S7-Graph

martin3885

Member
Join Date
Aug 2005
Location
Saarland
Posts
144
Hi, the problem is following...
I did use in S7 Graph in one step an instruction N(german nomenclature, unfortunatelly I don't know english) to set a bit M0.0. And as I thought this bit should be set when the step is active. In FB1(STL) I check the status of M0.0 to so something.
Other possibility of setting this bit only for one step is useing instruction SM0.0 in the step and in the next step to reset it with instruction RM0.0.
But I noticed that there is a difference between those two methods. In first case the program doesn't work properlly, it hangs, and behaves itself so as M0.0=False. It looks like this bit was constantly set and reset when the step is active.
 
What condition are you using for the Transition. I've had this sort of problem when I had Transitions with no conditions - i.e. the Transition was permanently True. It looks as if in some circumstances this can lead to the Step going straight through without actually completing all the instructions.

The simplest way to be sure in this sort of case is to use M0.0 True as a part of (or all of ) the Transition requirement.
 
hi, no I block the transitions by other bit. It is set as soon as other function is completed, and it lasts a while, so it's impossible to go throug this step without setting a M0.0. In VAT I can see that all the time M0.0=TRUE.
 
Sounds as though this may be a case of double addressing, depending on where in the cycle the Merker is being cleared after being set in S7GRAPH the VAT may show the bit as permanently set. You can define with "Variable -> Trigger" when the Variables are read into the VAT, the default appears to be at (before) the start of the cycle. If the instruction that is clearing the Merker then comes before you're program which is trying to check it's state, then you won't see this in the VAT.

Have you double-checked that the Merker isn't being written to somewhere else, including BYTE, WORD and DWORD access? This is easily checked in the X-Ref listing - unless you're using indeirect addressing somewhere, then life gets a bit more complicated!
 
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