Bluetooth to LK201 bridge for DEC video terminals

This repository contains Zephyr RTOS firmware code and KiCad hardware files for a device which interfaces a DEC VT to a Bluetooth keyboard via an emulated LK201 interface.

The vtbt board has a socket for an ESP32-C3-DevKitM-1 which provides the microcontroller and Bluetooth functionality (as well as 3.3V regulation). The vtbt provides power from the keyboard interface's 12V supply, performs RS-423 level conversion for the serial interface and has the four LEDs and magnetic buzzer found in a real LK201 because Bluetooth keyboards are unlikely to have those.

I built this after I spent great effort to obtain a VT420 but then neglected to use it due to frustrations with the infamous LK401 keyboard. Once I got the vtbt capable of "self-hosting", I used it with my VT420 to finish developing the firmware over a serial console. I found that the terminal actually provides a very enjoyable and relaxing experience with a decent keyboard!

What works

Everything documented in the VCB02 Video Subsystem Technical manual is implemented including:

• Beeps, keyclicks, beep and keyclick volumes
• The four LK201 LED indicators
• Per-key-division auto-repeat, up-down, down-only modes
• All mode-setting operations for changing modes and auto-repeat timings

What doesn't work

I've only tested the vtbt with DEC VT420 terminals.

The key mapping is set up for the VT420's default "<> Key Sends ~" and "~ Key Sends ESC" settings. It may need adjustment for other terminals or configurations.

Similarly, Alt keys, which are disabled by default on the VT420, are not supported, but support could be added.

Compose keys have not been tested.

Usage

1. Build and flash the firmware to an ESP32-C3-DevKitM-1 connected over USB.

   From a Zephyr environment:

   west flash

2. Mount the ESP32-C3-DevKitM-1 on the vtbt with the USB port facing the vtbt's modular connector and the ESP32-C3 and antenna on the side with the LEDs.

3. Connect the vtbt to a DEC terminal with a straight-wired 4P4C module cable. (I'm working on a new revision of the board which uses rollover wiring so that it works with the once-ubiquitous phone handset cable!)

4. Turn on the DEC terminal. The vtbt will display the terminal's power-up self-test sequence on its four LEDs followed by a beep.

5. Connect to a Bluetooth keyboard.

   The firmware indicates Bluetooth status via the ESP32-C3-DevKitM-1's RGB LED.

   Blue means it is searching for a Bluetooth keyboard.

   Green means it has found a keyboard and is awaiting authorization. To authorize, type 123456 on the keyboard.

   Orange means it is connected to a keyboard and processing keyboard events.

   The firmware stores bonded devices and automatically reconnects to them.

Development

The vtbt's modular connector blocks the ESP32-C3-DevKitM-1's USB port, but for quicker iterations a pair of pin headers as spacers will make it taller.

vtemu.py in this repository can be used as a basic emulation of a DEC terminal for testing.