S7-300 opn db

jwilcoxson · Sep 7, 2014

Hi everyone,

If I am writing a S7-300 program in STL, why would I need to use

Code:

OPN DB2
L DBW1

when I could use

Code:

L DB2.DBW1

KalleOlsen · Sep 7, 2014

You don't "need to". The choice is all yours.
I normally use the latter.

Kalle

jwilcoxson · Sep 7, 2014

Ok, so maybe I would just use OPN if I was going to use the particular DB several times and wanted to save myself some typing?

L D[AR2P#0.0] · Sep 7, 2014

Please be aware that there is an implicit OPN with any fully qualified DB access. In the example below, the transfer will write to DB2, not DB1

Code:

OPN DB1
L DBW 0
L DB2.DBW2
+ I
T DBW 2

jwilcoxson · Sep 7, 2014

Thanks, that is really good to know, I wouldn't have assumed that. So that means that this:

Code:

L DB2.DBW1

Is exactly equivalent to this

Code:

OPN DB2
L DBW1

mk42 · Sep 7, 2014

jwilcoxson said:
Thanks, that is really good to know, I wouldn't have assumed that. So that means that this:

Code:

L DB2.DBW1

Is exactly equivalent to this

Code:

OPN DB2 L DBW1

That is correct.

I believe that

Code:

L DB2.DBW0
L DB2.DBW2

is also the same as

Code:

OPN DB2
L DBW0
OPN DB2
L DBW2

You might theoretically save a tiny bit of processing by using OPN, but it is negligible. Most people find that using the fully qualified name (DB2.DBW0) is much clearer and easier to read. The hardest part of STL is making code that other people can follow. I think there are a very few specific situations where using OPN allows you to do something you couldn't program other ways, but most of the time I think people prefer the DB2.DBW0 style.

L D[AR2P#0.0] · Sep 7, 2014

Execution time and memory useage may become important to you at some stage. Using DB numbers 255 and below also uses less memory.

Code:

OPN DB1
L DBW 0 
L DBW 2
+I
T DBW 4

uses less memory than:

Code:

L DB1.DBW0
L DB1.DBW2
+I
T DB1.DBW4

JesperMP · Sep 7, 2014

One important note.

With "OPN" etc. you wont have any symbols, apart from the DB symbol.

Code:

OPN "myDB"
L DBW 0 // absolute addressing !
L DBW 2
+I
T DBW 4

Code:

L "MyDB".MyINT1 // symbolic addressing !
L "MyDB".MyINT2
+I
T "MyDB".MyINT3

IMO the eventual gain in execution speed is far less important than that abolute adressing is more error prone and more difficult to maintain.

Btw. You can program symbolically with SCL, and the SCL compiler will take of when to use "OPN" in the generated STL code.

Peter Nachtwey · Sep 7, 2014

You need to try the different examples listed above in debug mode

You can see the DB register gets changed with the OPN DB1 or with the L DB1.DW0. The DB register stays the same until changed again.

L D[AR2,P#0.0], I forget. What restores the DB to the original state after a return from a call? Upon entering a FB or FC I would save the old DB away if I needed to change it and restore the DB before returning so that it would be in its original state.

KalleOlsen · Sep 8, 2014

@Peter
No need to worry, the OS stacks it in an out for us.

Kalle

Manglemender · Sep 8, 2014

OPN DBXXX
L DBWXXX

In many cases the construct above is the result of code having been translated from S5.

Nick

S7-300 opn db

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