Siemens S7 1200/1500 use of OB's

boneless

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boneless
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Hi all!

Short question, when using Simatic Manager and S7 300 CPU's, one had to add OB's to keep the CPU from stopping in case of hardware or IO errors.

I was under the impression this was no longer needed with the new line of CPU's, but I had a weird CPU stop the other day. I got it back to run after I added the OB's, but that could have been a coincidence.

Anyone have any documentation if these OB's are needed or just there to run code if I want to?
 
To add something that wasn't explicitly stated in L D[AR2,P#0.0]'s post, it is true that many of the error OBs in the 1500 are not required to keep the PLC in RUN, like HW or network errors.

However, there are exceptions, like the Program errors. The Cycle Time error OB also works similarly to the 300s, where it allows the PLC to ride through 1 scan that's too long, but it still goes to STOP on the 2nd in a row.
 
Hello again,

So the reason I posted this thread was that I have a PLC out there that stops. We have now found out it happens when one of the analog signals goes below 4 mA (3.7 mA or something).

I have never seen this happen.. I have added OB82, but have not marked wire break detection. Anyone ever seen this happen before?
 
On the S7-1200 the broken wire detection is/was just on the outputs, you need to unset 'Enable underflow diagnostics' for each analogue input channel. I tend to unset them and then create my own alarm if the channel drops below about 3500.

Just thinking again and I know that the 'overflow' doesn't cause the S7-1200 to stop, I just get a red LED that clears when I fix the fault. I have seen problems when using the built in scaling functions, they do/did cause the PLC to stop if you go outside the range. I use my own scaling function instead.
 
Last edited:
That is what we do, but in this case my CPU goes into STOP whenever a broken wire is detected.

I am wondering if this may have something to do with an active/passive mix-up.
 
What does the diagnostic buffer tell you about what happened. Could you post it?

Usually it'll tell you exactly why it went into STOP.
 
No Scale or Norm blocks in use, that is not the issue, we have our own scaling blocks that include alarms etc, which are in use all over the program.

No diagnostics at the moment, we have teamviewer access, but it is not working due to license issues. I will have a guy on site tomorrow, but was hoping to figure out a solution before hand.

It just baffles me, never had a 1200 go into stop before.
 
Quick update.

The error was indeed due to the client powering the loop while it was already powered by us.

Thanks for all the help :)
 

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