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. From coff at tuhs.org Tue Mar 17 16:37:31 2026 From: coff at tuhs.org (Ori Kuttner via COFF) Date: Tue, 17 Mar 2026 08:37:31 +0200 Subject: [COFF] VT100 TX but no RX? In-Reply-To: References: Message-ID: 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 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. > From coff at tuhs.org Tue Mar 17 21:01:14 2026 From: coff at tuhs.org (Dan Cross via COFF) Date: Tue, 17 Mar 2026 07:01:14 -0400 Subject: [COFF] VT100 TX but no RX? In-Reply-To: References: Message-ID: On Mon, Mar 16, 2026 at 4:32 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... 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. From coff at tuhs.org Tue Mar 17 09:22:46 2026 From: coff at tuhs.org (segaloco via COFF) Date: Mon, 16 Mar 2026 23:22:46 +0000 Subject: [COFF] VT100 TX but no RX? In-Reply-To: References: Message-ID: Thanks for the help folks, turns out there's some corrosion on the STP (standard terminal port) edge socket inside. The pins are supposed to short across the socket when a card isn't inserted, so corrosion or bent pins can cause trouble elsewhere. After some scraping a loopback test of tying RX to TX finally works, so now I can get back to messing with the signal chain. Loopback actually doesn't work on the other end either so maybe both sides have their own problems...but VT100 is looking a lot better now thanks for the pointers! - Matt G. From coff at tuhs.org Mon Mar 23 19:05:05 2026 From: coff at tuhs.org (Warren Toomey via COFF) Date: Mon, 23 Mar 2026 19:05:05 +1000 Subject: [COFF] Ping Message-ID: Just a test of the COFF mailing list. Cheers, Warren From coff at tuhs.org Mon Mar 23 20:26:48 2026 From: coff at tuhs.org (=?utf-8?q?Cameron_M=C3=AD=C4=8Be=C3=A1l_Tyre_via_COFF?=) Date: Mon, 23 Mar 2026 10:26:48 +0000 Subject: [COFF] Ping In-Reply-To: References: Message-ID: ping -c 1 www.warren.com.au 1 packets transmitted, 1 received, 0% packet loss, time 0ms rtt min/avg/max/mdev = 15.469/15.469/15.469/0.000 ms :-) -------- Original Message -------- On Monday, 03/23/26 at 09:05 Warren Toomey via COFF wrote: Just a test of the COFF mailing list. Cheers, Warren From coff at tuhs.org Tue Mar 24 03:32:41 2026 From: coff at tuhs.org (Bakul Shah via COFF) Date: Mon, 23 Mar 2026 10:32:41 -0700 Subject: [COFF] VT100 TX but no RX? In-Reply-To: References: Message-ID: <41CC5812-14B8-44F0-BA2A-359AFD08227E@iitbombay.org> > On Mar 17, 2026, at 4:01 AM, Dan Cross via COFF wrote: > > On Mon, Mar 16, 2026 at 4:32 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... > > 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! I didn't add anything useful, not having carefully read that Matt was already using a MAX232 which does the level shifting. Though for 3.3V devices a MAX3232 would be better. I have seen this only one direction working in serial as well ethernet connections so checking the connections should be the first thing to try! From coff at tuhs.org Tue Mar 24 03:42:48 2026 From: coff at tuhs.org (segaloco via COFF) Date: Mon, 23 Mar 2026 17:42:48 +0000 Subject: [COFF] VT100 TX but no RX? In-Reply-To: <41CC5812-14B8-44F0-BA2A-359AFD08227E@iitbombay.org> References: <41CC5812-14B8-44F0-BA2A-359AFD08227E@iitbombay.org> Message-ID: <5I8xrVVLJLbKun31v2M6v4yIMe8PXLFZAy6af3rY-kVhAABRe4VKbTPkG6FoGzQtooZGMDRLrWW-4DrucROyFC1-YIVqa5zQ9dJ_ruaMBvE=@protonmail.com> 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. From coff at tuhs.org Tue Mar 24 03:46:51 2026 From: coff at tuhs.org (Dan Cross via COFF) Date: Mon, 23 Mar 2026 13:46:51 -0400 Subject: [COFF] VT100 TX but no RX? In-Reply-To: <5I8xrVVLJLbKun31v2M6v4yIMe8PXLFZAy6af3rY-kVhAABRe4VKbTPkG6FoGzQtooZGMDRLrWW-4DrucROyFC1-YIVqa5zQ9dJ_ruaMBvE=@protonmail.com> References: <41CC5812-14B8-44F0-BA2A-359AFD08227E@iitbombay.org> <5I8xrVVLJLbKun31v2M6v4yIMe8PXLFZAy6af3rY-kVhAABRe4VKbTPkG6FoGzQtooZGMDRLrWW-4DrucROyFC1-YIVqa5zQ9dJ_ruaMBvE=@protonmail.com> Message-ID: On Mon, Mar 23, 2026 at 1:43 PM segaloco via COFF 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.