S7-300 First scan?

Do I have a first scan bit in S7-200 ?

I only saw it in low end products like S7-200 or SLC500, Micrologix, etc.

If not, how can I make one like that?

Thanks.

Set a flag in OB100, then reset it at the end of OB1.
Pp

But I have other OBs like OB35 I set to 3ms cycling time may keep 2 or 3 scan cycle and OB34 which is 200ms and the bit will go off before it runs.

Simple answer is "No," but you can create a 1st scan bit. This would be for S7-300/400. (Haven't used S7-200 in a few years and it used to be programmed by S7-MicroWin, not Simatic Manager and was a little different).

Depending on CPU, you have 3 OBs that run at startup; Cold, Warm and Hot.
The main difference is if you restart the CPU by going from program to run or cycle the power.
In the past I have SET a bit in the start up blocks and then after a 2-second delay in another block (FC or OB1) RESET the same bit. This gave a pulse to the SCADA system to tell it the PLC was starting up.

Any questions, please ask.
SMY

Douyi

You have to decide what it is you want to detect. Is it the first scan of OB35, or the first scan of OB34, or the first scan of OB1 ...?

If you need to check on all these possible OBs and react differently, then use multiple bits with each one assigned to a different execution OB. Set them all true at the same time in the start-up OBs (note what SMY-UK said about multiple start-up OBs) and then unset each one at the end of its own OB once that OB has executed.

Ken

I use OB100 and moved first scan logic into there, it works. Since there is not too much things related to first scan, I avoid using several first-scan bits.

The down effect is it may confuse reader or troubleshooter.

Thanks guys.