Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: Davek0974 on July 08, 2016, 04:31:07 AM

Title: Control inputs - active low/high?
Post by: Davek0974 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?
Title: Re: Control inputs - active low/high?
Post by: garyhlucas 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