Siemens S7-300 start up bit and always on bit?

I have done a couple S7-200 programs, but this is my first S7-300. I can't seem to find any documentation on a startup bit(on for just the first scan) or an always on bit? can anyone help me out with this? Thanks.
Don_t_tell

"Speak no evil, hear no evil, see no evil"

You could just set a bit in the startup OB, and it will remain set forever until you reset it. For instance:

SET
= M10.0

That will set M10.0 unconditionally, and you can then use it anywhere you want.

With that being said, there really isn't any need for an "Always On" or "Always Off" bit in S7. Instead, you can use the CLR and SET instructions. These simply set the RLO to 1 or 0 unconditionally, and you will be all set (no pun intended). I use them liberally throughout my programs.

There is no first scan bit, just the OB100,
first scan OB. There is no "always on" bit like GE plc's.

Always Off Always Off Always Off
-| |-----------|/|---------( )

Always On Always On
-----| |--------------------( )
|
Always On |
-----|/|--------|


Put this code in ob1

Let Always Off=M1.0
Let Always On =M1.1

There is also a byte called "OB1_SCAN_1" in the local data for OB1. If it has the value 1 it's the first scan, if it's 3 then it's on a subsequent scan.

That OB1_SCAN_1 is only on at startup after a cold restart.
You have to do something else in the other situations.

I set a 1st scan bit in OB100 (hot restart), OB101 (Warm restart) and OB102 (cold restart).

thanks!

thanks.

I'm so glad I found this site. You all are very helpful.