[NTLK] Newton ROM card circuit board files reverse engineered

[Matthias Melcher]

I had a bit more time to put into the ROM board to clean up the schematics view. It is actually possible to understand now what is going on. So here are my findings. And yes, we can build reprogrammable ROM boards, probably even boards that carry additional Flash for the OS. Exciting.

"The ROM board connectors offers address lines up to A24, giving it access to 32MB blocks of memory. Two different chip select signals, ROM_CS_0 and  ROM_CS_1, make two of those blocks available. ROM 0 supports only read functionality, wheras ROM 1 allows for read and write, and even includes a delay line to insert CPU wait cycles if needed.

The populated ROMs are organized at 2*4Mb*16bit, adding up to 8MBytes, which are mirrored into the area covered by ROM_CS_0. The unpopulates pads can alternatively hold 4 of the same type of ROMs at 4*2Mb*8bit for 16MB of ROM space. Interestingly, the 4 ROMs are used in 8 bit mode, requiring the NewtonOS and possible licensee ROM Extensions present in the same ROM. I would have expected two sets of ROMs in 16 bit mode, with two slots taking the original ROM, and two ROMs holding the licensee extension. 

Well, and I had hoped in my wildest dreams, that the empty pads were there to simply have additional Flash RAM soldered onto them, which should be supported by the OS for the ROM_CS_1. But they are not. Adding Flash to the ROM board would require a redesign."

 - Matthias