Originally Posted by Al_The_Man  In the realm of PLC inputs etc, the term Active Low or High, usually means what the status has to be to activate the named Function of the input.
Take for example over travel switches used in systems like Fanuc or Mitsubishi etc, these are usually dedicated inputs that have to have 24vdc to disable the OT condition, the OT occurs when the NC switch opens and the input goes to a logic 0, This is then Active Low for the assigned condition to occur.
Al. |
Al,
Then the issue is what is the signal you are tracking. Given a NC limit switch that is high when closed, one could say that it is "active high" for the condition - "within limits" (i.e., the limit switch is closed and powered) but if you are considering the condition to be "over travel" the signal is "active low" when you trip the limit switch (or power to the switch fails).
Alan