Bus and bus protocols

Up to now, we have dealt with digital and analog components. GPIO logic is much like ON/OFF switches. It is not good for exchanging complex information. Data exchange in between intelligent devices can not be done by GPIO logic. Then we dealt with sensors of analog type. Analog sensors generate voltage on output pin proportional to measured units. Reading information from them is straightforward. New generation digital sensors do a lot more than this. They accept and respond to supported commands. They can have the capability to do signal processing, data storage, analog-digital conversion, auto-calibration, and so on. They generate more information than digital HIGH/LOW or analog voltage equivalent to measured units. An example is GPS coordinates information by GPS sensor or current time information by the RTC clock. GPIO/analog logic is not enough to transfer this complex data. Besides these smart sensors, many output devices expect complex information for their work...