From coff at tuhs.org Tue Mar 17 06:24:24 2026 From: coff at tuhs.org (segaloco via COFF) Date: Mon, 16 Mar 2026 20:24:24 +0000 Subject: [COFF] VT100 TX but no RX? Message-ID: 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. From coff at tuhs.org Tue Mar 17 06:57:41 2026 From: coff at tuhs.org (Bakul Shah via COFF) Date: Mon, 16 Mar 2026 13:57:41 -0700 Subject: [COFF] VT100 TX but no RX? In-Reply-To: References: Message-ID: <8D664281-537E-452D-A71E-53A9D1E185AB@iitbombay.org> You need a TTL<->RS232 converter. > On Mar 16, 2026, at 1:24 PM, segaloco via COFF 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.