How to make an inverted stop button in winCC 2008

Hallo all.
Im trying to make an inverted stop button in winCC flexible 2008. The reason for this is, i want to make sure that the machine stops, in case of power loss to the panel. I somehow need to make a function in winCC, that sets a bit 1 in the plc all the time i think. Unless the button is activated, or the power fails. Picture below shows my idea.

Regards jydepower.

Hej jydepower.

I suggest that you set it up as a "life-bit monitoring".

In the S7, you let one bit or counter invert every second for example.

In the HMI, you monitor the bit or counter by assigning it to a tag ("life_bit" for ex.), On the tag you add a SetValue function that is triggered by ChangeValue event on the tag. The SetValue copies the tag value to another tag ("life_bit_return" for ex.)

In the S7 the program monitors "life_bit_return" by means of oneshots on the rising and falling edges. When no rising or falling edge is encountered within 5 seconds for example, you can set a global variable "HMI_failed" for example. You can use that variable to halt the machine.

There are other methods, but this one is the most straightforward.

Hej jesper.
Thanks for the reply. It almost works. I cant get it to work unless i add the life_bit tag to an object. A box or something.

The tags in winCC should be able to work like a bit in the plc right, or do i get this wrong?

You have to set the tag to update cyclically, as opposed the default setting "on demand".

Works perfect.
Thanks once again Jesper.

This is exactly what I do with all my HMI's. One thing to keep in mind: During the installation and testphase I keep the monitoring time at atleast 1 minute. It's very annoying if the installation suddenly stops if you execute a download to your HMI after correcting a misspelling noticed by an operator.