Well, the
Retrochallenge 2015/07 event started a couple of days ago. As usual I'm off to a slow start. This weekend started a day early, giving me a little time to get going on my project. So far I've done very little, but I thought it might be useful to review the outline of the project and discuss some possibilities...
Outline
The point of this project is to exploit some features of the
CoCo cartridge port for the potential benefit of game cartridges. Therefore it is reasonable to question what such features are available. This includes
interrupt signals,
address decoding signals, and audio input.
I have discussed the interrupt-based timing sources available on the CoCo in my other blog posts over the years. The
Hsync and Vsync signals are a bit inflexible and difficult to use for audio playback in particular. Vsync is too slow to drive the
DAC, and Hsync is much faster than needed -- especially since it connects through the
PIA
that feeds the IRQ signal on the
6809. The CART line on the CoCo
cartridge port is actually tied to the PIA that feeds the FIRQ (i.e. "fast IRQ") signal on
the 6809, potentially making it a great candidate for an audio playback
timer signal.
The cartridge port provides two separate address decoding signals. One is the standard
ROM chip select used to access program code on cartridges. The other is designed for accessing the
floppy drive controller hardware. But since a game cartridge typically is used without other cartridges, that signal can be used for whatever purpose a cartridge likes. In particular, it can be used to provide access to other hardware bits that a game might need.
The CoCo cartridge also has a line that can feed an audio signal directly back into the CoCo. This could be used to enable audio generation hardware directly in the cartridge itself.
Apparatus
(Note: the apparatus does not work -- do not attempt to replicate this from the picture!)
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| Breakout Circuit for CoCo Cartridge Port |
The purpose of this experiment is really just "proof of concept", so I won't be bothering for now with building
PCBs or any other permanent design artifacts. Instead, I only want to have electrical access to the CoCo cartridge port as a means of interfacing to hand-built circuits on a
breadboard.
Some time back another CoCo person produced some simple PCBs that breakout the cartridge port to a 40-pin
IDC connector. More recently, the popularity of the
Raspberry Pi platform has led to the availability of some small PCBs that adapt between a 40-pin IDC connector and a 40-pin
DIP connector compatible with a breadboard. I combined these two items to adapt from the CoCo cartridge port to a solderless breadboard.
I often like to use a "
monitor" program plugged into the the cartridge port when doing CoCo development. In order to facilitate that, I have added a "
Y cable" to the mix. This should allow me both to use the "monitor" program and to have access to the cartridge port signals for circuit development.
Ideas
If I haven't lost you yet, then I probably don't need to tempt you further! Nevertheless, I will provide a couple of teasers to whet your appetite...
In order to facilitate audio playback on the CoCo, a reasonably paced
signal is needed. Use of the FIRQ line is desirable, since that can be
the most efficient mechanism for handling high frequency events. The
more flexible interrupt and timing mechanisms of the CoCo3 illustrate
that a moderately paced FIRQ signal can provide pleasing audio playback.
So, a simple idea is to use a simple timer circuit (e.g. a
555 wired as an astable multivibrator) to provide such an FIRQ signal.
A normal CoCo cartridge can provide almost 16
Kb of program storage. That may seem like plenty(!), but larger programs can easily consume that with
graphics assets,
audio samples,
map data, and even just
program code. One approach to extending available storage is
bank switching. This can be done by using the
I/O address range in the cartridge to control the bank switching hardware in order to multiplex the ROM address range onto a larger memory.
Adding audio hardware to the CoCo seems like a simple way to extend its gaming capabilities. This may have been expensive in the past, but for today's hobbyist I think this option seems quite reasonable and could make for a very collectible game cartridge. Anyway, I hope to conduct the exercise of building such a circuit as the month progresses.
If I get this far, I may have one more idea that would have seems a bit ahead of it's time when the CoCo was in stores. Still, it might be a cool option in today's market. For now I'll keep this last one under my hat...stay tuned!
