RS-232 Keyboard Matrix Circuit for up to 512 switches/buttons

Features:

up to 512 switches/push buttons
- should suffice to control 3 CDUs, overhead, pedestal, and many other home cockpit controls
costs for components less than/around 10 Euros/Dollars (excluding switches and diodes)
uses 1 RS-232 Serial Port
based on a PIC 16F628 / 16F628
uses several 74LS138 (inverting 3-to-8 line decoder/demultiplexer) to realize a scanning keyboard matrix
possible to start out with a smaller variant (eg. 64 switches/buttons) and extend in steps of 64 up to 512
needs a diode (eg. 1N4148) for each switch
- for each switch
  - sometimes you can find buttons with a diode alreay built in (eg. order-no: 47-420032, available from www.pollin.de for 0.05 Euros each)
- or one for each rotary switch (if you put the diode in series with the common terminal)

Software/Firmware will be GPL licensed.

Firmware (PIC assembly language) currently work in progress
Software on PC side will be very simple to design, I will only create something for Linux
- If you want something for Windows, someone else will have to create that, but I'll link to it if somebody does
- I don't know how this could work with M$FS, since most solutions use keyboard emulators, and since 512 keys might be a problem to assign in such an emulator

Here's a preliminary circuit schematic:

Click to enlarge [30k, 1090x827]