Modern computers do not respond to energy represented in any form other than voltage generally, not even current. However, The Figure shows the generalized "outside world" as an external system which is propelled by continuous (analog) energy sources (mechanical, electrical, thermal, etc.) and feeds back analog signals that represent some form of energy (eg: temperature, voltage, current, pressure, stress, strain, position, velocity, acceleration, etc.) which is, in general, not a voltage. The closed loop of Figure clearly represents some problems in terms of interfacing the small digital voltage signal world of the computer to the analog energy world outside. Signals fed back from the external system to the computer need to be:

(i) Converted from their original form to voltage
(ii) The raw voltage levels may need to be isolated from the external system
(iii) The raw voltage levels need to be scaled to a level compatible with the
needs of computer circuits
(iv) In the event that the scaled voltages may suffer from unwanted spikes (as a
result of spikes in the energy levels received from the outside world),
protection circuits need to be considered to prevent high signals from
damaging the computer
(v) The scaled voltages need to be converted into a digital form.

The other major problem with connecting digital computers to the outside world is that the circuits within the computer are not only low in energy consumption, but also incapable of providing a large amount of energy. We know that signals within the computer are represented by digital voltages, somewhere in the order of those shown for TTL circuits. However, we also need to appreciate that the major limitation of those circuits within a computer is that they are unable to provide large amounts of current. Typically, a digital circuit can provide less than a milli-Amp of current. This means that digital circuits can only provide a few milli-Watts of power. The actual amount depends upon the type of digital circuit but, in any event, is generally much less than the energy normally required to drive many external systems.

Consider the case where a computer is required to control a power station. The external system is a generator driven by a rotating turbine. The signals fed back to the computer may include items such as turbine speed, output voltages, etc. Based upon these signals, the computer needs to provide a controlled driving force that will cause the turbine to rotate at a given speed - clearly the computer is not going to supply the Mega-Watts of mechanical power that are required to rotate the turbine. Rather, the "driving force" produced by the computer is a very low level electrical signal that is used to drive actuators, valves, etc. that control the flow of steam or water to the generator turbine. The problem is then how to convert the computer's controlling signals into some form of energy that can ultimately drive the external system.

Assuming then, that the computer circuits are unable to provide the required driving energy for an external system, there are a number of interfacing steps that may need to be carried out in order to achieve the required outcome. These include:

(i) Conversion of the computer's internal digital voltages to analog form
(ii) Amplification of the analog voltages to higher energy levels
(iii) Isolation of the computer circuits from the external system
(iv) Conversion from the analog voltage form to the required final energy
(mechanical energy, thermal energy, etc.).