The following illustration shows a simple PLC, as it might appear from a front view. Signal connection and programming standards vary somewhat between different models of PLC, but they are similar enough to allow a “generic” introduction to PLC programming here. Although some PLCs have the ability to input and output low-level DC voltage signals of the magnitude used in logic gate circuits, this is the exception and not the rule.
PLCs are industrial computers, and as such their input and output signals are typically 120 volts AC, just like the electromechanical control relays they were designed to replace. Thus, an industrial electrician or electrical engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC to perform the same control functions. In an effort to make PLCs easy to program, their programming language was designed to resemble ladder logic diagrams. It also has many output terminals, through which it outputs “high” and “low” signals to power lights, solenoids, contactors, small motors, and other devices lending themselves to on/off control. Ladder Logic and Programming PLCsĪ PLC has many “input” terminals, through which it interprets “high” and “low” logical states from sensors and switches. The purpose of a PLC was to directly replace electromechanical relays as logic elements, substituting instead a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks. Other engineering firms developed their own versions of this device, and it eventually came to be known in non-proprietary terms as a PLC, or Programmable Logic Controller. As an acronym, it meant Modular Digital Controller, and later became the name of a company division devoted to the design, manufacture, and sale of these special-purpose control computers. In the late 1960’s an American company named Bedford Associates released a computing device they called the MODICON. The History of Programmable Logic Controllers Instead, digital computers fill the need, which may be programmed to do a variety of logical functions. 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. 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. Before the advent of solid-state logic circuits, logical control systems were designed and built exclusively around electromechanical relays.