The IOSTRING is a modular physical user interface system which consists of a series of three basic board modules designed around the Atmel AVR AtMega88 MCU. Each module type can handle switches, pushbuttons, rotary selector switches, rotary shaft encoders, potentiometers, LED’s, LCD displays, and an alphanumeric LED display. Dual footprints are provided on each board to accommodate a large variety of combinations and device types.
Designers simply assemble IOSTRING modules together in a daisy-chain configuration to form an arbitrarily complex user interface panel. The best part of it is that the host only needs a single serial port to interface to all the IOSTRING modules.
For more detail: Modular User Interface System using ATMega88 microcontroller