plc analog input module is continuously changing physical

plc analog input modules are continuously changing physical quantities such as voltage, temperature, pressure and speed, such as PLC, can not directly handle digital output plc‍, analog input module requires use of A / D converter converts the analog input signal into a proportional to the digital. PLC in digital (such as the output of the PID controller) with analog output module requires the programmable logic controller plc‍ converter to convert them into the corresponding digital proportional voltage or current for external actuators (eg electric control valve or drive) to use.