Re: Did all plastic C128Ds have a missing U35?

From: Johan Grip <*ogun_at_ogun.org*> Date: Mon, 25 Mar 2024 10:40:32 +0100 Message-ID: <59bcf718-b2bb-47b8-bb69-6011f3d0c932_at_ogun.org> · ogun_at_ogun.org

On 20/03/2024 15:57, Jim Brain wrote:
> Maybe this is more common, but someone sent me pics of a C128D (non DCR, 
> plastic case) with an unfilled U35 ROM socket.  Unit works perfectly. 
> Jumper J4 (I think is the one) is shorted, so U32 is handling ROM1/ROM4 
> duties.
> 

Hmm, I wonder how that works. J4 connects A15 to the highest address pin 
on U34. Except there's a bug on the C128(D) board that connects A14 to 
it instead.

But without J3 also being closed, the chip selects would still behave as 
if there were two ROMs in the system.

 From the PLA:
...
assign rom2 = !ms0 && !ms1 && ms3 && rw && aec &&  a14 && !a15;
assign rom3 = !ms0 && !ms1 && ms3 && rw && aec &&
               ((!a14 && a15) || (a14 && !a15 && !rom_256));
...

I'm going to have to so some testing on this to figure out what's going on.

//Johan