![]() ![]() An example of ladder logic can be seen in See A Simple Ladder Logic Diagram. This eliminated the need to teach the electricians, technicians and engineers how to program a computer - but, this method has stuck and it is the most common technique for programming PLCs today. The first PLCs were programmed with a technique that was based on relay logic wiring schematics. When the input coil is not energized the normally closed contacts will be closed (conducting). Normally closed contacts are shown with two lines with a diagonal line through them. Normally open contacts are shown as two lines, and will be open (non-conducting) when the input is not energized. The output contacts are shown with two parallel lines. Relays are normally drawn in schematic form using a circle to represent the input coil. The normally closed contacts touch when the input coil is not energized. The contact that closes when the coil is energized is called normally open. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. When a voltage is applied to the input coil, the resulting current creates a magnetic field. A relay is a simple device that uses a magnetic field to control a switch, as pictured in See Simple Relay Layouts and Schematics. Modern control systems still include relays, but these are rarely used for logic. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced. The decision to use the relay logic diagrams was a strategic one. As mentioned before, ladder logic has been developed to mimic relay logic. Ladder logic is the main programming method used for PLCs. Reliable components make these likely to operate for years before failure.Trouble shooting aids make programming easier and reduce downtime.Computational abilities allow more sophisticated control.Flexible and can be reapplied to control other systems quickly and easily.Cost effective for controlling complex systems.Most of this is because of the advantages they offer. PLCs have been gaining popularity on the factory floor and will probably remain predominant for some time to come. The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). It is common to use relays to make simple logical control decisions. These relays allow power to be switched on and off without a mechanical switch. More recently electricity has been used for control and early electrical control was based on relays. In the past humans were the main method for controlling a system. Control engineering has evolved over time.
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