The block diagram of Figure is a functional representation of a type of digital control system, in which G and H serve the same function as in any feedback system. Note that the error signal is sampled and a digital processor is used. The controller in this system is the digital processor whose output, reconverted to an analog signal, becomes the excitation for the block G.

As usual, G is the subsystem that provides the output to be controlled. A central computer which controls several functions could be used as a digital processor; or a microprocessor (special- purpose computer) designed for the particular control function may also be used as a digital processor. Large, high-speed computers with their speed, memory, and computational ability, as well as programmability, are utilized for central control in large automated manufacturing facilities. The control process may be summarized as follows:

1. The computer is programmed to indicate the sequence of operations required. Data are fed continuously from various monitoring stations on the progress of the process.

2. By comparing the actual and desired performance, the computer generates a new set of instructions to correct for the deviations.

3. The new set of instructions are usually converted to an analog signal, which in turn forms the excitation applied to the machines that actually do the manufacturing.