The keyboard has two interfaces: I2C for MCU or 4D display and 8x8 for vintage CPU, like Z80A. The keyboard has 16-digit LED display 0.28" high and 8-bit I/O port accessible via I2C. The keyboard use tactile switch from CT and is controlled with an MCU with FIFO, debouncing function and has a sound generator. The 16-dogit display is multiplexed and can be drived with Maxim 7219 or 7221 controller. Keyboard is powered from +3.3V and has size 7.3" x 3.8" (186 x 96mm).
Next step is development a MCU board to complete the RPN calculator.