This is an experiment to add a USB keyboard to the Vectrex using *only* a Raspberry Pi (which many people
have) and wires - no electronic components, not even resistors.  Honestly it shouldn't be expected to work,
but it does :-)

The Pi connects to two pins in the second controller port corresponding to two of the controller buttons; it
breaks any keyboard key value down to 8 bits which it sends serially - each bit being set up, then a 'clock'
or 'enable' bit being used as an edge trigger (both rising edge and falling edge) to indicate that the data
bit is valid.  As long as there is a sufficient gap between bits, the actual speed is not important and does
not have to be the same between bits.

vectrex.c is a sample receiver program to run on the Vectrex, and linux.c is the sender program which reads
from the keyboard and sends each key to the Vectrex.

Since I used a very old and cheap Pi 1 A for this test, the pin numbering for the 26 pin header is slightly
different from current pies.

There's a photograph in here showing the simple connection to the Pi - a ground connection from pin 8 on
the controller port to Pi GND, physical pin 20 (GPIO 25), and from two button pins (1 and 2) to Pi physical
pins 16 and 18 (GPIO 4 and GPIO 5; Broadcom 23 and 24).