In an analog control system, the controller consists of traditional analog devices and circuits, that is, linear amplifiers. The first control systems were analog because it was  the only available technology. In the analog control system, any change in either set point or feedback is sensed immediately, and the amplifiers adjust their output (to the actuator) accordingly.
In a digital control system, the controller uses a digital circuit. In most cases, this circuit  is actually a computer, usually microprocessor- or microcontroller-based. The computer executes a program that repeats over-and-over (each repetition is called an iteration  or scan). The program instructs the computer to read the set point and sensor data and then  use these numbers to calculate the controller output (which is sent to the actuator). The  program then loops back to the beginning and starts over again. The total time for one pass  through the program may be less than 1 millisecond (ms). The digital system only “looks”
at the inputs at a certain time in the scan and gives the updated output later. If an input  changes just after the computer looked at it, that change will remain undetected until the  next time through the scan. This is fundamentally different than the analog system, which  is continuous and responds immediately to any changes. However, for most digital control  systems, the scan time is so short compared with the response time of the process being controlled that, for all practical purposes, the controller response is  instantaneous.
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