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Author Topic: Control inputs - active low/high?  (Read 1326 times)

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Offline Davek0974

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Control inputs - active low/high?
« on: July 08, 2016, 04:31:07 AM »
Seems the safest way is to have the limit switch loop as active low i.e the switches are all in series and are normally closed, this way a broken wire will trigger a stop instead of being ignored.

But control inputs - cycle-start buttons, home switches etc - active low or high?

My PLC stuff was always active high - normally open contacts that supply a voltage when and action is needed - this way a short to ground of the supply wire will blow a fuse, short to ground of the return will do nothing, only a supply-return short could be dodgy as it will be seen as a request for action.

Does this all remain true in the world of CNC?
Re: Control inputs - active low/high?
« Reply #1 on: July 09, 2016, 08:06:50 PM »
Good electrical practice is good practice CNC or not. I build waste treatment plants, always have to think "if it fails what would be best?". So high level float switches are always wired normally closed