[COFF] VT100 TX but no RX?

[Dan Cross via COFF]

On Mon, Mar 16, 2026 at 4:32 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...

What Bakul said. I would expect something of the VT100 era to want to
signal at RS-232 levels (+/- 3-15V), not TTL (0-3v3/5v). In
particular, you need the negative voltages for the mark signal. Level
shifters to convert are readily available, though!

        - Dan C.