What is the reason plc cpu modules

(1) We live with analog instruments, is the pressure, level, temperature and other signals into 1-5V ,4-20mA ( there are other signals , such as 0-10V ,0-5V , etc. ) and then to the PLC processed .
2.PLC not know the size of the specific  Nanda Programmable Output Module voltage or current signal .
So, we use a number to indicate its size, then tell PLC, PLC in order to know the voltage and current signals input specifically how much.
For example , we measured the temperature is now 20 degrees ( range 0-50 degrees , the output range is 1-5V), a temperature sensor generates a voltage value at that time is (5-1) * 20/50 = 1.6V ( instrument characteristics must be linear so it can be counted ) . After the PLC analog input module receives input of 1.6V , which will be converted into a digital value. For Nanda digital input module  example , the resolution we are using the analog input module is 0-4000 of the corresponding 1-5V . Well , 2.0V should correspond ( 4000-0 ) / ( 5-1 ) * 1.6 = 1600. That is , PLC , after reading the analog input module to its 1600 figures will be able to know it , from the temperature sensor to sensor temperature is 20 degrees