i had typed an explanation, but the internet crashed on me and it was lost. PLC=programmable logic controller BCD=binary coded decimal a PLC takes inputs from switches, sensors, buttons, or touchscreens and based on the program will turn on outputs such as lights, motors, valves or indicators on a touchscreen. with a PLC and touchscreen, you can literally create any type of control system you can imagine. i do automated machine design and get to play with these all the time. very cool stuff. you don't have to have a touchscreen though. you can have just switches and lights.