🚀 go-pugleaf

First, the keyboards are from the 1200XL and not the 800XL as I said.

Second, the TI keyboards once offered were gobbled up as soon as they hit
the shelves.  If you want an Atari keyboard I suggest polling the RS stores
in your area by telephone ASAP.

The keyboard uses a membrane switching matrix actuated by mechanical keys.
The major difference with the 400 keyboard is that the 1200XL KB includes
the 2 MOS chips that encode the keystrokes. On the 400, the chips are on
the motherboard.  Interfacing requires removing the chips from the
1200 KB and bringing the proper connections out with a 22 wire cable which
can plug into the KB socket on the 400.  The Atari Technical Reference
shows the mapping between the keyboard socket and encoding chip pins.  
Comparing foil patterns is a more difficult way of figuring things out.

Good Luck -
Jim