The Amiga Museum

The first “big box” Amiga, full of the new Zorro 2 expansion slots and a Video slot…  Released in 1986, this model was only produced until 1987, when it was replaced with the Amiga 2000-B.  This was the first Amiga to have Kickstart in a ROM chip, meaning that the Kickstart Disk was no longer necessary in order to use the machine, it came with the then new Kickstart 1.2.  This model is identical on the outside to the Amiga 2000-B, but on the inside we consider it different enough to have it’s own dedicated page here at The Amiga Museum.

This is the original (rev 4 – just 4, not 4.0 or 4.1 etc) Amiga 2000 mainboard, designed by a team at Commodore in Germany in 1986. About 60.000 Amiga 2000’s were sold with this mainboard.
It was soon replaced by the “cost reduced” Amiga 2000-B (rev 4.x) board, designed by Commodore US and Dave Haynie, which nonetheless preserved the original layout/form factor (positions of slots, ports, connectors etc).

Component-wise it’s very different from the US rev 4.x (which is based around the Amiga 500 rev5 design).
This one however is based upon the Amiga 1000. Actually, it’s just the Amiga 1000 and the concept Zorro II backplane add-on integrated together onto a single mainboard.
Therefore it inherits most of the Amiga 1000‘s characteristics. It uses the same “thin” Agnus chip as the Amiga 1000, so no option to upgrade to a 1 or 2 MB capable Agnus exists, it has no “Gary” chip but uses discrete logic for its functions and instead of using the Buster chip to arbitrate Zorro II access it uses a PAL-based approach.

Compared to the later Amiga 2000-B motherboards, it’s of higher quality – it uses a 4-layer PCB design vs a 2-layer design used by the US models.
Actually, part of the “cost reduced” nomenclature is due to the transition to 2-layers, the other part being higher circuitry integration in the form of dedicated chips: The functions of Gary (address decoder, bus arbiter, floppy motor control, reset), Buster (Zorro II bus arbitration) and Fat Agnus (local DRAM controller) are implemented in more expensive discrete logic in the original 2000-A, just like on the Amiga 1000.

Picture of the Amiga 2000-A mainboard (will open in a new tab).

But in other comparisons it comes short:
– ROM socket has the wrong pinout (like in Amiga 500 rev3/5) so it only takes 256 KB ROMs (Kickstart 1.2 and 1.3) without modification. It originally shipped with Kickstart 1.2.
– The CPU slot (named the MMU slot here) is limited in function compared to the US models, and typically the 68000 must be removed when a processor upgrade card is installed there, eliminating a “fallback mode” possibility.
– The video slot is also limited – no digital RGB or other signals exist there, just the ones already to be found on the external analog RGB connector. Scandoublers/flickerfixers which rely on the 12-bit RGB signals will not work.
– The Zorro II subsystem is not as stable, some cards may not coexist as well as they do on later models.
– No option for $C00000 “slow” RAM, although there is an official Commodore card for the MMU slot which provides (among other possibilities) true Fast RAM at the same $C00000 address (so it won’t conflict with the Zorro II expansion 8 MB space. The “slow” or “ranger” 512 KB RAM expansion on the Amiga 500/Amiga 2000-B was mapped to the same address to emulate this).
– No composite video out (either colour as in the Amiga 1000 or monochrome as in later Amiga 2000-Bs).
– No option to turn the audio filter off (just like in the Amiga 1000 and early rev3 Amiga 500).

Also noteworthy is that many of these (rev4 based) Amiga 2000-A systems came with a Cherry keyboard (small function keys, red Amiga keys) with a Philips MCU instead of the 657x MOS ones universally found thereafter. These keyboards typically don’t work with the PC Key (or equivalent) adapters.