20-key keypad

This is the 20-key keypad for the I/O bus (see the specs for the reZet80 I/O bus).

The keys '0' to 'F' are connected via a 4-bit matrix.
Additionally 4 special keys (ENTER, BACK, ESC and SHIFT) are available.

Bill of materials:
- 20-key keypad
- PCB 5x7 cm (at least 22 rows)
- 22-pin right-angled male connector
- 4-pin male connector
- 8-pin male connector
- 4-wire cable female/female
- 8-wire cable female/female
- DIP-14 socket
- 2x DIP-20 socket
- 74x04 hex inverter
- 2x 74x373 octal latch
- 8x resistor 4k7
- capacitor 100 nF