Re: PET 2001 RAM question

Den Wed, 31 Jan 2018 00:01:58 +0100 skrev Francesco Messineo
<francesco.messineo@gmail.com>:
> On Tue, Jan 30, 2018 at 11:18 PM, Mia Magnusson <mia@plea.se> wrote:
> 
> > Addition:
> >
> > Also as 6502 afaik always drive the adress bus, you anyway need bus
> > separation between the 6502 and the video circuit, so the video ram
> > is anyway separate from the main ram. (The same is true for VIC-20,
> > it has buffers for both the adress and the data bus which is
> > disabled during the video halv of each cycle).
> 
> sure, but I was talking about the few 2001 boards that use 2114 as
> main RAM too, so there's no address conflict in this case.
> Making sure the /WR signal to the RAM happens only when it's supposed
> to happen was too obvious, but I was rather asking why
> the selects wheren't qualified with PHI2, and the answer is that 6502
> knows when to read the data bus anyway.

Yea, but it isn't that obvious why the main 2114's are on the
"processor" side of the buffers. They could just aswell have been on
the "video" side of the buffers, as the internal memory on a VIC 20.
Then they would have needed PHI2 qualification too.

> I'm repairing a few 2001s lately.
> By the way, I can now read 6550s and 6540s in a custom microcontroller
> unit, so I can dump/test all the 6540s easily out of a PET board.

Nice!

Btw I had a look at the schematics. Except for _SEL0_ missing on the
memory expansion interface, it seems like it wouldn't be that hard to
add a 62256 (32k*8 SRAM) to both act as an expansion and as replacement
for broken motherboard main RAM. You'd need one 8-to-3 line encoder
for most of the select lines, and an AND gate to generate a kind of
_SEL0_ line from the enable pins of removed motherboard RAMs, and maybe
(if you want to remove just one broken 4-bit SRAM IC and keep the
other) some logic controlling two 4-bit bus buffers. (The simplest
solution would be to cut _SEL0_ on the motherboard so internal ram is
never selected, or if internal ram i socketed just remove all those
IC's).

Not that historically correct, but adding 6116 (2k*8) or maybe 6264
(8k*8) would probably be been historically correct as a home made
project that someone could had done back in the days.

At some point in the near future I should have a go at something like
this, as I happen to have a 4k 2001 which most likely have some broken
ram (2114) or rom as it starts but crashes into the TIM entry point no
matter what I try to do. (TIM itself doesen't really work, but the full
screen editor works so I know the CPU runs code and the video hardware
is good). One goal is to avoid any permament modifications if they
aren't absolutely neccesary, but adding a ram expansion board that fits
the expansion connector seems like a good thing to do as it can be
removed whenever I might want to.

I know that there are some existing projects where you put a small PCB
between the 6502 and the motherboard, and while that would be an
elegant solution, it seems historically more incorrect than adding a
62256 to the expansion connector. But maybe I'll change my mind
later. :)

> I've discovered that my own two 6540 spares are actually only one
> spare by now :/

Oh, sorry to hear that :(


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