How to detect a new ladder has been loaded

[MalSnaize]

Hi Team

As a rule I try not to preset values at power-up so the PLC will retain it's state after a power fail. This is particularly useful when I'm running timed or timer functions which just pick up where they left off after a power glitch. Sometimes I use SB 2 when I do need to reset stuff after power up.

Now the real question - Is there a way of detecting when a new programme has been downloaded?

I have been trying to use SB75 to detect when a new ladder has been loaded; that's when I definitely want to do a full reset of values to known states.

As my very first action in Main I've tried testing the bit being Set, Negative Transition, Positive Transition and for each case I've SET an MB (which has a power-up reset state).

Nothing seems to move that MB.

So I'm wondering, does SB75 actually go high in first scan and drop at end of scan? Maybe I'm misunderstanding what SB75 is for.

Is there perhaps a better way to know I have a new ladder downloaded to the PLC? 

I possibly could have a current-version MI and a last-version MI and make sure I change the current-version power-up value to check for version change but that's a little mes

Oh also, I'm using a V1210.

 

Many thanks for any guidance coming my way

 

cheers Mal