How would you write this?

mort · Nov 23, 2004

I want to write a simple ladder logic sequence.
When the input comes on, the output turns on.
When the input turns off, the output turns on.

I believe we will need to have the output on a timer so that we can adjust the "time on" and make sure the signal is sensed by another controller before it turns off.

We don't know if the input is a short pulse or is held on for a long time.

I know this is simple, but my brain goes flat every time I try to write it. Thanks.

akreel · Nov 23, 2004

mort said:
When the input comes on, the output turns on.
When the input turns off, the output turns on.

Can you clarify that? When does the output turn off?

Do you mean to say that whenever the input changes state, the output comes on for a short period?

randylud · Nov 23, 2004

confused?

-------]  [----------------(   )------------

----------------]/[-----------(  )---------

The above will do:

When the input comes on, the output turns on. When the input turns off, the output turns on.

I believe we will need to have the output on a timer so that we can adjust the "time on" and make sure the signal is sensed by another controller before it turns off.

When you figure out just what you need to do for sure here, your brain will probably kick in and help you out. Remember, you can not write a program for any situation you do not fully understand! So, try your best, post some of your work and we can help you from there.

mort · Nov 23, 2004

<<Do you mean to say that whenever the input changes state, the output comes on for a short period?>>
Yes, that's right. The output comes on for a short period of time and the "on time" is adjustable. Thx

mort · Nov 23, 2004

Got it! Thanks alot.

Steve Bailey · Nov 23, 2004

I'm not quite sure what you want the output to do. Do you want it to turn on for a brief period every time the input changes state?

If your PLC has one, this might be a place for an off-delay timer. trigger the off-delay timer by parallel one-shots. The parallel one-shots should be the positive and the negative transitions of the input. The preset of the timer will be the duration that the output is on at each transition of the input. If the input is on for less time than the timer preset, you'll only get the output to turn on once.

Terry Woods · Nov 23, 2004

The following is very generic... your mileage might vary.





            INPUT SIGNAL

      +-------------------------+

      |--+                      |--+

   ---+  |                      +- |--

         |                         |

         |                         |

         |  +-----+                |  +-----+

         +->|/////|                +->|/////|

      ------+/////+------      -------+/////+-----

             Timer                     Timer

               |                         |

               +-------------------------+--------> Output

If Input goes ON or OFF...
...Start a timer...
...While the Timer is running, turn ON the Output.

The following assumes that you have transition detectors (if not, they can be easily made).

If the Input Signal changes state while the Timer is running then the Timer will reset at the end of the time as initiated by the previous transition.



  INPUT       RST

 JUST ON      TMR

---|/\|---+---|/|--------( RUN TMR )

          |

  INPUT   |

 JUST OFF |

---|\/|---+

          |

          |

  RUN TMR |

---|\/|---+





 RUN TMR   +-------+

---| |-----| Timer |-----( Time Out )

           +-------+



 RUN TMR   Time Out

---| |--------|/|--------( Output )

mort · Nov 26, 2004

Thanks very much for all the responses.

How would you write this?

mort

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akreel

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randylud

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mort

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mort

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Steve Bailey

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Terry Woods

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mort

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