[COFF] VT100 TX but no RX?

[Dan Cross via COFF]

On Mon, Mar 23, 2026 at 1:43 PM segaloco via COFF <coff at tuhs.org> wrote:
> 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.

Glad to hear that it's working now.  As Bakul kind of noted, but was
too polite to point out directly, I, too, missed that you were already
using a part that does the appropriate level shifting. :-D

        - Dan C.