Programmable Logic Controller. The "brains" behind almost every show control and automation system. Essentially a sophisticated series of inputs and outputs, PLCs use ladder logic to determine, based on the inputs, what state in which to set outputs. For instance, when an operator presses "GO" to execute a scenic unit move, the PLC will scan the E-Stop bus to verify no E-Stops are pressed, an intrusion alarm circuit to make sure no one or no thing is in the path, and one or more dead-man enables to make sure a human is watching for safety, then it will instruct the motors that actuate the scenic unit to move. Once the unit has reached a predetermined limit, via switch or counter, the PLC will stop the unit. A factory's automated assembly line works in the exact same fashion, which is from where the entertainment industry acquired the technology. For more information, see Chapter Nine of John Huntington's book, Control Systems for Live Entertainment, 3rd Edition.