simatic S7 help

sayahan2003

Member
S
Join Date
Jun 2006
Location
Calgary
Posts
185
Dear friends,
i have a question about first scan of PLC. When i write a program, how can i initialize some state. I mean, during first scan, i dont know what is the situation of the states. if i want to force the PLC to goes to a specific state in first scan, without pressing any inputs and just by powering on the PLC, what can i do? i hope u undrestand my problem.
 
Search in the online help for imformation on blocks OB100,OB101 and OB102 these blocks are called during CPU restart. I believe tghat you should be able to use one or all of these to initialize any values .
 
Hello sahayan?

Didn't we discuss this point (initializing of values for S7 PLCs) just 2 days ago?

What is different in this question?

Daniel Chartier
 
thank u.
but plz look at the following ladder:
quest..jpg

i cant undrestand what is the "first scan" and how it will active. can u explain that?
 
First scan means that whenever the PLC goes from stop to run in first processes OB100 (OB 100 is called first scan) In this OB you can set any bit to initialize your prog. These bits can then be reset in for exeample OB1 (cycle).

hope this helpes you.
 
First scan

Hi,

well if you want the "first scan" to be up for the whole duration of the first scan, you will have to set it to "1" in OB100 and then reset it to "0" in the end of the OB1(last rung). You could reset it also in the second rung of OB1.
 
You can reset it best in the last network of your OB1, then you know that your complete program in OB1 has been "scanned".
 

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