reZet80 PIONEER - alternative 20-key keypad

This is the alternative 20-key keypad for the KEYPAD bus (see 20-key keypad with 74C923 key encoder for the KEYPAD bus specs).
The keys '0' to 'F' plus 4 special keys (ENTER, BACK, ESC and SHIFT) are connected via LSTTL chips.
This circuit requires polling the keys constantly.

The card uses two I/O ports in the 8-bit I/O space (see IO decoding specs).
It could also be implemented to use only one I/O port but it would require one extra demultiplexer and currently we have plenty of free I/O ports.

Bill of materials:
- reZet80 PIONEER keypad card (prototype)
- 10-pin right-angled male connector
- 2x 10-pin male socket
- 10-wire cable female/female
- 2x DIP-20 socket
- 74LS244 8-bit buffer
- 74LS374 8-bit flip-flop (or 74LS574)
- 20 key switches (FUTABA MA41)
- 4x resistor 4k7
- 2x capacitor 100 nF