Plc ladder logic tutorial

Written by vicki elander
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Plc ladder logic tutorial
PLC Ladder Logic is used to control the machinery and process that makes noodles. (noodle image by red 2000 from

PLC is the acronym for a programmable logic controller. These little machines are programmed with software referred to as ladder logic. Ladder logic differs from other typical programming methods; it is a visual, pictorial method. The finished result looks very much like a ladder, and is the reason it is referred to as ladder logic. PLCs, together with their ladder logic programming, are used to run machinery and/or processes autonomously or with operator control through buttons or touch screen.

Skill level:

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Things you need

  • PLC programming software

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  1. 1

    Determine the requirements of your system. This includes the input and output devices needed.

  2. 2

    Determine the sequence of operations for your system. This includes determining the conditions required, if any for the output devices, as well as what will happen when the input devices are activated. A flow chart can be helpful in this step.

  3. 3

    Order the rungs of the ladder logic program to follow the sequence of operations. Digital input devices are devices that are either on or off and will be NO or NC contacts on the left side of the rungs. NO contacts look basically like --| |-- and NC contacts look basically like --|\|--. Digital output devices are devices that will be turned on or off by the program and will be a coil that typically looks like --O-- on the right side of the rungs.

  4. 4

    Assign addresses to the input and output devices. These addresses tell the PLC where to look for input signals and where to send the output signals. Each physical device is wired to a location on a module in the PLC that has been chosen based on the physical requirements of each device. The addresses in the software point to the physical modules--like a map.

  5. 5

    Get the communication cable for your PLC and connect it to your PC. Load your completed ladder logic program on the PLC and run it to test your program. Make any changes, if necessary.

Tips and warnings

  • The wiring of the physical devices needs to be done by a qualified electrician.
  • In a ladder logic program, each rang is required to have an output device. It is possible to use an internal memory address for these when necessary.
  • The sequence of operation is important, as the program is executed top to bottom and left to right. It executes quickly, but only one rang at a time.
  • Refer to the manual for your PLC for all procedures and commands available. Many advanced commands are available for most brands.

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