Siemens Load DB Number

cjd1965 · Nov 26, 2013

Hi I am looking at a FB written by someone else

there is the code

Code:

L     DINO
T     #DBNB
OPN   DB [#DBNB]

I understand this is reading the DB number from the ACC and copying it into the variable DBNB and then opening the DB with the number DBNB...

My question is "why?" I always access the DB directly because it is related to the FB to which it is assigned.

I assume I am missing something very clever

PeterW · Nov 26, 2013

Only Instance DB's are assigned to FB's.

Global DB's are not assigned to FB's.

A typical use for indirectly accessing a global DB, could be communications, where different links are assigned to different DB's or recipe's, many different reasons.

L D[AR2P#0.0] · Nov 26, 2013

Why not show some of the code that accesses the global DB - this may give a clue.

cjd1965 · Nov 27, 2013

Hi all
I have revisited this... the DB's are instance ones. I will lost 1 FB and its DB later. I cant see that the access other than normal is used.

cjd1965 · Nov 27, 2013

Archive on 1 FB and its DB

L D[AR2P#0.0] · Nov 27, 2013

My guess is that at some stage the FB's interface was out of date and the code was giving unexpected results. The programmer decided to write to the instance DB using a hard coded address and then put the jump in to disable the write. The interface was updated and the instance DB regenerated and everthing started working. The debug code was never removed.

cjd1965 · Nov 27, 2013

Hi Thanks for the reply. I realise there are are so may solutions but this style is used in almost every FB... sometimes there are writes to bytes for string access but it just seems redundant in most cases. Most blocks don't have the unconditional jump

Siemens Load DB Number

cjd1965

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L D[AR2P#0.0]

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cjd1965

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cjd1965

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