VT100 to USB Converter Firmware
Version: 1.0 Last Updated: 28 July, 2013
1.0 About
This project contains source code and schematics for a DEC VT100 terminal keyboard to USB converter.
The purpose of the converter is to allow a DEV VT100 terminal keyboard to be used as a USB input device on a modern computer. This is, of course, almost completely useless, but that doesn't mean it's not fun.
2.0 Description
The converter uses a Teensy 2.0 development board powered by an ATmega32U4 microcontroller, which this firmware targets. In addition to the microcontroller, the converter uses an external HD6402 UART, an external clock generator, and a voltage comparator for signalling with the VT100 keyboard.
The firmware is written in AVR C, and can be compiled with the avr-gcc tool chain. The resulting firmware should be loaded onto the Teensy with the Teensy loader.
3.0 Key Mapping
Most keys should be self-explanatory. All alphanumerics and standard punctuation characters map to what you'd expect. Several keys, though, need special attention.
| VT100 Key | USB Keyboard Key |
|---|---|
| NO SCROLL | Scroll Lock |
| BREAK | Pause / Break |
| LINE FEED | Enter / Return |
| PF1 | F1 |
| PF2 | F2 |
| SET-UP | Keyboard Config (see 4.0) |
The VT100 keyboard is notably deficient in modifier keys compared to a modern USB keyboard, so some sacrifices had to be made. PF3 and PF4 do not map to F3 and F4. Instead, they map as follows
| VT100 Key | USB Keyboard Key |
|---|---|
| PF3 | Alt (PC) / Option (Mac) |
| PF4 | Windows (PC) / Command (Mac) |
4.0 Keyboard Configuration
The VT100 keyboard's built-in speaker can be used to generate key-clicks and an audible bell if the options are set in the USB converter's firmware. The options are persisted in the AVR's non-volatile EEPROM between restarts.
4.1 Bell (SET-UP + B)
To toggle an audible bell when the ^G (control-G, ASCII BEL) character is sent, hold down SET-UP and press B. The first LED (labelled L1 on the keyboard) will illuminate when the audible bell is enabled.
4.2 Key Clicks (SET-UP + K)
To toggle key clicks, hold down SET-UP and press K. The second LED (labelled INSERT on the keyboard) will illuminate to show that key clicks are enabled.
5.0 License and Credits
This project is Copyright © 2013 by Seth Morabito, and is distributed under the MIT license. See the file LICENSE.txt for details.
This code makes use of the Teensy USB Keyboard Example Code
(usb_keyboard.c and usb_keyboard.h). This code is Copyright ©
2009 by PJRC.COM, LLC, and is also distributed by PJRC under the MIT
license.