simatic S7 Help

sayahan2003 · Jun 12, 2007

Dear Friends,
Pls tell me what is the problem of the following ladder? it ask me to add some logic in network 4 but i dont want to do like this. i want to use MCR and just activate and deactivate some coil.

Marshy · Jun 12, 2007

You could always put an 'always on' contact in series with it?

dchartier · Jun 12, 2007

Hello sahayan;

Step 7 does not allow an unonditionnal coil assignment on a network:

As Marshy point out, you need a preceding logic input on the network; minimum an "Always ON" bit will do. You can create this bit in OB100, for example.

Hope this helps,
Daniel Chartier

sayahan2003 · Jun 12, 2007

thank you for your help.
But can you tell me why i have to create this "always on" bit in OB100? i mean, i must create it in special OBs?

dchartier · Jun 12, 2007

Hello sahayan;

Just a matter of covenience. OB100 is the cold restart OB, it is a good place to initialize your program variables if required. Setting the "Always ON" and "Always OFF" bits there means they will be set once, and remain in their initial state as long as I need them
You could use OB1, or use a FCx block to initialize them, but OB100 is easier.
Hope this helps,
Daniel Chartier

sayahan2003 · Jun 12, 2007

Dear daniel,
thank you so much for your help:beerchug:

JesperMP · Jun 12, 2007

I prefer to put the Always_on and Always_off bits in some cyclically scanned code.
You risk that they gets messed up by some other error.
For example overlapping words (writing to MW10 writes to all bits M10.0 .. M11.7, MD10 writes to all bits M10.0 .. M13.7 etc).
Or if the bits are changed from an HMI.
Or some indirect adressing gone wrong.

sayahan2003 · Jun 12, 2007

Dear jusper,
thank you but i dont undrestand what you mean.

JesperMP · Jun 12, 2007

Sorry for being unclear.

If you set the always_on and _off bits in one of the startup OBs, then they can be modified by some of the mentioned errors.
If you set the bits in some cyclically scanned code then they will be set back to the right values.
It is not a guarantee against problems, because the faulty code can be scanned later in the program. But it is better than just setting the bits once.

I hope that it makes sense.

sayahan2003 · Jun 12, 2007

Dear jesper
Great. thank you very much

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