Siemens S7/TIA v18: Is there a "first scan" in TIA portal?...

[Mas01]

Hi, I want to initialise some tags on first scan only. As they are related to wall-clock time, I want to do the initialisation when the PLC first goes into RUN mode, rather than give the tag a 'start up' value in the datablock.

Is there a built-in tag that I can use for this?

I *think* it might be %M0.0 but I'm not sure.

Thanks

[MarkNightingale]

Go to properties of the CPU.

Go to System and Clock memory

Here you can enable System Memory bits. And then it is possible to select a MB.

Then you have a bit for First scan.

[JesperMP]

For S7-1200/1500 you can activate the system bits. I think you can find them in the CPU properties under "system/clock bytes" or something to that effect. It is described in the dialog what are the merker addresses and their function.
In S7-300/400 you have to use OB100, OB101 and/or OB102.

[mk42]

Originally Posted by Mas01

Hi, I want to initialise some tags on first scan only. As they are related to wall-clock time, I want to do the initialisation when the PLC first goes into RUN mode, rather than give the tag a 'start up' value in the datablock.

Is there a built-in tag that I can use for this?

I *think* it might be %M0.0 but I'm not sure.

Thanks

Couple options, in theory:

• OB1 has an "Initial Call" bit, which the help says is true after startup, or after the OB has been redownloaded.
• You can activate the system byte, which includes the First Scan bit you mentioned.
• You can create a startup OB (OB100 by default) which is called as the PLC starts up, before the first call of OB1.

[Mas01]

Ah, cheers, all.
Sorted...