[Coco] DragonCoCo

[Phill Harvey-Smith]

Hi all,

I've just managed to modify a Dragon 64 to be dual personality, both as 
a Dragon 64 or a CoCo.

This involves two circuits, one is a 27256 (or bigger) that replaces 
IC18, and has the normal Dragon rom in the lower 16K and a slightly 
modified** CoCo 2 rom in the upper 16K, A14 is connected to a switch so 
that you can select between them.

** The reason for the modified rom is twofold, firstly the Default CoCo 
rom writes to one of the PIA echoes in the FF04-FF07 area, on the Dragon 
64 this is where the ACIA is mapped, so this has to be avoided.
The second reason is that the CoCo 1/2 have to detect what RAM chips are 
fitted (as does the Dragon 32), whereas the Dragon 64 will always have 
8x4164 64Kx1 chips, so the ram detection code is removed.

The other part of the circuit is a little board containing 2 4053 analog 
multiplexors, that sits in the keyboard cable and who's job it is to 
re-map the Dragon keyboard layout onto the CoCo one. The select inputs 
for these two chips, pins 9,10 and 11 are wired together and connected 
to the A14 switch of the rom circuit.

There is one mod to the main board that is required for this to work, C5 
and C2 must be removed, on the Dragon these sit between the PA0 and PA1 
lines of the PIA and ground, these are the lines the joystick buttons 
are connected to, without removing these the first two rows of the 
keyboard will not work in CoCo mode. The CoCo has these two caps on the 
other side of the L1 and L2 inductors inline with the buttons, so it may 
be possible to re-connect them there.

Note at this time I have done no testing of how this affects the joystick.

This way I can power on with the switch in one position and I have a 
Standard Dragon 64, which will play nicely with standard Dragon 
programs, and will boot into all ram basic by typing EXEC 48000. In the 
other position I have a CoCo 2, with 64K CB 1.2 and ECB 1.1.

I have designed a circuit that will hopefully plug into the printer port 
and convert it to a bit-banger, which will work in CoCo mode, though if 
the correct software where written there's no reason it shouldn't also 
work in Dragon mode.

Please also note that all the above was done on a UK Dragon 64, I do 
have a couple of Tano machines, so at some point I'll try it on them but 
see no reason why it would not work. However some of the components may 
be labeled differently.

Cheers.

Phill.

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

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