Amiga Floppy Swapper

Here is a small project that I wanted to get dob for quite a while. It is a little PCB that can be used to swap how the Amiga sees the floppies. In one state everything is normal, and if a jumper is set, then DF0: and DF1: are swapped, so that you can boot from an external floppy drive.

What makes this design differ from other designs is that it is controlled by a single jumper or a normal STSP switch. This makes it controllable via the Key-Tap, another one of my little projects.

How it works

A Floppy drive is controlled by a bunch of electrical signals, among which one signal is the most important, it is called /SEL. If this line is high, the drive ignores all other control signals, and if it is low, it responds to them. In the AMIGA, the even CIA chip generates four select signals /SEL0/SEL3 responsible for the drives DF0: to DF3: To swap the floppy drives we can electrically swap the /SEL0 and /SEL1 line and the AMIGA will not be aware that we changed its view of the world. In simple Floppy swapper PCBs this is simply done with a clever (DPDT) switch arrangement. Here we choose a dual 4-channel multiplexer chip 74HCT153 (here is the Nexperia datasheet) to do the work. While it seems overkill to use an IC where a simple switch would do, this has some advantages:

  1. We can control the multiplexer by one signal. This control signal is much simpler: If we ad a pull-up resistor it is sufficient to either let the signal be pulled up for one state or connect it to GND via a jumper or a SPST switch to get into the other state. Furthermore this signal can be generated by the Key-tap so that you do not need a mechanical switch at all.
  2. The select lines do not have to be run via long connections to a switch and back. This reduces the risk of short circuits, noise pickup and possible damage to the CIA.
  3. The 74HCT153 and a SPST switch is actually cheaper than a DPDT swich.

The main disadvantage is that you can not realize this circuit without a PCB. Fortunately, here is my design which you can use freely. KiCad schematics, PCB and the resulting Gerbers are in my Github repo here:

https://github.com/jmA500/amiga-goodies

Look into the folder floppyswap for the files for this project. I used the gerbers in amiga-goodies/floppyswap/Gerber/floppyswap-20210313-0042.zip to produce the PCB in the image above. Building it should be straigtforward, you need only a couple of passives, the socket for the CIA and pin headers to connect to the socket of the even CIA.

Look at the schematic above. The lines SEL0 and SEL1 are coming from the CIA. Since Eb and Ea are grounded, both outputs of the multiplexer are always enabled. Channel a controls the line connected to the internal DF0: and Channel b the select for the first external drive. The binary representation of the input at S0,S1 selects which input of the multiplexer gets connected to its output. Since we want the swapper to be inactive if the jumper is unset, the multiplexers are connected, so that SEL0 and SEL1 are unswapped if S0 is high (I3a and I3b are connected to Za and Zb respectively) , and swapped if S0 is low.

If you have a PCB, building and using should be straight forward. You only need a few passives together with the multiplexer and socket plus pin header for the CIA and connection to the Mainboard. All parts are in this list at reichelt.de.

Key-Tap: A hardware configurator for the Amiga 500

Update 2021/03: GIT repository available!

In this post I want to provide the details of an older project I published  in 2014 on a1k.org. This little piece of hardware is used to provide some means for configuring an Amiga 500 with the keyboard. This is to replace at least some of the small switches that are commonly used to configure the extensions in an Amiga. The circuit listens to the keys pressed on the Amiga keyboard and then adjusts some of its output pins. These pins connect to jumper headers on the extension boards and hence provide some means to configure these extensions without opening the case or drilling holes into it.
Note however, that this project is not for beginners. If you have other extensions than I have, you will likely have to adapt the firmware to your needs. Also, you have to figure out whether your extensions are configurable by means of an external MCU, that is if is suffices to drive a signal statically high or low. Switching a dynamic signal is not possible (without extra hardware). For example, I have seen some kickstart switches that use a switch to route an enable signal to either of the two ROMs.

In any case you need a possibility to program the AtTiny4313 micro-controller with an in system programmer (ISP). These are cheaply available and connect to USB or parallel port. A lot of information regarding this micro-controller can be obtained on the excellent websites http://www.avrfreaks.net/ or http://www.mikrocontroller.net/ (German).

In addition, if you are not running a Linux machine similar to mine there might be some extra difficulties adapting the code to your build environment. In particular I am not using AVR studio and this distribution does not include any project file for it. I am using avr-gcc, make and avrdude to build and flash the images. However, pre-built firmware images are included, if you are happy with the firmware out of the box.

The PCB files and the firmware are open source,  so build your own!

Continue reading Key-Tap: A hardware configurator for the Amiga 500