Interfacing Computers Via Hardware

When we talk of interfacing computers to external systems, we are generally endeavouring to create closed control loops that enable a digital computer to generate (compute) some desired driving force, based upon the feedback obtained from the outside system. Sometimes, of course, we don't need a closed loop. This is particularly true if we only use a computer based device as a data logging or monitoring system, or if we use the computer to drive a system independently of the feedback (ie: as an open loop controller). However, regardless of the application, the same sorts of issues need to be addressed as a result of the incompatibilities between the "Newtonian-analog outside world" and the "digital computing world". The issues are shown schematically in the below Figure.


When observing the above Figure the most important point to understand is that modern computers will only respond to incoming digital voltage signals and will only generate outgoing digital voltage signals. The levels of these digital voltages are normally in the order of those shown for TTL circuits, although the actual levels depend upon the specific architecture of the digital circuits within the computer.