The PLC Ladder Diagram programming language is used to program PLCs (programmable logic controllers). PLCs, or programmable logic controllers, are digital computers that perform control functions, typically in industrial applications. Ladder logic is the only language for programming a PLC directly modeled after electromechanical relay systems. The PLC Ladder Diagram has three rungs. The CPU "scans" or runs the program from left to right and top to bottom. The symbols on the rungs are graphical instructions. PLCs typically include a timer, counter, latching, and advanced logic instructions.