The Amiga 500 Rev.5 Kickstart upgrade.

A friend came over with his beloved Amiga 500 and wanted me to help him get Kickstart v 3.1 installed. It turned out to be a rev.5 motherboard.

The kickstart is basically a part of the Amigas OS, and the software is stored on ROM chips. The Amiga 500 has only one ROM, but other models might have two. You replace it by taking out the original and replace it with a new one. I just programmed a 27c400 EPROM for this.  (See my other post about my Kickstart adapter and how to use it.) A legal license can be bought at Hyperion. After programming the EPROM, its usually only a matter of swapping chips, but not on the older A500s.
The rev5 and older motherboards has a small signal routing bug in it, that the user usually never get any problems with unless they want to upgrade the Kickstart. If you look at the schematics from Commodore, you will notice they actually put “oops” in there, and thats why we used to call it “the oops bug”

You can rectify this bug by bending up pin 31 on the new ROM and connect a wire from pin 1 and pin 31 on the ROM. It is important that the pin 31 does not touch the socket. Next you have to solder yet another wire to keep the EPROM in 16-bit mode. This time its from pin 21 (VCC) to pin 31. As usual, I hate to hack OTHER peoples hardware like this, so I decided it was time to finally make a PCB for this bug. (my own A500 had to endure me just hacking up a socket for this bugfix. Shame on me.)

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This time I wanted to try using RED PCBs. Not bad at all. As you might have noticed there are more on this adapter board than only a socket. The old rev5 PCBs also has had a history of throwing out some random “guru meditation” errors and even some times they even refused to boot after upgrading the ROM. This was also true on mine. But what exactly was going on?

I decided to compare the schematics on a rev6 with the schematics on a rev5. I found that Commodore had added resistor nets on the data lines between the ROM socket and the CPU on the later revisions. So I decided to REALLY destroy the old socket solution on my own machine and added some pullups to the datalines. All the problems suddenly was gone. So I decided to put the “missing” pullups on the socket. After hours of testing  my friends and my own Amigas, both ran very stable.

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These are the parts needed. The correct resistor nets was really hard and expensive to source. I made a star on the PCB to match the “dot” printed on the resistor net so I wont forget the orientation.  The dot and star indicates the VCC pin, by the way. I printed SOCKET and Chip to remember where to put the “turned” pins (socket) and where to put the ROM (chip). In hindsight it would probably be better if I used the words Pins and Socket instead…

This project with PCB and parts can be purchased on ebay:  https://www.ebay.com/usr/kirsti_73
or from Sellmyretro.com