Siemens TIA Portal - Tag query

I came across this online test question about tags - has anyone taken a Siemens course and know what is meant by "scope"?

Scope would normally mean from where the tag is accessible.
If the tag is declared as a parameter of an FB or FC, it would only be accessible within that FB or FC (*).
If the tag is declared as part of a shared-DB, then it is accessible from everywhere.
So I interpret it such that the scope is implied when declaring the tag, and it is therefore not specified.

*: You can actually also access Instance-DBs from everywhere, but that is yet another can of worms.

BTW, the scope has to be ticked to be correct (NB order changes each time you do the test)

JesperMP said:

So I interpret it such that the scope is implied when declaring the tag, and it is therefore not specified.

Click to expand...

I agree. I think the scope answer is one of the wrong ones. Sounds plausible to a programmer, but just isn't how Siemens does things.


Unless they're referring specifically to the Declare Tag dialog, where you do get to select local vs global, but that'd be awfully specific without saying so.

Scope is usually local or global i.e. local to the block, global is across all blocks.
Initial value is a value set initially when downloaded to the PLC
Data Type is for example integer word, real (float) double integer, bit etc.
Symbolic name is it's symbol i.e. can be refered to by symbol or actual address.
Not sure what they mean by absolute address but assumes this refers to actual address in memory but could mean Bit area, DB area I/O area, this is not clear however, assuming this means common to Siemens that's probably what it means, in some other PLC's for example the absolute address refers to it's location in memory i.e. D100 could be 24565 decimal. Like Mistsubishi the data is fixed in banks within the memory area, Siemens is slightly different, for example DB's are like files on a disk, when a block is downloaded it is placed in spare memory, there is a sort of file allocation table (FAT) where the absolute address is stored, when you call a DB the PLC looks at the Fat table to find the actual start address, this applies to PB's,FB's, FC's as well (not quite so sure on S7 but was for S5 so assume it is similar).

I am concerned that the initial values are not specified !?

Tag for %I0.0 ?

Yeah your right, If I remember correctly, the move is towards symbolic addressing rather than absolute, RW have symbolic i.e. tags are referenced by the symbol (so assume they must have some sort of lookup for the physical address. this means they can be accessed outside by say an HMI, many other platforms have not moved to this symbolic format so for example Siemens (I believe) symbolic only tags cannot be read from an HMI, this definitely goes for Mitsubishi symbolic only addresses are allocated at compile time (using part of existing variable addresses reserved in the compiler) so there is no way for an external device to access them, however, I think some ofthe latest or up and coming systems might either go completely away from absolute or have a table of symolic addresses that point to actual addresses.
In mitsubishi at least the docs use scope to determine if it is local or global so I think it's an interpretation difference.