Help on OB86 STEP 7

Hello TheViet;

You seem to have understood the basic principle behind the diagnostic OBs of Step 7: if a fault is detected by the CPU and the corresponding diagnostic OB is loaded in the PLC program, then this OB is called and should prevent the cPU going to STOP mode. There is no need to add programming inside the diagnostic OB for it to operate this way.

So, normally, if a Profibus slave (such s your drive) goers offline and OB86 is loaded to the CPU then your BF LED should light up but the CPU should remain active.

Unless another fault is present that is not covered by OB86...

Very often when you are working with a Profibus slave you will have in your program functions that will read and write to the addresses of the slave. If these addresses re not responding (because the slave is no longer present) then a programming fault occurs, which would be covered by another diagnostic OB, such as OB 122.

I suggest you open the diagnostic buffer of your CPU and see if any of the logged messages contains a line such as "Required or Missing OB, OB XXX). This will indicate the OB xxx that you should create and download to the CPU to prevent the PLC going to STOP.

If a Profibus slave is regularly faulted (or for long periods) uor you need to remove it so that it is no longer polled, and you do not want faults to appear in your CPU, then you should look at SFC 12, D_ACT_DP:

Enabling a disabled DP slave with SFC 12 "D_ACT_DP"

https://support.industry.siemens.com/cs/ww/en/view/19730984

What should you pay attention to when operating a DP slave temporarily on a Profibus DP network?

https://support.industry.siemens.com/cs/ww/en/view/15014658

Hope this helps,

Daniel Chartier