Trying to understand this wiring loop

Discussion in 'General Electronics Chat' started by Alice24, Oct 7, 2014.

  1. Alice24

    Thread Starter New Member

    Apr 22, 2011
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    Hi guys, I have a PLC with inputs and outputs...from some reason it shows that X0 has a PNP transistor, while the other inputs have switches...why does the first input need a transistor and not the other ones?

    I attached the specific wiring loop I'm talking about to this topic

    And here is a link to the the entire instruction sheet of my PLC:
    http://www.delta.com.tw/product/em/...ual/SA Instruction Sheet-English-20060112.pdf


    -Alice
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    This diagram shows that the PLC input accepts signals from devices that pull-up to the +24V rail. The diagram gives examples of a PNP transistor or a switch being used to provide this input.

    In the world of process control and PLCs these are "source" or "PNP" signals
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    428
    that inpout is being used to detect if the 24 volt power supply is on. most plc input can be used with solid state or contact switching, just be carefull with the polarity and voltage using solid state inputs.
     
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  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Just to add it is a safety feature to prevent outputs that could be turned on when a logic low is input if the input P.S. failed, the PLC logic could interpret these inputs as off and inadvertently turn on the separately powered outputs.
    Max.
     
  5. Alice24

    Thread Starter New Member

    Apr 22, 2011
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    Let me see if I get it right. As soon as the "gate" at the transistor opens up, there is a potential difference of 24 volts across every input (X0, X1, X2...)?
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    No, the other inputs are totally independent of one another, I am not sure why they used a transistor, a direct connection from the 24vdc would have done the same thing.
    Once the 24vdc is present, the transistor directs 24v to input 0.
    Max.
     
  7. Alice24

    Thread Starter New Member

    Apr 22, 2011
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    Doesn't that mean that X0 is always on, or that without X0 being on, the other inputs can't turn on?
     
  8. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    No, its is just a example how you can use it, nothing more. This transistor may for example represent a pnp type proximity sensor.
    And you can connect it to any input you want. Stop worrying about this.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    X0 is always on and it would be used internally in the logic to ensure that one or more critical outputs were not turned, as outputs are generally fed from some other supply.
    Max.
     
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