Raemixx500 is an Open Hardware remake of the Commodore Amiga 500+ mainboard, revision 8A.1, which fully passed all tests.

For more information please see https://github.com/SukkoPera/Raemixx500.

Differences from Original

The initial objective was to come up with a new mainboard as similar to the original one as possible, while including minor modifications that would improve its usability. This mainboard was designed with reasonable - not maniacal - accuracy to the original design. Most care was taken in the positioning of components that ought to be in a certain position (i.e.: screw holes and I/O connectors), to ensure drop-in replaceability. Other components and tracks are "more or less" there, but as the board was wholly laid out from scratch by hand, don't expect sub-millimeter accuracy.

Some modifications were soon added in, in the hope that they would be useful. Let's make it clear straight from the beginning that most of these improvements and modifications are completely optional and actually disabled by default. This means that you can just ignore them and everything will work as it did on the original A500+ board. If you want to take advantage of them instead, you will need to do some work.

Modifications introduced in V2

• Buffered Video Synchronization Signals: This will make the VSYNC and HSYNC signals going out from the DB23 connector somewhat "stronger", which should improve video quality and also allow using fully passive (i.e.: just wires) RGB to VGA cables (but please note that the signal will still be 15 kHz so a scandoubler will still be required with most VGA monitors). To enable this, cut both the JP99H/V jumpers, solder a 74HCT14 SOIC chip at U94 (yes, it's surface-mount, but the available space was limited) and a 100-330nF ceramic capacitor at C94. Note that if you want to get back to unbuffered (why???) you will both need to unsolder U94 and close the jumpers again, otherwise you will definitely damage your Denise and/or buffer. One small caveat: you'd better decide what to do with this mod before you solder the nearby components, as space around U94 is really tight and (un)soldering it with the other components in place is not a task for the faint-hearted.

• ESD Protection for the Joystick Ports: While connecting controllers blindly is still not recommended, you can add some diodes that should hopefully avoid damage to the internal chips whenever you do so. These won't hurt in any case, so why not? Just solder some AT1042 chips at U90-U93. Yes, they are small but you can make it.

• Hard Disk LED: If you use an internal expansion board that provides an IDE connector, you will feel the need for a LED flashing whenever the disk is accessed. If you connect a wire from pin 39 of your CF adapter to the new HDACT connector right in the middle of the board, the Floppy light will also flash when the hard disk is accessed. To get this to work, solder a 2n3906 transistor at Q99 and resistors R98 (10k) and R99 (4.7k).

• Alternative Footprint for the Line Filter: The original line filter (LF1) was probably a custom part, but in any case it's impossible to find these days. Luckily a functionally-equivalent part is available (Laird Z131B-10) but it has a different footprint, so the latter was added at LF99. Be very careful when soldering this, as it is very easy to make a solder bridge between the pads of LF1 and those of LF99 and short out the power rails.

• Alternative SMD Footprints for the JFETs: Q321/331 are a bit hard to find these days in the TO-92 package, so alternative surface-mount SOT-23 footprints were added (Q921/931). Choose the package you prefer and only solder one of each.

• More Accurate Dimensions: While V1 was already pretty close to the original board, the new version should be right on the money. Placement of the audio and mouse ports was also improved.

• More Detailed Silkscreen: The values of all resistors, capacitors and chips are now printed on the silkscreen, which makes the assembly process quicker and less error-prone.

• Improved Routing and Ground Plane: Some tracks were slightly altered in the hope of achieving improved signal integrity. Lower impedance to ground also resulted in all chips running a few degrees Celsius colder.

• One more change lies in the license: the project is now released under CC BY-NC-SA, which is not too different in practice from the license we were using before, except that any commercial use is now prohibited. I know this makes the project non-free, strictly speaking. While I am sorry for that, that's what the behaviour of the "community" has led me to. Blame them, not me.