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opening instance and shared data block in STL

Created by: JabbarKamali at: 2/15/2021 8:35 PM (9 Replies)

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2/15/2021 8:35 PM	Rate (0)
JabbarKamali Posts: 51 Rating: (0)	Hello Guys in STL manual we can read that, only two DB can open at the same time, one instance and one shared. DB and DI represents shared and instance data block respectively. 1. I create an instance DB10 and open it in STL with shared notation rather than instance: OPN DB10 However, Simatic s7 open data block regardless of it is shared or instance! why is it so? if there is no difference between syntax why it has declared two different notations for DBs? 2. if we open DB10 with OPN command no need to prefix dbw and dbx with db10. so why this code works? A DB10.DBX 2.0 L DBW 2 T MD 10 thank you in advance
JabbarKamali Posts: 51 Rating: (0)
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2/15/2021 9:06 PM	Rate (0)
@CIONCO Bronze Member Joined: 1/17/2013 Last visit: 2/25/2025 Posts: 136 Rating: (23)	After the statement OPN DB10 any call for DBW, DBD or DBX without previous DB number specification will apply to DB10 while any call for DIW, DID, or DIX will apply to the currently open instance DB

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2/16/2021 12:18 AM	Rate (1)
Piotr.M Platinum Expert Joined: 6/24/2013 Last visit: 11/14/2025 Posts: 1696 Rating: (433)	Hi OPN DBxxx - opens data block as shared DB OPN DIxxx - opens data block as instance DB warning access DBxxx.DBWyyy implicitly opens DBxxx:- see example below: OPN DB10 L DBW20 T DB30.DBW40 L DBW50 // reads DB30.DBW50!!!!!
	Last edited by: Piotr.M at: 02/16/2021 10:03:08 Last edited by: Piotr.M at: 02/16/2021 10:45:29
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2/16/2021 6:00 AM	Rate (0)
JabbarKamali Posts: 51 Rating: (0)	thank you! is it possible to open a Shared Data Block in STL via OPN DI command or vice versa? I tried this and it seems to me it works.
JabbarKamali Posts: 51 Rating: (0)
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2/16/2021 6:32 AM	Rate (2)
qwazee Platinum Expert Joined: 3/30/2020 Last visit: 12/11/2025 Posts: 5656 Rating: (1082)	JabbarKamali thank you! is it possible to open a Shared Data Block in STL via OPN DI command or vice versa? I tried this and it seems to me it works. Hello. This topic is aiming onto a technical point. < TL;DR > Any DB can be opened as OPN DI or OPN DB. The OPN instruction is not concerned if the DB is a linked (instance) or unlinked (global) type DB. The instruction see a DB as a memory area which needs to be opened in one of two ways: shared or instance memory block. Calling an FB the linked DB is opened as a instance type memory block. That happens in the background without us needing to do anything. It is possible for the user program to open any of the DBs as a shared or as an instance through the OPN instruction. It is handy where there is an exchange of data needing to flow between two memory blocks. But the memory block can only be accessed as an absolute address. (see notes by Mr. Piotr.M and @CIONCO) CAUTION.. Inside a FB the OPN DIxx instruction will cause the originally linked memory block of the FB be replaced. This can cause many unwanted problems if you are not careful. Symbolic access could attempt to point to memory that is not there anymore. In turn this can stop the PLC. Exchange of instance memory blocks in an FB can be a powerful tool for those who understand the concept and takes responsibility for its consequences. Do these additional notes help?
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2/16/2021 8:03 AM	Rate (0)
jacek d Platinum Expert Joined: 9/23/2005 Last visit: 1/9/2026 Posts: 5154 Rating: (772)	Piotr.M T DB30.DBW40 L DBW50 // opens DB30.DBW50!!!!! It does not open anything. DB30 is already opened by a fully qualified access one line earlier.
	Regards, Jacek What is confusing you is the fact that you never see a number but always its digit representation. So you end up thinking that the number is the representation. - Andrej Bauer Any claims against me - based on whatever legal reason - resulting from the use of any code and/or ideas presented on this forum shall be excluded. #StayWithUkraine
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2/16/2021 10:04 AM	Rate (0)
Piotr.M Platinum Expert Joined: 6/24/2013 Last visit: 11/14/2025 Posts: 1696 Rating: (433)	Thanks. Jacek. Corrected it reads not opens.

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2/16/2021 10:32 AM	Rate (0)
JabbarKamali Posts: 51 Rating: (0)	thank you for your respond. I have DBxx and this is instance one. STL manual says to open instance DB use "DI" like this: OPN DIXX however, I tried(while this is used when we have shared DB): OPN DBXX why this works? I have created an instance DB and in my program I say open it as a shared. how come? OK you say OPN doesn't mind about shared or instance. if there is no consequence to open shared DBs as an instance and vice versa why manual says different notation for them?
JabbarKamali Posts: 51 Rating: (0)
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2/16/2021 10:41 AM	Rate (0)
jacek d Platinum Expert Joined: 9/23/2005 Last visit: 1/9/2026 Posts: 5154 Rating: (772)	JabbarKamali OK you say OPN doesn't mind about shared or instance. if there is no consequence to open shared DBs as an instance and vice versa why manual says different notation for them? Read once again answers carefully. There are consequences.
	Regards, Jacek What is confusing you is the fact that you never see a number but always its digit representation. So you end up thinking that the number is the representation. - Andrej Bauer Any claims against me - based on whatever legal reason - resulting from the use of any code and/or ideas presented on this forum shall be excluded. #StayWithUkraine
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This contribution was helpful to 1 thankful Users qwazee

2/16/2021 7:28 PM	Rate (1)
Rescobedo Silver Member Joined: 11/5/2012 Last visit: 8/14/2025 Posts: 490 Rating: (65)	hello Jabar, look at this treat, Fritz made a really good explanation. Regards.

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