PLC Ladder Logic Basics

by Vicki Elander

PLC Ladder Logic is the programming language of programmable logic controllers (PLCs). This differs from almost all other programming languages because it is a visual program, called ladder logic because it looks very similar to a ladder. The power rails are on the left and the right and the rungs with instructions run between them.

Digital Inputs

Digital inputs are also referred to as contacts, and can be either normally open (NO) or normally closed (NC). Visually, they will appear in the program as NO: --| |-- and NC: --|/|--. With a normally open contact, power flows through the contact when the input is energized. If the input is a switch, it is energized when the switch is turned on. A normally closed contact is just the opposite, and power will flow when the input is not energized. To use the switch example again, the NC contact passes power when the switch is turned off.

Digital Outputs

Digital outputs are also referred to as coils and will appear in the program as: --( )--. When power flows to the coil, the output is activated. If it is a light, then the light will turn on.

Instruction Blocks

Instruction blocks vary from PLC to PLC--each software program has a slightly different look to it as well as some that are only found on one manufacturer's equipment. There are analog inputs and outputs that read or send a variable number. There are math blocks that will do basic math functions such as add, subtract, multiply and divide. There are comparison blocks to look for numbers greater than, less than or equal to other numbers. There are timers and counters and other various instructions that you can use to program your system.

About the Author

Vicki Elander has been writing software documentation and technical manuals since 1993. In 2008, she wrote product reviews for Elander has a bachelor's degree in electrical engineering from the University of North Dakota.

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