USB adapter for an old VT100 keyboard


Ah, the VT100, the first dumb terminal that was controlled with a microprocessor. This ancient beast from the late 70s is quite unlike the terminals you’d find from even five years after its vintage – the keyboard connects via a TRS quarter-inch jack – the electronic and code design of this terminal is a bit weird. [Seth] was up to the challenge of making this mechanical keyboard work as a standard USB device, so he created his own USB adapter.

On the little quarter-inch to USB adapter, [Seth] included an HD 6402 UART to talk to the keyboard, along with a Teensy dev board and a few bits of circuits stolen from DEC engineers. The protocol between the keyboard and terminal is a little weird – first the terminal sets a bit in a status word, then the keyboard scans all the key rows and columns in sequence before telling the terminal it’s done. Yes, this gives the VT100 full n-key rollover, but it’s just weird compared to even an IBM Model M keyboard that’s just a few years younger.

[Seth] finally completed his circuit and wired it up on a perfboard. Everything works just as it should, although a little key remapping was done to keep this keyboard adapter useful for Mac and Windows computers. It’s a wonderful bit of kit, and any insight we can get into the old DEC engineers is a wonderful read in any event.

Vidias below.

23 thoughts on “USB adapter for an old VT100 keyboard

    1. It’s mechanical, but it’s not a buckling spring. All I know is I’ve written 200k words on my Model M, and barely any on my VT100 keyboard.

      …An adapter may help with the VT100, though…

      1. I used to type on various mechanical keyswitches (Apple Extended Keyboard II for the longest time,) but then I got my survived-a-house-fire 1391401 Model M for $20. Never even looked back.

    2. VT52 has MUCH better quality keyswitches and crisper screen than VT100, but 1974 technology means that the keyboard is not detachable, also limited scrolling functionality and LOTS of padding means a slow experience. VT100 needs padding too but not so much.

      VT100 keyboard craps out soon enough due to lack of strain relief on the “guitar cord”.

      VT52 buzzed a mechanical relay for the ^G sound; unsurpassed coolness.

    1. It can be done with the Soarer (XT/AT/PS2/Terminal to USB Converter with NKRO). I’m running mine on a Teensy. Works great on my model M. not sure about the VT100. I prefer my model M though.

  1. Stupid, stupid, stupid.. (ME, not you)
    I’ve been using my VT220 to tour the interwebs in text with no pics.
    Gotta get this built and bring me up to speed.

    Well done sir….I SAID, WELL DONE!!

  2. I never seen this model of keyboard before. Granted my oilfield work experience never had me around terminals, and the keyboards used with them, much less use one, to develop any nostalgia. Different strokes for different folks, but I never have assigned a dollar value to my leisure time. Then again not many would have gambled $15 that the old high impedance headphones I ran across the other day work. I really should toss a inexpensive DMM in my pickup toolbox.

    1. VT100 is from the days of elegant engineering when thermal engineers roamed the hallways. Gear for close human proximity had no fans; even the TTL designs that could cook toast, were designed like chimneys. The original Mac was the ultimate expression. Today nobody cares, slap a fan in there and who cares about the noise.

    1. That, and the fact that it was a VT100 means its pretty much the definition of “not a dumb terminal.” Dumb terminals were basically “glass teletypes,” they just output what they were fed, no going back. The VT100, and others, supported escape sequences that basically made the screen rewritable in place, as used by any UNIX/Linux app that relies on CURSES

    1. For years I used an old Dell ps/2 mechanical keyboard and drove everyone in the office nuts with it, until the birth of the Intel MacBook stuck me with USB. I fingers pined for the incomparable touch of mechanical keyswitches until a friend tracked down a decent USB to ps/2 adapter. The rest of my team mocked me for the longest time until one of them discovered that unicomp (the makers of the original model M) are still making buckling spring keyboards. Now the whole team drives the rest of the office completely mad!

  3. Mmm, I remember noticing as a kid (we had one of these in the basement) that you could mash the keyboard and even while holding down a palmful of keys, pressing another key would register.

  4. Fascinating project. I have used a Teensy and Soarer’s Converter for an IBM XT keyboard, but now I am interested in older terminal keyboards, such as the DEC VT100, Televideo 950, and others. Is your approach generalizable to other terminal keyboards?

  5. Fascinating project. I am typing this on an IBM XT connected to a Mac via a Teensy and Soarer’s Converter. However, I am now interested in getting older terminal keyboards, such as a Televideo 950, up and running with modern computers. Is your approach generalizable to terminal keyboards other than the DEC VT100?

    1. Torn apart a VT100 keyboard here.

      The keyswitches have two metal leaves, separated by a plastic spring-loaded spacer. Depressing the key pushes the spacer down, allowing the leaves to spring back towards each other. Officially, they’re Hi-Tek Linear switches. See here:

      > and what keyboards today emulate it?
      None. As far as how they operate, they’re unique. You’ll probably get the same feeling from MX Reds, though. To answer your next question, no, you won’t be able to salvage keyswitches from an old VT100 keyboard.

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