PLC Logic Standalone Logic Modules

In most of today's modern electrical control scenarios, it is common to see some kind of controller or PLC that is monitoring the state of certain electrical components and providing control over several electrical circuits. This controller has the ability to process the information coming in and send output commands in response. This is all done using some kind of programming language, like ladder logic. However, before controllers were widely used, or in situations where the functions of the electrical circuits were quite simple, relay logic was used.  

So what is relay logic? Relay logic consists of relays wired up in a particular fashion to perform various switching operations to one or many different electrical circuits. The method in which the relays are wired will impact the overall function of the control that is provided. Relay logic circuits use a combination of relays and other components such as switches, motors, timers, actuators, contactors, lights and many other types of electrical hardware. Relay logic control uses basic ON/OFF functionality, which is quite simple, but can perform very complex control tasks depending on how many relays are used and the configuration of the wiring.  

This type of control uses different parts of the relays for different functions.  For example, the coil portion of the relay can be used as a discrete input point. When the coil receives voltage, it will perform a switching operation.  Depending on the relay, this might mean the relay switches just one circuit or it could switch many circuits. This switching can then control other relay coil inputs or it might turn on a light or motor. It all depends on the need of the control circuit.