[Coco] Looking for a Coco3

[Phill Harvey-Smith]

Sylvain Rousseau wrote:
> Hello,
> 
> I got a Coco3 again in 2002 and now I want to try to make a 512k
> upgrade kit for it using SRAM (not DRAM) and a CPLD but I'm afraid to
> burn it.  So, I'm looking for a cheap Coco3 that don't need to look
> well but it should works.  If my 512k SRAM upgrade kit works, I want
> to make it public via this CoCoList.

I can't help with the CoCo3 I'm afraid, however I am currently working 
on a similar project for the Dragon 32/64 (and by extension should be 
pretty easy to adapt for the CoCo 1, though poossibly trickier for the 2 
as there where several different models :( ), to replace the 4116/4164 
memory chips with a single 128Kx8 SRAM, as I have a whole bunch of them 
that I got essentially free :)

The main problem that I have encounterted so far is that 4116 and 4164 
chips have seperate data in and data out pins, the data in are connected 
to the CPU bus, but the out are connected with a LS244 on the CPU side 
and a LS373 on the VDG side, this should be easy enough to solve using 
something like an LS245, with the data from the RAM being on the VDG 
side and gated by E and R/W (or derivitives).

This of course may not be a problem for the CoCo 3 as the Gime may well 
handle this. I know cirtain CoCo2 models had a special LS buffer chip to 
do this and others did it via the 6847T.

You will of course also have to de-mux the row and column addresses, but 
this should be pretty easy as you just need to latch one half on ras and 
the other on cas which will give you the complete 16 bit address for 
your sram.

I'm about to make the board for this so I'll let people know how I get 
on. Eventual hope is to implement a CoCo3/Dragon Beta like paged memory 
system, using a small sram as a memory mapper (something like a 6116 
should do the trick I figure).

Cheers.

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.