[Coco] Dragon to CoCo Conversion....

[Phill Harvey-Smith]

Hi,

I seem to recall about a week ago someone was enquiring about converting 
a Dragon to a CoCo. I have since done some experiments and been 
reasonably successfully.

As a basis I used a spare Tano Dragon 64 board, what applies to this 
would also apply to the standard British Dragon 64. To convert a 32, 
would be rather more involved due to a number of factors however not 
impossible.

To do the conversion you will need a blank 27128, and access to an Eprom 
programmer, into this eprom you will need to blow a copy of Extended 
Color basic ($8000-$9FFF), and a *MODIFIED* copy of Color basic 
($A000-$BFFF). You will then need to remove IC18 (the Dragon ROM based 
basic ROM), and replace it with your ECB/CB Eprom.

The reasons that Color basic needs to be modified are as follows :-

1) The CB rom, assumes that the PIA at FF00, is mirrored at 4 byte 
intervals up to FF1F, however on the Dragon 64, there is also an ACIA at 
$FF04-$FF07, so this fails, however the PIA+ACIA are mirrored, but at 8 
byte intervals, so adjusting the offsets in the ROM enable it to init 
the PIA correctly.

2) The Dragon 64, as it only comes in one memory configuration, does not 
need the PIA line to detect the type of RAM chips, so it is used to 
select the second ROM, containing the RAM basic image. The upshot of 
this is that CB detects the machine as only having 16K, and so has to be 
patched to assume 64K.

I have produced a listing of Color Basic from the unravelled PDF, file 
that can be assembled to produce the above file. If anyone wants a copy 
of this (I also have ECB, RS-DOS, Dragon 32/64, DragonDos & SuperDos), 
or has somewhere that they can host these feel free to ask.

Also due to the keyboards being wired differently I have needed to 
re-wire the keyboard, this should not be a problem if doing a one way 
conversion.

I have not yet implemented the bit-banger, however, since the 3 PIA 
lines used are brought out to the Dragon parallel port, and there is a 
+5V/0V supply also on this port it should be possible to replicate the 
CoCo op amp circuit to implement this. Better still using one of the MAX 
series of RS-232 voltage converters.

I have successfully booted the CoCo version of Nitros on this setup 
(with an RS-DOS cart), which seems to work correctly.

Of course there is no support for the Dragon parallel port and ACIA, in 
the CB/ECB roms, but it should be perfectly possible to use them under 
Nitros providing the correct drivers where available.

Hope that's useful to some of you....

Phill.

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

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