Siemens: Tag data from TP1500 to CPU1513

Quick & basic question, TIA v15:

Is it legitimate to directly reference (load) the PV in a counter instruction from a numeric entry field in the HMI, or does it require an intermediate tag to convey the value?

Thanks.

http://www.plctalk.net/qanda/showthread.php?t=407

You could go directly but I would personally use an intermediate tag for future use if I want the value to change from somewhere else.

Just a thought.
Regards,

Thank you, gclshortt. I guess that so long as it's in the processor then it's accessible (subject to check-boxes in DBs, where needed), but I follow your reasoning.

In general, people recommend against writing directly to the instance of an FB/instruction. The biggest reason is that if the value appears out of nowhere, and especially if it doesn't show up in the cross reference, someone will have a heck of a time troubleshooting things down the road.


That said, if it's well documented, and especially if it is done consistently, then it can sometimes make things simpler and clearer.

mk42, I can see both sides of that argument but (at least in TIAv15) the cross referencing is very good, including all HMI uses so I'll stay with it this time.

DairyBoy said:

mk42, I can see both sides of that argument but (at least in TIAv15) the cross referencing is very good, including all HMI uses so I'll stay with it this time.

Click to expand...

Yeah, the HMI cross reference is better than it used to be. I swear it was amazing in like 10.5 or 11, went away, and then came back again recently.


If you have an external HMI (separate portal projects, third party, etc), then you don't get that xref advantage. Portal only recently gained the ability to cross reference tags in an instance DB to both their internal use as FB locals, and external use as a "global" DB. This helps make it less crazy, but I dpn't think it handles nested multi-instance stuff.