The AMIGA 2000 was launched in 1986 and was aimed at the professional productivity market. The original version was designed in germany, and was actually based on the the AMIGA 1000, not the AMIGA 500. However, later that year Commodore US redesigned it, and the new model (referred to as AMIGA 2000-B) was indeed the high-end version of the AMIGA 500 that the AMIGA 2000 is known as. There are probably more variations and revisions of this model than any other Amiga. Not only were at least 15 separate motherboard revisions produced, the model spans 3 versions of Kickstart and two chipset generations. Several sub-models (AMIGA 1500, AMIGA 2000HD and AMIGA 2500) were also produced. It is usually possible to tell the difference between the OCS and ECS versions of the AMIGA 2000 by the colour of the front panel LEDs. The OCS versions have a red power LED and a green HD LED, while the ECS versions have a green power LED and an orange HD LED. The easiest way to separate an AMIGA 2000-A (the AMIGA 1000-based german version) from the later, AMIGA 500-based models, is to see if there is a composite connector beside the RCA sound connectors at the back. If there is none, it's the original version. All Amiga 2000s were sold with 1MB of RAM, but how this was implemented varied over motherboard revisions. The German (model A) designs have 512K chip RAM on the motherboard and 512K Fast RAM on an expansion card fitted to the CPU Fast slot. The US 4.x (model B) motherboards had 1MB of RAM soldered to the motherboard with 512K configured as CHIP RAM and 512K as Fast. The Fast RAM on these models can be reconfigured into Chip RAM by upgrading the Agnus and cutting some connections). Rev 6.x ("model C") boards have 1MB of RAM soldered to the motherboard, all of which is configured as CHIP RAM.

Early AMIGA 2000's were fitted with a Buster Tower which sits in the buster socket. This is because the buster was faulty, it was missing a term signal in one of the backplane buffer control signals. The tower contained an additional PAL which fixed this problem.

Motherboard revisions according to Dave Haynie (Commodore Engineer)

"The AMIGA 2000 Model A was a german design, like an AMIGA 1000 in a different box. 512K CHIP and non-standard processor and video slots. Yes, The orginal AMIGA 2000 was designed in Germany. It was based on an integration of the AMIGA 1000 motherboard design and the example Zorro II backplane from "Schematics and Expansion Specifications", the AMIGA 1000 hardware manual. It used the thin Agnus, which handled only 512K of DRAM. They added a "Genlock" slot, which was essentially just the 23-bit video signals on an internal connector, and the "MMU" slot, which was essentially just the AMIGA 1000 external edge connector on an internal slot. The machine shipped with 512K of Fast RAM in this slot, though the case was slow and had some reliability problems. About 60,000 of these machines were made, so I've heard.

AMIGA 2000-B Redesigning The German Model:

This was the original AMIGA 2000-CR (Cost Reduced) that I designed. I took the AMIGA 500 chip set and integrated most of the control logic for the expansion bus into a gate array called Buster (the original thin Buster). This board was code-named the "B2000", not out of any special letter-sequencing, but because it was made from some AMIGA 500 parts -- the AMIGA 500's code-name was the "B-52".

To the German design, I added a second video connector, which brought out the remaining 8-bits of digital the video signal and some parallel port lines (just in case someone needed a way to control something out there). I also designed a "coprocessor interface" for the internal CPU slot, which allowed processor cards to be added without the need to remove the 68000. And of course, given the much higher integration (thanks due to Buster, Gary, Fat Agnus, etc), I was able to fit 1MB of RAM on the motherboard, even though I went to a 2-layer board (management's demands for cost, not my idea, though I did make it work OK.)

REV 3.9:

About 200 pre-production B2000s were released, as "Dealer Demo" units, never meant to be sold. That's labelled Rev 3.9, of course some dealers sold them, since the AMIGA 2000 was hot. These may have problems with CPU slot devices since, like the orginal AMIGA 2000, there was no additional buffering on the clocks that drove that slot (and timing is very critical in many CPU slot applications).

REV 4.x Series:

The Rev 4 AMIGA 2000 was around most of the system's life. This was really just the Rev 3.9 with all the finalized changes. Extra buffering splits Agnus clocks between internal, expansion, and CPU slots. Rev 4.1 immediately corrected one missing signal on the secondary video slot (no plain Rev 4.0 boards were released). Rev 4.2 was some kind of production fix, I think it removed some overzealous FCC hacks, like the extra filter capacitors on the keyboard connector. Rev 4.3 tweaked a few pullup resistor values, to adjust for differences in the new CSG-made Gary chip (a full custom part) versus the Toshiba-made Gary chip (a gate array). After some 150,000 or so AMIGA 500/AMIGA 2000s, the CSG Gary was phased in. Rev 4.4 I believe made some changes to filtering of the TICK line, to account for some new power supplies that had been particularly noisy on that line. Rev 4.5 increases the value of the pullup resistor on the BAS* line of the expansion bus; there was a strange problem in conjunction with expansion bus DMA turnaround (what happens after a DMA device gives the bus back), BAS*, certain vendors of the 74ALS245 buffer that sits between BAS* and the local bus's AS*, and accelerator cards that could cause crashes. Add a 1K or so pullup to pins 11 and 20 of U605 on pre-4.5 boards to fix this problem (this was the only severe problem addressed by any subrevision).

REV 6.x Series:

The Rev 6 AMIGA 2000 was never called the AMIGA 2000C, except maybe by geeks on usenet :-).

The Rev 6 AMIGA 2000 changed the DRAM array from 256K x 1 parts to 256K x 4 parts. Some layout and FCC changes were made, I don't know just what, I was kind of out of the picture at the time (Rev 5 was just Rev 4.5 with the DRAM changes, nothing else. Rev 6 did more, I wasn't really involved). The original Rev 6 board has some noise problems on the expansion bus, possibly due to a new production of 68000s from Motorola that happed to be used at the same time. Rev 6 also had some extra noise on its time-of-day clock. Rev 6.1 corrects the clock noise, Rev 6.2 addresses the expansion bus noise.

Source : http://www.amiga-hardware.com/