Well, here's something to look at.
Internal 64 K RAM memory expansion, CRU I/O bank enabling and VDP access delay logic. Load interrupt and reset switches also included.
Peripheral expansion box with real time clock and digital/analog I/O board. Later, cards with real time clock, like CorComp's Triple Tech, became available, but there was nothing to buy when I built my clock card.
Analog joystick with buttons, 7-segment digits and LED. It's not a modification of the 99/4A itself, but connects to it. Inside there's a dual port 4*4 bit memory, which holds the numbers to show on the display, LED status and controllable leading zero ripple blanking for the display. This is accessed by a four bit data bus, two bit address bus and one WE signal from the computer. The rest of the wires are for power supply and analog signals from the joystick. The electronics inside the joystick are built on three separate PCBs, stacked on top of each other.
Edited by apersson850, Tue May 23, 2017 4:09 AM.