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This is the fourth segment of the first volume on “How to Program a PLC”. In this series, we will examine an application and then create the code (logic) for the control of the discussed application. In this segment, we edit the logic to accomodate I/O memory locations and more compact code.

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Page edited by E.C. (Google).

2 Comments


  1. Ela
    Oct 18, 2015

    Yeah just figured it out while trying to fall asleep… a limit switch will provide 1 (true) until it reaches the limit… thx anyway :)


  2. Ela
    Oct 17, 2015

    Hi. Thanks a lot for all your videos. I’ve learned a lot. But I do have a question right now and it would be great if someone could answer..
    You have the next situation:

    MOTOR UP is being powered up by CR_UP
    CR_UP(last piece – third line of the program) is TRUE when the UP_PB has been pushed, the CR_DOWN is false and the UP_LIMIT is TRUE.
    This line of the code tells the door to go up, and stop when the limit has been reached. But until the door sets the UP_LIMIT sensor off, it still needs to be moving upwards. So basically the initial conditions in order to get the MOTOR_UP functioning, are not met.

    I have seen this many times and I just cannot understand the logic.. Maybe you could help me understand.
    In my opinion, this doesnt work. Because in order to get the UP_Limit sensor triggered, the door has to move initially
    (even if it is a few inches). But our program says that, in order to get the MOTOR_UP on(and the door moving), we first need to have the UP_LIMIT Sensor triggered. So how does it actually work? What am I missing?

    THANK YOU VERY VERY VERY MUCH IN ADVANCE!!!

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