S7-300 PLC CPU 314 - First Scan Bit

SelimHS

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Join Date
May 2018
Location
Denmark
Posts
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Hello Everyone!

As the title suggests, i would like to know how to program a "First Scan Bit" in S7-300 PLC CPU314. I made a sequence with 3 cylinders, which you can see in the file i attached. You will also notice in the file, that there's a first scan bit in the beginning. This is what i would normally do, if i was using a S7-1200. But sadly, there's no first scan bit in S7-300. You have to make it yourself.

I read something about OB100 (warm restart) which is the only thing you can use in S7-300 for a "First Scan Bit". The problem is, i'm confused to what should go inside the OB100. What should i put inside? :(

I should also mention, that i'm a beginner. So don't be too hard on me. :unsure:
 
You'll have to dig this a bit more, but the first OB being run when the PLC starts is OB100 (or similar, I think 100 is warm restart). Inside that OB, set a bit to 1.
At the end of OB1, reset the bit to zero.

Though I would think that OB1 has a first run bit, but don't quote me on that.
 
Same bit address on all instructions:

1st rung in OB1:
--|/|---------------------(S)--


last rung in OB1
--------------------------(R)--

Or this for the last rung in OB1:
---| |--------------------(R)--
|
---|/|-------
 
Last edited:

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