[Coco] drive wire in coco system rom

[Tormod Volden]

On Sat, Oct 18, 2014 at 8:19 AM, didier derny wrote:
> Is there hack to install drive wire into the coco system rom ?
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> I saw the coco 2 can use 64k or 128k eprom.
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> Is there any way to pack the coco2 system and drivewire in the same rom ?

I believe you mean an HDB-DOS ROM. Drivewire is a protocol, supported
by HDB-DOS, NitrOS-9, etc.

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> I did something similar on another machine (not a tandy)
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> By replacing the eprom by a small board + larger eprom + a GAL + 2
> "floating" wires

I think it can be even easier than that. Look at Figure 4-7 (page 30)
of the Coco 2 service manual
(http://www.colorcomputerarchive.com/coco/Documents/Manuals/Hardware/Color%20Computer%202%20Service%20Manual%20%28Tandy%29.pdf)

Let's say you want to replace both the original basic and ext.bas ROMs
and insert an EPROM instead (with a 28 - 24 pin adapter). That's how I
understand your question talking about the "system rom".

You'd want your EPROM *CE to go low if either Y1*,Y2* or Y3* goes low.
So use a three-input AND gate. Then connect A13 and A14 of your EPROM
to A and B (S0 and S1) and make sure you lay out the 8KB banks in your
EPROM accordingly (empty, ext. basic, basic, hdbdos). For a larger
(64KB) EPROM, just repeat the bank pattern and/or tie A15 accordingly.

What is easier, especially if your basic ROM is soldered, is to just
replace the ext.bas ROM. Then you can use a normal two-input AND gate
instead, for Y1* and Y3*. You would only need B (S1) to select the
right 8KB bank in the EPROM (A13). Layout: ext.basic, hdbdos
(repeated, or tie down higher address lines of the EPROM).

In all cases, make sure you don't have a ROM in an external cartridge!

Maybe somebody already tried this and can confirm or correct.

Regards,
Tormod