[Coco] CoCo 1 64K upgrade failed :(.

[Phill Harvey-Smith]

Ryan Pritchard wrote:
> I got to thinking this afternoon that there should be nothing preventing me
> from taking 41256 chips and merely grounding out pin 1 (A8) to keep the
> signal LOW (or is it HIGH? I don't remember my electronics logic that well)
> to allow me to use the chips to provide access to the 64K using the A0
> through A7 lines with the state of A8 always locked.  A0 - A7 on the DRAM is
> MA0 - MA7 according to CoCo Technical Manual.

My answer to that would have to be 'maybe' I have done this on a Dragon 
32 board and it did seem to work ok, however it is out of spec as the 
74LS783 SAM can only do 128 cycle refresh, where as the 41256 type chips 
need 256 cycle refresh however like I said I tried it and it did seem to 
work, so may be worth a shot.

IIRC the 256 cycle refresh was later implemented in the 74LS785 SAM, 
some of the later versions of the CoCo 2, used this chip as they also 
used 4464 (64Kx4) rams, which are also 256 cycle refresh.

Cheers.

Phill.

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

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