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Vectrex - INTERFA2.TXT

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T85/08/13. 19.17.59. VECTREX. MYBASMT. Jeff Woolsey.

Blue-sky Vectrex Interface design notes.

The S100 Vectrex interface continues to have noise problems with its cable.
The problems have gotten worse since I reworked the memory array. Unless I
can correct the problems soon, I may be tempted to begin work on the second
version of the Vectrex interface -- a more general purpose, but perhaps less
versatile one.

I envision a board that plugs into the game port. Initially it would have had
its own CPU, a serial port, and some dual-ported memory. I revised the design
somewhat, considering that the Vectrex already had its own intelligence. What
I have, then, is a board with a ROM, some 16K (or more) of RAM, a serial port,
and some logic. The ROM contains an Interface Monitor which allows a terminal
(or whatever) attached to the serial port to do normal monitor-type things,
like read and write memory. It will also have a mode where it can load S-
format files, and jump to them, and so on. Because part of the purpose here
is to retain the capabilites of the S100 interface, the Mark II should behave
in rather a different way.

When the board is plugged into the Vectrex, and the Vectrex is reset, it will
jump to the ROM on board, which at 0 contains the Interface Monitor. It looks
like an ordinary game cartridge. When it starts up, it monitors the serial
port for commands (or possibly one of the game ports can be treated as a
serial port?) The ROM will have code in it which will get it out of the way,
and allow the interface to look like a pile of RAM (with some status bits that
control write protect and so on.

Perhaps the ROM operation will be to copy itself into internal RAM, or to the
high end of the itnerface memory. It will then set some bit that makes the
ROM disappear from the address space, so that the RAM can be written (with,
possibly, a game image). The monitor can then jump to a specific address, or
to the Executive to have it start the game. The code in the ROM can be made
position independent, facilitating a mechanism which might just allow
relocation of the ROM somewhere else in the address space.

One thing that doesn't appear feasible in this implementation is to wrest
control of the Vectrex from the running game to the Interface Monitor. In the
S100 interface this is done by halting the Vectrex and writing all over the
interface memory. In this version the only thing we could possibly hope for
is that interrupts (from the serial port, however that's configured) still
work. Most games, however, either trash the interrupt vectors, disable
interrupts, or do something else with the ports (like run a light pen or an
imager). This advances the case for offboard intelligence.

The problem I'm trying to eliminate here is noise on the long cable between
the Vectrex and the interface, and one obvious thing to do is put the
interface right outside the Vectrex. We lose some capability by doing this,
however, as I have outlined above. However, one would not need any real sort
of support system to do a few other things.

There is also the matter of the serial port (such as it is). Ideally, it is
autobaud and everything. However, the Vectrex does not provide +/- 12 volts
at the cartridge connector, and it appears that the pins on the two ports are
only TTL levels. What's a designer to do, particularly when one of the goals
is to work with an unmodified Vectrex?