[Coco] CoCo "user" port?

[Phill Harvey-Smith]

Leon Howell wrote:
> Phill Harvey-Smith wrote:
> 
>> I believe when I checked a few months ago the PIA lines that are
>> used for the CoCo Bitbanger, all appear on the Dragon Paralell
>> port,
> 
> This gives me an idea for an interesting hardware project.
> 
> Are the other PIA lines used on the Dragon paralell port used for
> anything in the CoCo 3? If not, why not build an adapter that expands
> the port to a full paralell port, that can also bit-bang serial? If
> the address is the same, we could easily use dragon printer drivers.

The Printer Data lines are shared with the keyboard matrix, I believe 
the strobe and other handshaking signals use the same lines that the 
CoCo uses for the bit-bang port.

> How hard would it be to make the port bi-directional?

It would cirtainly be easy to rig up a latched output port with an LS573
or similar. I guess input could be done with a Paralell to serial chip
and strobed in with one of the handshake lines.

It would also be relitively simple to build a little circuit that 
plugged into the paralell port and gave you a CoCo style bit-banger, 
especially as the port has +5v and Gnd available. Might actually give 
that a go at some point, of course there would be no ROM support for it, 
but I guess you could borrow routines from the CoCo ROM, or if you where 
  running something like Nitros, use the already existing CoCo modules.

On that subject, the input lines seem to be routed through a Diode and 
then an LM339 opamp (U14 on the CoCo tech ref schematic), and the output 
through a 741 opamp (U15). Are these just being used as level shifters 
from the +5V logic of the CoCo to the +/-12V that RS232 uses. If they 
are then almost all you would have to do is route the lines through a 
MAX232 or similar chip and to an apropreate connector. This could of 
course also be fitted inside the Dragon's case as there is plenty of room.

Cheers,

Phill.

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

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