[Coco] An interesting Korean CoCo2 - 26-3134 (no 'A' or 'B')

[Gene Heskett]

On Thursday 15 November 2012 23:47:30 Brian did opine:

> Greetings CoCoNuts!
> 
> I recently (early October still counts for recently, yes?) acquired a
> CoCo2 from evilBay. I just got around to dis-assembly and was met with a
> motherboard I can't identify - it's marked 20261044. I have looked thru
> Sam's Computer Facts for the CoCo2 which covers the 26-3134, but, the
> circuit board layout isn't even close; the diagram has the CPU just to
> the left of the cartridge slot. The CPU on this board is under the
> keyboard, closer to the middle of the board and there are two white
> connectors sticking out and two TMM41464P-15 chips (64k x 4 RAM) in
> between those connectors.
> 
> So, not being able to I.D. the board from that publication, I opened up
> Tandy's Little Wonder (thank you Frank!!) and started reading the CoCo2
> section. TLW describes this board pretty close since it does mention the
> two white connectors and a 'RAMSIZE' jumper (J6), which this board has.
> However, there's also jumpers J2 thru J5, apparently used to select 64k
> or 128k? J6 has been 'jumped' and all of J2-5 has been jumped to select
> 128k, and there's a wire running from the ECLK pad near the CPU to an
> SN74LS02N which has been piggy-backed on top of IC13 - and since I can't
> find a board layout I have no earthly idea what it is. Additionally, to
> the left of the CPU is J1, another place to choose either 64k or 128k.
> Finally, this machine has been upgrade to ECB - it reports as much at
> boot (along with a bunch of RFI noise...). To the right of the ECB ROM,
> the motherboard is stamped 128k/64k ROM.
> 
> Doing a Google search for 26-3134, I came across Techno's site
> (http://users.digitalindigo.net/~techno/coco2.html) which mentions a
> board that sounds similar to mine; however, there's no mention of 128k
> markers on this board. Additionally, he lists the 20261044 as being in
> both a 26-3134A and 26-3127B. He also mentions that this board has a 28
> pin socket for the ROM chip, which was originally shipped with a 24 pin
> ROM containing Color Basic (mine has been upgraded to ECB). Now based on
> only the type of RAM that's installed (dual 64k x 4), I'm pretty sure
> I'm looking at a 64k, ECB upgrade to what was once a 16k CoCo2 with
> Color Basic. Are all of these other jumpers and the wire from ECLK
> necessary for it to use a 128k (27128) EPROM?
> 
> The machine does work, and other than this particular motherboard not
> being very well documented, I don't think there's any great mystery - I
> guess it's more of a curiosity thing. If I get the time over the
> holidays, I may do a component layout of the board so at least that
> exists.
> 
> 
> Brian

Brian:  Those 64k/128k markers are for rearranging the rom access, either 
for a 64k (64 kilobits, aka 8 kilobytes) or 128k=16kilobytes roms.  Nothing 
to do with how much memory is in it, which appear to be a pair of 64kby4 
bit rams, so two work side by side to supply all 8 bits.
> 
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Cheers, Gene
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
My web page: <http://coyoteden.dyndns-free.com:85/gene> is up!
I can't decide which WRONG TURN to make first!!  I wonder if BOB
GUCCIONE has these problems!