The hardware can be patched in several independant parts.
Each part is described in a separate chapter.
The chapter 'Electronics' describes voltages and buffering of the ports
on an elementary level, just to understand the possibilities and the other chapters.
The following chapters describe the modifications that can be
Electronics for in- and output (General)
GPIO (bitwise I/O)
De following chapters describe possible hardware expansions
to be performed
However: some external hardware can depend on some internal hardware.
i2c - PCF8574 (8 bit paralell I/O)
For remarks please