[Coco] Coco ProtoType Board

[Joel Ewy]

Mike Pepe wrote:
> Hi James,
>
>> 2) some form of flexible address decoding included along with
>> buffering. It is still unclear as to whether just the Coco 3 I/O
>> range be flexible or also the range from $C000 to $FFDF also be
>> included. 
>
> I think the PLD would be nice, but overkill.
>
If it's a 100-pin fine pitch SMT that only uses a few outputs and leaves
the rest unconnected, has 150 registered macrocells that are permanently
wasted, and takes the price right out of the impoverished CoCo hobbyist
range, then I would concur with Mike that a few 74xx chips would be a
better option.

On the other hand, if the PLD can be re-programmed in situ, (I think at
least some CPLDs can be??) then "overkill" here means "extra potential
features".  Certainly you don't need more than a few TTL chips just to
decode some addresses.  But if you can get more logic in the same space,
and something approaching the same price, and if that logic can be
appropriated by the user, then I would see it as worthwhile.  If the
thing could be made to mimic an MPI, for instance, or could be
programmed with internal registers or counters, then it would be a
significant improvement over a dedicated address decoder circuit.  If
the extra I/O lines were brought out to .1" headers, it would seem like
useful potential, not overkill.

A reasonable compromise, in the event that the PLD in question is not
reprogrammable in circuit, would be to try to use up as much logic as
possible on potentially useful functions, and make it available through
jumpers or DIP switches, and MPI-like internal selection.

Of course the main purpose of such a board would be for hobbyists to
attach their own gadgets.  But a simple expansion kit could turn it into
an enhanced Multipak replacement for the less technically inclined.  I
have a CoCo 3 in a mini-tower case and have used a ribbon- Y-cable to
hook up a Disto SC-II/4-in-one and an RS-232 pak.  I'm not going to tear
apart my one MPI to try to wedge the board in the mini-tower.  I don't
think it would fit anyway.  But if I had an inexpensive project board
that had MPI circuitry built-in, that would be another story.  I would
also burn several of my favorite cartridge games on a 27512 and stick
that in one of the "slots".  Wouldn't it be nice if the logic to switch
banks in a larger EPROM would be built into such a device?  I'm
confident I could add that myself.  I have the hardware and software
(and I think, the knowledge) to program PALs and GALs.  But if the
capability is already present in a board I could purchase for, say,
<$50, I would rather save myself the time and effort.  And most CoCo
hobbyists probably don't have a universal device programmer.

Another cool thing you could stuff into extra logic would be a
mini-expansion-bus like Disto's.  In fact, if you could make it
compatible, that would be really cool.  I've used the MEB on my SC-I to
wire up a quick-and-dirty stepper motor interface, and a hobby servo
interface.  Wouldn't it be nice to be able to prototype a simple project
on an MEB built-into one of these prototype boards, then plug it into
existing Disto hardware when it's ready for use?  You don't have to put
in all the connectors, at manufacture time, but if there is extra space
left in the PLD, put the logic in!

JCE