Before the advent of solid-state logic circuits, logical control systems were designed and built exclusively around electromechanical relays. Relays are far from obsolete in modern design, but have been replaced in many of their former roles as logic-level control devices, relegated most often to those applications demanding high current and/or high voltage switching. Systems and processes requiring “on/off” control abound in modern commerce and industry, but such control systems are rarely built from either electromechanical relays or discrete logic gates. Instead, digital computers fill the need, which may be programmed to do a variety of logical functions. In the late 1960’s an American company named Bedford Associates released a computing device they called the MODICON . As an acronym, it meant Mod ular Di gital Con troller, and later became the name of a company division devoted to the design, manufacture, and sale of these special-purpose control computers. Other engineering ...