A4000 motherboard layers? and A3000?

[freqmax]

How many PCB layers is the A4000 motherboard made up of?

Perhaps anyone knows this for the A3000 too?

[Castellen]

Both machines have four layer boards.  Signals are mostly routed on top and bottom layers with the two internal layers predominantly carrying +5V and GND.  Some signals are routed on the internal layers.

Here's a diagram I once drew: http://amiga.serveftp.net/images/ViaRepair.jpg

[freqmax]

My thought was along the line that the FPGA projects are not fully compatible yet with the originals. And there exist lot of broken Amigas where only the motherboard is the broken part but the chips are ok. So then the idea is how feasable it would be to create a replication of the original motherboard.

Only sockets and ports need to retain their mechanical position. The rest could be optimized. Perhaps new PCB technique allow 2-layer board for the same job?

[Hanzu]

My thought was along the line that the FPGA projects are not fully compatible yet with the originals. And there exist lot of broken Amigas where only the motherboard is the broken part but the chips are ok. So then the idea is how feasable it would be to create a replication of the original motherboard.

Only sockets and ports need to retain their mechanical position. The rest could be optimized. Perhaps new PCB technique allow 2-layer board for the same job?

Yes it would be great to have new motherboards that would still keep the old backward compatibility. However 2-layer board as motherboard would be bad move. 4 or more layers are needed because of fight against interference and not because of no space or lack of routing skills of old developers or autorouter programs. Ofcourse redesigning some components to smaller SMD size and putting resistor networks instead of resistors could leave some more space for better ESD and radio interference protection. With modern design developer or developing team would probably go nuts when trying to balance with too many things to improve and keeping enough backward compatibility with additional hardware and software and without backward compatibility it would be pointless project since we already have better next generation Amiga-likes.

[billt]

My thought was along the line that the FPGA projects are not fully compatible yet with the originals. And there exist lot of broken Amigas where only the motherboard is the broken part but the chips are ok. So then the idea is how feasable it would be to create a replication of the original motherboard.

Only sockets and ports need to retain their mechanical position. The rest could be optimized. Perhaps new PCB technique allow 2-layer board for the same job?

What about something like gb1000, completely new motherboards? Then include a Mediator/Prometheus like bridge and have native pci slots instead of isa... (or then bridge pci to pci-express slots too)

2 layers? please no. the inner power/ground layers are important for a few reasons.

1. signal integrity really benefits from signal trace being close to a solid power or ground plane. Not doing so makes eeverything significantly more difficult. Signal return current wants to be as close to the signal trace as possible, and solid reference planes are best for this. lose the planes, and you have to be tremendously careful about layour to avoid return current problems. yes, parts of these boards surely need this, even if a lot is still slow.

2. The first source of signal energy when a trace changes values is the power/ground plane capacitance at chip power pins. more is better. then local decoupling caps, then regional bulk caps, then the power supply.

3. power and ground routing become quite easy. having so sneak traces for them between signal traces on 2-layer would make it nightmarish.