[COFF] VT100 TX but no RX?

[segaloco via COFF]

Replying all just to confirm again that this is all good now.  The
culprit was corrosion on the pins of the STP port on the mainboard
inside, the pins normally short across the slot when nothing is
inserted.  I had to straighten a few out and scrape them a bit so they'd
make a good contact again without a card in place.  I suppose another
option would be a jumper card so the connection is made over thru-holes
in the card rather than having to rely on the pins themselves to bridge.

Ultimately it's a matter of STP being a serial bus connection rather
than parallel.  If the STP slot is messy, communication suffers.

The VT100 works beautifully now, although it turns out my MAX232 thing
was *also* on the fritz.  Ordered a new FDTI-based cable and all is
good.  I can plug the VT100 into my modern FreeBSD box and work, or
launch SimH and really get a classic experience!  It also works fine
with both of my Mac Tutor units, so I intend on experimenting with some
300 baud Dataphone modems next.

Thanks for the input folks, what got me unstuck as far as diagnostics
are concerned was the suggestion to just loopback the VT100's
communication lines, see if it can drive itself.  When it couldn't, I
knew to troubleshoot the board, not something downstream.

- Matt G.