[Coco] RE: [Color Computer] Looking for a Coco I 64k?

[Bruce W. Calkins]

I surprised someone has not recalled the differences of the chips 
between the "E" board and earlier CoCo 1 and "F"/285 and CoCo 2.  The 4k 
and early 16k chips used in the CoCo 1 needed 12 & 5 volts.  From the 
beginning 64k chips used only 5 volts; the 12 volt line became the high 
order address line.  The capacitors would have destroyed that addressing 
information, so they had to go.  The jumpers on the "E" and later boards 
took care of addressing that line.  The "D" and earlier boards needed to 
have trace cuts and jumpers to achieve that address change.

The "E" earlier and boards did not have provisions to address both 
halves of the 64k RAM, until a spare gate on the LS7402 was added to the 
addressing scheme.  The first upgrades had you bending pins up and 
linking them to that gate.  The slicker method was to identify the trace 
and jump the gate to the bus serving those pins.  (I always hated 
bending pins.  I have a few times soldered directly to the broken stub 
to save a chip.  Always a weak joint waiting to fail.)

Years ago I was given a much hacked 256k "D" board, sans case & power 
supply rigged up to run on a car battery.  (I still have it in storage.) 
The 256k migrated to a "F"/285 board.  Unfortunately, the software hack 
needed to boot OS-9 in the 256k environment won't work with NITROS9. 
Perhaps someday I'll understand enough to convert that hack.

Bruce W.


> p.s.  I seem to remember having to do some minor "surgery" on the "F"
> board when I installed the 64K chips -- clipping a few capacitors if I
> recall properly.
> 


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