Basic knowledge: switching volume and analog quantity

Switch only two opposite working condition, such as high level and low level, power and power relay coils, contacts connected and disconnected, PLC can be directly input and output switching signals. Some PLC (such as SIEMENS S7 Series) the switch is called the digital quantity.
Analog is a physical quantity, such as continuous variation of voltage, temperature, pressure and speed of PLC, direct analog processing can not, need to use analog input module in the A/D converter, the analog to digital conversion digital.PLC is proportional to the input signal (such as PID in the output of the controller) need to use the analog output module in D/A converter to convert them into a proportional voltage or current and the corresponding figures for external enforcement agencies (such as the electric control valve or inverter).
So what is the amount of switching? What is analog?
The switch is not energized, is the power, or the 1 and 0,1 on behalf of power, 0 representatives of power. For example, button, switch, time relay, current pressure relay, which belongs to the category of the input, the output of contactor, relay, solenoid valve, etc.
Analog is a continuous variable, for example, inputs are: sensors, (many kinds) output: servo motor, solenoid valve, distance, speed, and other control signals