OB40 vs OB35.

Hello bb_;

The best explanation of the S7-300/400 OBs (since you placed this question in in the Step 7/Step 7 Lite category, these are the types of CPUs you should be using) is the Standard and System Functions manual, chapter 1:

https://support.industry.siemens.com/cs/ca/en/view/44240604

Below is a quick explanation of OB 35 (Cyclic interrupt) and OB 40 (Hardware interrut).

OB 35 interrupts the processing of your program every 100 ms (adjustable in the OB 35 parameters) to force your CPU to do something you want done exactly at this time, at every configured cycle.. We would program our PID claculations in OB 35 since very precise timing is required for the PID blocks to provide adequate resultsl.

OB 40 interrupts the processing of your program everytime your hardware components (you must configure which one) triggers the hardware interrupt, then OB 40 will force your CPU to do what you have configured in that OB.

I do not believe that transfering a value from one point in your program to another should be linked to a hartdware trigger, so maybe OB 40 would be an overkill.

My experience with Siemens PLC communications tells me that forcing some transfer on a very strict frequency might not work well, due to network delays that could de-synchronize the values you want to transfer.. I am not sure OB 35 would be what you need.

Why don't you transfer your values as part of OB1, your main program cycle OB, and see how it results? Generally the speed of the CPU scan cycle is fast enough that the transfer would not be detectable by the user.

Hope this helps,

Daniel Chartier