Programmable Logic Controllers (PLC) are programmed using desktop or laptop personal computers (PC) with manufacturer's software. The manufacturer's software uses a standard Relay Ladder Logic (RLL) language, which is a variety of instructions performing many different types of operations. A typical RLL program to toggle a light on and off from a momentary switch is shown below.

A series of contacts and coils are needed for each lighting circuit that requires a toggle on/off control. If the application is to raise and lower motorized drapes, than a momentary input with a pulsed output would be required for both the up and down functions. The RLL logic would incorporate a timer instruction to ensure an adequate pulse for each drapery motor unit.

Programming the above and many other functions on the PLC via RLL language code from a PC is very easy. There are a few drawbacks though in programming this way. One is that you have to have a desktop or laptop personal computer. Taking a desktop computer with monitor to a job site to make changes to a program would be very difficult. The other drawback is that every project becomes a custom program, which requires a fair amount of RLL programming code.

Programmable Logic Controllers with built in LCD display and keypad panel eliminates the above drawbacks. Assigning various inputs/outputs and functions is done through the LCD display and keypad panel by entering pre-assigned codes. For example, the toggle and pulse functions mentioned above. Entering the code "14" for the first input of the PLC would operate the toggle function on the fourth output. If the code "26" was entered for the second input, then the sixth output would pulse. For the above codes to work the PLC would require a "generic RLL program" to be installed prior to entering codes on the LCD display and keypad panel.