Thank you!
...to everyone who responded to my questions, slightly OT as they were.
I found that I had already downloaded the two books referenced for the
M100 hardware, and while I would need to study things more, they did
answer my questions.
As noted, unless the system as was shown in that Ebay listing was using
serial IO, then it was likely using the expansion connector on the
bottom of the machine (I forgot about that one!). In my opinion, given
the way the rainbow ribbon cable is positioned (to me, it looks like the
main I/O cable) - and the number and type of I/O "modules" that were in
the briefcase, I think it is most likely it -is- using the expansion
connector in some manner.
Likely as not, it is probably using it in a similar manner as the
"Inside the TRS-80 Model 100" book mentions (and provides schematics
for), which is by "simply" tapping into the address and data lines, and
decoding things as the "unused ports" that are available for access, but
not connected to anything (though mention is given about a RAM and disk
drive expansion hardware - which I know Tandy offered; heck, I think I
recall one being sold on Ebay at one time - for more than I could
afford, lol!).
To me that would seem like the most straightforward approach, as the
hardware would be simple to implement; likely only one or two 8255s
could be hung off those lines, in a manner similar to doing the same
with the IBM PC parallel port (to provide simple I/O expansion beyond 8
bits without needing to interface to the ISA bus; there are plenty of
examples of this out there, which many here probably have seen).
Of course in this case, you wouldn't be using a parallel port - so it
would be closer to building an 8255 ISA card for custom I/O - but using
a 40 pin DIP header connector of course...
Anyhow - thank you all again for the education; I'm not making any
promises (truth be told, it likely won't occur), but perhaps during my
new current period of unemployment (ugh) I might find some time to
explore this information more...
Andrew L. Ayers
Glendale, Arizona
phoenixgarage.org
github.com/andrew-ayers
