How I added an A2000-style CPU slot and Zorro II expansion slots to the A500
by Daniele Gratteri - Modena, ITALY
- About me
I am a 19 years old boy who graduated as an electronics and telecommunications technician on June 2004 and, now, I am studying electronic engineering at the university of my city.
I have been interested in electronics and in computers for many years. My first computer was a Commodore VIC 20, I received it in the 1990 as a gift from a cousin of mine who had bought the Amiga 500. Three years later, in 1993, my father bought me my first Amiga, an A600. I used the computer mainly for gaming and word processing. I remember that, when Windows 95 came out, I didn't know what it was :-)
I bought my first PC clone in 1997, it was a 486. I bought it because, at school, we started to program with the LOGO language and I was unable to find LOGO for my Amiga. A year later, I went on the Internet for the first time, using a 28.800 bps modem. On the Internet I learnt that the Amiga wasn't died, so I got interested in that platform again, thanks to the Italian Retrocomputing community too.
Now I own several Amiga computers: the A600, an A1200, an A4000T, a CDTV and a pair of A500 motherboards...
- The hack
Another Italian, Gabriele Peterle, managed to hack is A500 into a tower and he added to it a Picasso II video board, thanks to the hacked slot. You can find his project on Aminet (AddZorro2slot.lha). When I discovered this, I wondered about making an A500 with not only one Zorro II slot, but a complete busboard and, last but not least, an Amiga 2000-style CPU slot, for adding much more powerful turbo boards.
A friend of mine, Antonio, had already built a working Zorro II hack, following Gabriele's project and an Amiga Format article. He also tried to build a CPU slot for using A2000 boards but it didn't worked. So I started to build my own CPU slot board and managed to get it working with his Hardital Super Big Bang accelerator. After that, I modified his CPU slot board too. This was during the year 2002.
I was without an A2000 accelerator board, so I started to look on eBay for buying one. I wanted to buy a Commodore A2630 because I thought that, being Commodore, it would be more compatible, but all the boards I found were too expensive. At the end, I bought a GVP G-Force 040 accelerator with 12MB of RAM for just 30 Euro. When I received it, I tried to use it on my CPU slot but it refused to work, so I sent the G-Force to a friend for testing it.
After a while, I got my GVP card tested and, so, I had to start to modify the CPU slot board since, of course, the problem was there. Following the Amiga B2000 schematics, I added some integrated circuits for managing signals that the A500 expansion slot don't have and, so, I finally managed to get the G-Force working! With cache problems, but it worked, and it surely was a lot faster than a stock 68000 A500!
I also added 74LS245 chips for buffering the signals going to Zorro II slots. The most important thing, however, was the installation of a BUSTER, the controller chip used in the B2000 for proper operation of the Zorro II bus. Thanks to this chip, it is possible to use more than one card which used DMA (Direct Memory Access), if I have correctly understood the function of this integrated circuit.
So, after a lot of wire wrapping (I really don't know how many meters of wire I used, but they have to be a lot!), I finished my board and I was ready to connect an A4000 daughterboard, also bought on eBay, for using its Zorro II slots. I decided to use an existent A4000 board because I thought: why should I build my own riser card, when I can buy a better one? Also, an A4000 daughterboard comes with ISA slots, for BridgeBoards, and a video slot: it would be nice to have a working video slot on an A500, wouldn't it?
It was the 30th of October, 2004. I connected the CPU slot, together with the Zorro II slots, to an Amiga 500+ motherboard. I obviously installed the GVP G-Force, overclocked from 33 to 40MHz, and a couple of Zorro II boards, an A2091 SCSI controller and an A2088 bridgeboard. POwered the system and... IT WORKED! It booted OS 3.1 from a huge SCSI drive, connected to the G-Force, and various utilities showed me the Zorro II boards as properly configured!
After that, I made some photos to the setup. I also tried to remove the accelerator board for checking if Zorro II cards would work also in 68000 mode, but it seems they actually don't: maybe I missed some connections and an accelerator is required for taking the signals to the expansion boards, as a sort of "dongle". However, it isn't a major issue for me!
The "implementation" of Zorro II I made for my A500 is far from being 100% complete, in fact it is missing connections of various interrupt lines to the boards: this is because I noticed that all of the boards I own actually don't use these interrupts and, so, I decided not to complete that section.
That's all! I hope that, sooner or later, I will be able to make a printed circuit board out of this project so that, perhaps, the problem with the CPU caches will disappear.
- Do it yourself!
I wasn't able to draw a schematic because I have been quite busy recently. Also, I really didn't use any particular schematic for building my board: I just used A500 and B2000 schematics found on the Internet, and the Italian version of the Amiga Hardware Manual.
So, what I suggest to the people who may want to try to do this hack is to download the schematics of the two computers, compare them, accurately check the signals of the various connectors and how they are managed (I mean, if a signal is generated using some additional chip, and so on) and then... TRY! Try to build the board! If anything goes wrong, at worst the computer won't boot but it shouldn't be damaged. Be sure not to short circuit the voltages, of course!
- Useful links
Amiga Schematics and Manuals http://4x4.alphalink.com.au/schematics
Gabriele "RAY" Peterle Web Page http://digilander.libero.it/PeterGabriel
- The end
I hope this article was interesting for you. Special thanks to all the people who helped me with this design.