[COFF] VT100 TX but no RX?

[Ori Kuttner via COFF]

Do you have  an oscilloscope that you can connect to the RX line to see if
anything is sent there?

-- 
Ori Kuttner CEO Helicon Books
http://www.heliconbooks.com





On Mon, Mar 16, 2026 at 10:24 PM segaloco via COFF <coff at tuhs.org> wrote:

> Drawing a major blank and can't figure this out.  I got a VT100 some time
> ago, fixed up several things, but am still struggling a bit.  Was wondering
> if someone on list could give me some advice.
>
> Currently I've got the thing plugged into a little MAX232 adapter that
> level shifts from 5V to 3.3V, then into a USB TTL thing.  Sending
> characters from the VT100 is fine, I cat /dev/ttyUSB2 and every key I type
> on the VT100 shows up on my monitor.  However, any bytes I write back out
> to /dev/ttyUSB2 don't seem to go anywhere.  There shouldn't be hardware
> flow control involved, I can use the same adaptors with 3.3V devices in
> both directions just with TX/RX/GND, its just if the VT100 is on the other
> end, characters only seem to go one way.  If I launch getty against the
> device it seems to timeout.  For the record local mode works, so the
> general character generator/display are functional, I'm just not getting
> printing of stuff coming on the RX line.
>
> The odd thing is communication in one direction is totally fine, I get the
> expected bytes on my computer, I just can't seem to get the VT100 to accept
> anything back.  I've messed with the auto XOFF/XON settings to no avail,
> and have tried both low and high baud rates.  Consistently everything I
> type on the VT100 shows up on the computer but neither echoes back nor
> anything new coming from the computer.  Am I missing something painfully
> obvious or perhaps am I looking at maybe having a bad receiver circuit?  I
> might just pony up and order another basic video board, the traces are
> pretty squiggly from expansion over the years...
>
> - Matt G.
>