The Amiga 500 is my favorite vintage Computer and I a building a lot of extensions for it. Originally, it comes with 512k of internal memory. While this seems to be a lot in comparison to the 64k of a Commodore C64 , nowadays it is pretty tight. Especially, if you want to use newer C-compilers, 512k are not sufficient. Hence we build a memory expansion for the trapdoor slot. The advantages are that this is pretty easy to do and does not require much equipment. This project is Open Hardware, schematics, PCB-layouts and GAL Code will be available for download.
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!
This post describes how I repaired an A501 Trapdoor memory expansion. I explain how to diagnose the error and some tricks how this can be done with just an Amiga 500. The symptoms are that the Amiga does not boot and indicates a yellow screen.
The batteries used for the backup voltage for the real time clocks and SRAM settings in vintage computers are a real nuisance today. These were used in nearly all personal computers back in the 80s and beginning of the 90s and every single one started to leak and damage the PCB of the surrounding system. In the Amiga 500 they are used for the clock on trapdoor memory expansions. If you have one with such an ancient battery, it should be removed immediately. Here I describe, how a standard CR2032 coin cell holder can be fitted in place of the old battery to keep the clock running. Continue reading Changing the battery on a memory expanstion to a CR2032 battery
This Blog will document some of my Electronic Projects I am working on or have finished. I am mainly interested in creating new hardware for vintage computers to improve the fun and usability.