• Shawn Tierney commented on lesson Lesson 07: First Program 3 years, 3 months ago

    Good morning Jose,

    Thanks for your question.

    In this course, the address I:0/3 is wired to the Normally Closed contact of the Stop Button.

    The next question is typically “why” do we use a Normally Closed contacts on Stop Buttons? Why would we want the act of “pushing” the stop button to “Open” the contacts and stop sending power to the stop input?

    The reason is it's a safety measure.

    Imagine if the wire running to the stop button broke, or came loose, or was cut.

    Because the system is designed to see no power on the stop input as a stop command, any situation where power is lost on the stop input would result in the system stopping.

    This is a very important safety feature, just like how your car won't run without the key in the ignition – the machine won't run if the stop button is pressed OR the wiring from the stop button is broken/cut/comes loose.

    With that in mind, we know that no-power on the stop button input (i.e. a Zero in the data table) would equal the “Stop” button being pressed – which means the operator wants to stop the machine.

    We also know that with power on the Stop Input (a One in the data table,) the operator has not pressed the stop button, and therefore does not want to Stop the machine.

    This is why we use a XIC and not an XIO. We want a One from the Stop Button Input to allow the system to run, and a Zero from the Stop Button Input to stop the system from running.

    Note that these instructions do not care what type of contacts are wired to the PLC. All they care about is what the status of the input ciruit is – does the input have power or not.

    Here's an exercise to understand this better:
    – Wire the following devices to your PLC: a Pilot light, a normally open button, and a normally close push button.
    – Duplicate my motor control code from this lesson using the light as the motor, the normally open push button as the start, and the normally closed as the stop
    – After testing the code as I've written it, change the Stop button's instruction from an XIC to XIO to see the difference in operation.
    – Next, replace the normally closed Stop button with a normally open Stop button and test again.
    – Finally, power off the system, disconnect the stop button, and now see if you can start and stop the system (note – only do this with the pilot light representing the motor – and not a real motor – since you will not be able to stop the system)

    Hope this helps,

    Shawn Tierney,
    Instructor at The Automation School