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Thread: VT100 schematics and ROM check

  1. #1

    Question VT100 schematics and ROM check

    Hello,

    I've got a weird problem with my VT100. Sometimes the VT100 refuses to receive characters.
    Or while typing, the characters will show up seconds later onto the screen all at once. So it seems there
    is a character buffer filled-up. But for some reason it is not always displaying the characters.

    I wonder if there might be a corrupt ROM in the terminal. Is there an option
    to do a CRC check on the ROM's with a self test in the terminal?

    And I could not find schematics of the VT100. Does anyone know where to find these?

    Thanks in advance!
    Regards, Roland
    WTB: Case for Altair 8800 ...... Rolands Github projects

  2. #2
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    What is the VT100 connected to when you are typing?
    And when you say "typing", are you typing on the VT's physical terminal? Or on your PC on the "other end of the wire"?
    I would suspect that the "buffering" is happening at the other end, not in the VT. I find it unlikely that the VT would have that behavior.
    Try turning on "local echo" in the VT, and see what happens.

    Pete

  3. #3

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    Well I had the same problems on a PDP8 and PDP11. When typing text or a command
    or whatever, it does not always show up on the terminal. Or after a while a whole bunch
    of characters are showing up. Really weird behaviour and sometimes hard to reproduce.

    Tonight I connected it to my PC with Tera Term so see if this behaviour is on the
    sent or receive side. And it appears to be on the VT100 receive side...
    Sending characters from the VT100 to Tera Term is working fine.

    The first thing I noticed now is when I power up the VT100 and sent text to
    the VT100 nothing appears on the screen. Just a blinking cursor...
    After pressing a random key on the VT100 the last characters (I think 64 total)
    which I sent from Tera Term appear on the screen. So it is receiving the characters.
    But why showing them after a key press? And also the weird behaviour while typing...

    I've been searching in the settings for the terminal but I don't see anything that
    could explain this behaviour. But I'm open for suggestions!

    But just the behaviour of sometimes working and sometimes not is also suspicious...

    Regards, Roland
    WTB: Case for Altair 8800 ...... Rolands Github projects

  4. #4

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    There appears to be a set of schematics on archive.org https://archive.org/details/bitsaver...e/n47/mode/2up

    I find it surprising that you didn't find these, is this not your VT100 model or year?
    Collector of all things Digital, but mostly VAXes and corporate knick-knacks (books, mugs, etc).
    VAXes: VAX 4000-100A, VAXstation 3100-30, VAXserver 3100, VAXstation 4000 VLC
    Randoms: DECpc LPv+, Rainbow 100B, DECbridge 90, DECserver 300

  5. #5
    Join Date
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    Quote Originally Posted by Roland Huisman View Post
    Well I had the same problems on a PDP8 and PDP11. When typing text or a command
    or whatever, it does not always show up on the terminal. Or after a while a whole bunch
    of characters are showing up. Really weird behaviour and sometimes hard to reproduce.

    Tonight I connected it to my PC with Tera Term so see if this behaviour is on the
    sent or receive side. And it appears to be on the VT100 receive side...
    Sending characters from the VT100 to Tera Term is working fine.

    The first thing I noticed now is when I power up the VT100 and sent text to
    the VT100 nothing appears on the screen. Just a blinking cursor...
    After pressing a random key on the VT100 the last characters (I think 64 total)
    which I sent from Tera Term appear on the screen. So it is receiving the characters.
    But why showing them after a key press? And also the weird behaviour while typing...

    I've been searching in the settings for the terminal but I don't see anything that
    could explain this behaviour. But I'm open for suggestions!

    But just the behaviour of sometimes working and sometimes not is also suspicious...

    Regards, Roland
    So you have confirmed that when you press a key, a character is sent immediately over the serial port. That is good.

    How are you confirming that characters are actually being sent to the VT100? Not sure if I understand the setup. Are you testing with the VT100 connected to a PC with TeraTerm? And then testing by typing on each side?

    How about shorting pins 2 and 3 on the VT100 instead. That way, you are looping back outgoing characters, and do not have any other equipment involved in the loop. Since you know pretty much for sure that key presses immediately result in data transmitted, this would then tell you if characters received are immediately shown on the screen.

    I can't say I fancy having a complex thing like a PC with Windows, and TeraTerm, as a testing device here. I have no idea what it might be capable of doing.

    As for self test, the terminal is always performing that at power up, and if any problems are detected (including checksumming of the ROMs), the terminal will try to show this on the keyboard LEDs, if it is a fatal error (ROM checksum errors are fatal). If a fatal error is detected, then the terminal is totally non-operational.
    Non-fatal errors cause the terminal to go to local mode, and a character is shown at the top left, telling what kind of error you have.

    The VT100 do not have any kind of hardware flow control. Just so you know. So if your PC is paying any attention to such things, you might be in trouble. Also, wiring of a serial cable could cause funny things. I would really do the loopback on the VT100 itself. And if that works fine, then you know it's not the terminal. And then you can try troubleshooting where the problem actually lays after that. Or else it truly is in the VT100, at which point the UART would be the first suspect. But I would probably grab an oscilloscope and the schematics, and trace where signals are going, and what they should look like. It's actually not that hard most of the time.

  6. #6

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    Quote Originally Posted by DECtechGuy View Post
    I find it surprising that you didn't find these, is this not your VT100 model or year?
    Woah yes I find that surprising too! I found several manuals and not the schematic.
    I should have to looked at bitsavers right away...

    @ Bqt

    The terminal has x-on and x-off it seems I toggled it twice so it was still on.
    This seems to solve the problem. But now a big part of my keyboard that quits working.
    I can't even enter the setup... So I have to check the keyboard.

    I have a spare VT100 board from a VT180. I guess this board should work too.
    That is if the same ROM is on these VT180 terminal boards...
    Last edited by Roland Huisman; September 20th, 2020 at 06:21 AM.
    WTB: Case for Altair 8800 ...... Rolands Github projects

  7. #7
    Join Date
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    Quote Originally Posted by Roland Huisman View Post
    @ Bqt

    The terminal has x-on and x-off it seems I toggled it twice so it was still on.
    This seems to solve the problem. But now a big part of my keyboard that quits working.
    I can't even enter the setup... So I have to check the keyboard.

    I have a spare VT100 board from a VT180. I guess this board should work too.
    That is if the same ROM is on these VT180 terminal boards...
    Hmm, yes.
    Obviously, if you actually send an XOFF, then the remote system should not send you any data.
    However, just enabling xon/xoff in the setup just means that the terminal will be sending an xoff when the buffer is getting full, and an xon again when the buffer have more space. So I find it a little hard to believe you observed anything related to that if you were typing in TeraTerm. I don't think you can type that fast... (Unless you were running at 75 bps, and is a fairly good typist.)
    But maybe you just hit ^S on your VT100 by accident? Or hit the no-scroll key?
    But then I would not be able to explain why you would get characters at some later point without an xon, and only just by sending some random bytes, unless TeraTerm is funny enough to interpret any received byte as xon.

    Speaking of the VT180 - not sure if it's the same motherboard. The VT100 is different than all the other VT10x terminals at least (so don't try to swap roms between them). But interface wise, the keyboard connector and protocol is the same, so you can interchange keyboard just fine.

  8. #8

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    Quote Originally Posted by bqt View Post
    Hmm, yes. Obviously, if you actually send an XOFF, then the remote system should not send you any data. However, just enabling xon/xoff in the setup just means that the terminal will be sending an xoff when the buffer is getting full, and an xon again when the buffer have more space. So I find it a little hard to believe you observed anything related to that if you were typing in TeraTerm. I don't think you can type that fast... (Unless you were running at 75 bps, and is a fairly good typist.)
    Indeed, I did find this all very strange indeed.

    Quote Originally Posted by bqt View Post
    But maybe you just hit ^S on your VT100 by accident? Or hit the no-scroll key? But then I would not be able to explain why you would get characters at some later point without an xon, and only just by sending some random bytes, unless TeraTerm is funny enough to interpret any received byte as xon.
    No accidentality pressed keys. This problem was known for this terminal for at least a year. But now I wanted to get it back to life. But the strange thing is that I can't reproduce the problem at the moment. After toggling that bit in memory and saving the settings, and changing it back the problem is still gone. Maybe corrupt data in that memory?

    Quote Originally Posted by bqt View Post
    Speaking of the VT180 - not sure if it's the same motherboard. The VT100 is different than all the other VT10x terminals at least (so don't try to swap roms between them). But interface wise, the keyboard connector and protocol is the same, so you can interchange keyboard just fine.
    I checked the numbers on the ROMs and the PCB version. These are the same for the VT100 and VT180. So I swapped the boards and it works fine. VT100 is working with only the VT180 terminal board... So I have an easy solution if necessary... But I want to know that is/was really wrong now. Maybe the problem will come back because this error didn't show up always...

    Regards, Roland
    WTB: Case for Altair 8800 ...... Rolands Github projects

  9. #9

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    The keyboard was working again spontaneously and did also quit again tonight...

    Keys not working:
    SETUP
    3 4 6 7 9 - , (from the numeric part)
    Z C V M /

    These seem to be on the same scan line in the keyboard except the setup key.
    I think I first have to check what is happening on E5 (74LS00) in the keyboard.
    Because the signals from the setup key and the scan line for the other keys are routed trough that chip.
    And I will probably get another VT100 next week so then I can even swap the keyboard for testing.

    To be continued....
    WTB: Case for Altair 8800 ...... Rolands Github projects

  10. #10
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    Quote Originally Posted by Roland Huisman View Post
    I checked the numbers on the ROMs and the PCB version. These are the same for the VT100 and VT180. So I swapped the boards and it works fine. VT100 is working with only the VT180 terminal board... So I have an easy solution if necessary... But I want to know that is/was really wrong now. Maybe the problem will come back because this error didn't show up always...

    Regards, Roland
    Yes, I checked the manuals as well. The VT180 do use the original VT100. The VT180 boards cannot be installed in a VT101, VT102 or VT103. Actually pretty interesting/cool. The original VT100 was designed to be expandable, and the VT180 kit is actually using that capability of the VT100. The other VT10x terminals dropped the expandability.

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