plc analog input modules, What is a switch? What is Analog

Switch only two opposite working condition, such as high and low, the relay coil is energized and off, on and off an electric shock, plc analog input output io modulebe directly input and output switch signal. Some PLC (eg Siemens S7 series) will be called a digital switch. analog is continuously changing quantities such as voltage, temperature, pressure and speed, such as PLC, does not deal directly with analog, you need to enter the module a / D converter with analog unit, the analog to digitalconverter is proportional plc digital input output io moduleto the input signal. PLC in digital (such as the output of the PID controller) with analog output module requires the D / A converter to convert them into the corresponding figure proportional voltage or current for external actuators (such as electric control valve or drive) to use.