LK201 Breakout

A prototyping tool for DEC terminals that use the LK201 keyboard

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A 3D render of a square PCB. It has two black connectors, and a handful of small passive components surrounding one main integrated circuit in the middle.
  • It looks like this. Behold! Rendered in Blender, using the pcb2blender plugin.

A breakout board and serial-to-TTL IC for hacking/communicating with DEC terminals such as the VT220 and the LK201 keyboard. There are two RJ10 jacks to plug in the terminal and keyboard, and a variety of pin headers which allow hooking in either before or after level shifting.

This was originally intended as a stepping stone to a USB Keyboard interface, which would allow use of any modern class-compliant USB keyboard with the terminal. If this is something you’d be interested in, let me know…

The design

I used KiCAD. Here’s the Git repo.

Hardware git repo

And here’s the schematic embedded:

Schematic

How to use it

🚨 A disclaimer
I tested this on my own VT220 with no issues, but this is very much homebrew hardware and I can't guarantee you won't break something with it!
  • You can bridge pins on J5 “RS232” to electrically connect (loopback) the DEC and Keyboard, acting as a simple coupler. This allows you to sniff the serial traffic between the two on the J4 “TTL” header.
    • You can also achieve the same thing by jumpering the J2 “TTL Loopback” header, in which case the loopback happens the “other side” of the 232-TTL conversion.
    • In either case, you end up with 2 signals: DEC to Keyboard (on the DEC Out / KBD in pins) and Keyboard to DEC (on the KBD Out / DEC in pins)
  • With no jumpering, you can hook up a TTL-compatible thing to the TTL pins to emulate the keyboard.
  • You could do the opposite and emulate the DEC but you’d need to supply 12v to the connected keyboard by soldering a bodge wire. If I do a revB I’d put this on a header pin.

Here’s some useful documentation of the protocol the DEC and keyboard use.

Where can I get one?

I can probably help - email me! It’s sam@ this domain. Otherwise it’s pretty straightforward to build yourself.

Some warnings

  • Be careful not to connect between the RS232 pins (±6v) and the TTL pins (0-3.3v)! This will almost certainly break stuff.
  • Make sure you don’t swap the DEC and KBD RJ10 jacks - they have opposite power polarity and although this shouldn’t break the DEC or keyboard, it will put almost certainly blow up the Serial to TTL chip and any microcontroller you have connected.