By definition, control systems are computer systems that control some kind of physical process or machinery.

The control systems do this by using microprocessor processing power to perform binary logic, control algorithms and batch sequence scripts.

The control system integrates with the physical world through interfaces generally referred to as Inputs and Outputs or more commonly as simply 'IO'.

These interfaces can generally be broken into two basic types:

Analog and Digital

Traditionally, analog signals are inputted into the control system or outputted from the control system through circuitry that has a variable current signal between 4 and 20 mA at 24VDC,

For example, an analog signal measuring how open or closed a control valve would be at 4 mA when the valve is fully closed (0%) and 20 mA when the valve is fully open (100%).  Any other current value would be directly proportional to the percent the valve is open.

Digital Signals are easier to understand as they are ON or OFF signals.

For example, if a motor is running, the circuitry of the signal to the control voltage will have voltage and it will not have voltage if the motor is stopped .