opposed to between the cable and FDD) - this way possible collision with the EMI shielding sheet metal is entirely avoided.

Also, it's fun to make something new (even if reinventing the wheel).

Project files

This repository contains 3 main directories (look there for more details, assembly instructions, etc.):

• pcb_cia - KiCAD project of the bigger PCB which goes under the CIA chip.

• pcb_df1 - KiCAD project of the smaller PCB which goes between Amiga motherboard and the FDD ribbon cable.

• firmware for the ATtiny 404 MCU on the pcb_cia board (including programming instructions).