Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Terminal 'Common' Question

  1. Nov 6, 2013 #1
    Can someone please explain what exactly terminals labeled ‘common’ mean. I originally thought that common referred to any point that was meant to act as a common voltage point. As in, there could exist a common point of 24V that could act like a source, or a common point of 0V to act like ground. I’ve attached an example diagram of a terminal labeled with both a digital input common and a 24V common. Why exactly are there two different commons?

    From reading into this even more, I’ve began believing that 24V common is not meant to act a 24V source, but acts as the negative terminal for a 24V source. I know this should be simple, but it’s confusing me.

    Can anyone explain to me what each of the 5 terminals in the diagram are meant to do with emphasis on the terminals labeled ‘common’?

    Thank you

    Attached Files:

  2. jcsd
  3. Nov 6, 2013 #2


    User Avatar

    Staff: Mentor

    You are correct that the term "common" is generally used for the return, or negative terminal.

    So in this example, the 24V common terminal is the "ground" return for the +24V supply. And the Digital common terminal is the "ground" return for the digital signals.

    Different common terminals may or may not be tied together by the instrument. You generally need to check the specs/datasheet for the instrument to find out. Often they will be tied at a single point in the instrument, referred to as the "star ground" point.
  4. Nov 6, 2013 #3
    Thank you for the reply.

    Let’s say I wanted to have a pushbutton control the digital input. How exactly would this be wired? Would we need the common terminals?

    This is my guess.

    The +24V terminal could be connected to one terminal of the pushbutton and the digital input 0 terminal could be connected to the other terminal of the pushbutton. Let’s say the pushbutton is pushed and the internal switch is closed. The pushbutton should then be transmitting the 24V signal to the digital input 0.

    If I’m correct, would we even need to use the common terminals in this application?

    Because we aren’t really providing power to any load, is that why we wouldn’t need anything connected to the 24V common terminal? And the digital input terminal, is that used as a reference for the digital input signal? As in, we could send a 100V signal (probably shouldn’t though) to the digital input, and if we sent a 76V signal to the digital input common terminal, that would give us a 24V difference between the two terminals which would satisfy the digital input requiring a 24V input?

    I’m primarily trying to wrap my head around the use of these terminal in a push button situation.
  5. Nov 6, 2013 #4


    User Avatar

    Staff: Mentor

    Can you attach the datasheet for the device you are asking about? I'm confused what it means by DiOac taking an AC input.
  6. Nov 6, 2013 #5
    I would, however, the PDF is 41 MB which is way larger than what PF allows. The original screen shot I have attached was an excerpt from a variable frequency drive installation manual if that helps at all.

    Regarding the digital AC input, I would believe that it is simply used a digital input that monitors whether or not a 120V AC signal is present. I’m assuming that the input expects some other device to source that signal.
  7. Nov 6, 2013 #6


    User Avatar

    Staff: Mentor

    Ah, that makes more sense now. So if the 24Vdc digital input is active high, you would connect a pulldown resistor to the 24V return point, and connect a SPST switch between that input and +24V. If the input is active low, you would swap the positions of the resistor and pushbutton.
  8. Nov 6, 2013 #7
    You brought up the idea of a pull down resistor. Is it possible that there could exist an internal pull down resistor?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook