PLC Logic - Interlock and Permissive

I am not a wiz on PLC's maybe this link is of use

http://forums.mrplc.com/index.php?showtopic=15832

Hi there,

To put this simply - a Premissive is something you need in place to start a machine i.e. a switch must be closed. Once the machine is running you do not care anymore what state this switch is in or changes to.

An Interlock on the other hand is different. It is also known as a Trip. You do care about what state the Interlock is because if it changes you will trip your machine if it was running.

For more information about the terminology and standards try www.isa.org

Regards,

Craig

I think it's simpler than that, also, interlocks are applicable beyond trips.

A Permissive is something that must take place BEFORE another action or sequence of actions can be undertaken.

An Interlock is something that takes place as a RESULT of or as PART of another another action or sequence.

For example a latched Emergency Stop button would be a Permissive: the logic must be true (high, on) before anything else can take place, and must REMAIN true for the entire time, "permitting" the process to proceed.

If I have Start button going to a motor starter, which also has an Auxiliary Contact to indicate that the contactor has closed, the Start is a Permissive, the Aux. contact is an Interlock that I can use to seal-in around the Start button. So in this case the Permissive enables the action to begin, but the Interlock then takes place and allows it to continue after the Permissive is no longer necessary.

Do you check interlocks before starting the equipment or are they checked only after the equipment has already started?