How to do analog output wire break detection?

OB82 runs one scan when the diag event happens, and once again when that event clears.

You need to capture the event values during that one event, and save it to a DB, say "AI_DIAG_DB", "AQ_DIAG_DB", which includes the inputs to OB82 if you want to investigate it in real time (while monitoring online).

If you copy all the events, you may want to store an array of events, and increment the count (or reset to 0 if it reaches the end of the array), all onside OB82.  Then you can monitor the DB's array contents live while connecting and disconnecting wires to prove you are capturing wire break.

I find the DIAGs to be so incredibly difficult to use because they do not provide stock FBs/FCs/DBs/UDTs to mate up with each module that has diagnostics available.  They require you to detect 1) the device causing the error (LADDR), which is easy if you only have the main PLC, and maybe just one module.  It quickly becomes a chore to copy the system constant for each LADDR I want to monitor into the compare.

Oh yea, I have found using the 270 rather than the system constant who's value is 270 is often a problem for OB82.  My experience is that it really depends on which PLC / firmware you are using as to how picky it will be.

Try dragging in the system constant with the value 270 over top of the 270, and copy all the input tags shown in the interface to a global DB.  Add those DB values to a watch table.  Now cause the problem.  The watched values should show you something you may not have known.  Now clear the problem.  And now you know what it looks like when it clears.

That's how I have had success with DIAGs in the past.  Finding all the values for IO state / channel / multi-error handling and all that from the various manuals in advance is not quick.  That wire break with screwdriver / wire tug is incredibly easy to do, and it makes it 100% obvious how to detect it happened, and when it clears, what that looks like too.

Then you just point it to the correct input / output channel on that module and alarm or warn operators.

eam_rts

 Hello, I want to check wire break on analog output of SM 1234 . Found this info - 

#dP

See what you can extract from:

https://support.industry.siemens.com/cs/document/42320122/

https://support.industry.siemens.com/cs/document/109483251/

 I created diagnostic OB (see attached file), there are diagnostic events in diag. buffer and LED are blinking. Inside OB I copy IO_state or set/reset flag depending on bit4, but I don't see any changes in copy of IO_state, because I have channels with and without wire breaks. Seems like diagnostic OB just does not run...

Any idea how to do analog output wire break detection ? 

If you have an s7-1200 trainer kit or a spare s7-1200, mount one to a din-rail, pop on a power supply, wire up the plc power, turn it on, download your project to the plc and ignore all the errors.  Now on the analog output module, hopefully you have a spare.  Power off and drop that into the project.  Apply L+/M/ ground to it and power back up.

Now install a 500 Ohm resistor across a couple terminal blocks snapped onto your din rail.  Wire up from the resistor TBs to your AO terminals.

Go online and what your diagnostic interrupt OB's captured values in your watch table when you pull the wire off the TB for the resistor.

Now you know the answer for certain.

If you do not have hardware, ask if any systems are not running for a few hours that have the same AO module.  Add a DIAG error OB long enough to capture the error of disconnecting the AO output.  Just be sure you are fast enough not to cause disruption to their schedules :) 

eam_rts

Dear huggy, thanks for your comprehensive answer. 

For me, it's a bit complicated to check and test. I will keep in mind this for future needs.

These analog outputs are used to control valves and because I have feedback signals from the valves, I can judge if all Ok with control signals of AO 4..20mA if feedback are (no break) and (not equal to output ) for more than valve's full open/close time. In my case it would be simpler. 

Thanks for your valuable comments!