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Multiple Instance Data Blocks

Created by: FatRich at: 10/9/2008 2:02 PM (2 Replies)

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10/9/2008 2:02 PM	Rate (0)
FatRich Posts: 24 Rating: (2)	Hi Ourclient has issuedus with an "ICA Good Practice Guide". This guide details that Multiple Instance Function Block should be used. This means that only one instance DB is required for multiple FB calls. For example FB11 - Inlet Valves will be assigned instance DB11 FB11 will call the valve control block FB100 five times (as there are five valves). None of the valve FB calls require an instance DB as the FB's are declared in the static area of the parent FB11. I have aconcerns regarding this approach: 1. Online Monitoring - Is it possible to online monitor a multiple instance FB? When I've tested this sometimes it works & and sometimes it does not. I'm surprised it ever works, as the FB does not have its own instance DB to reference in the call environment i,e the instance DB contains several FB instances. 2. Reduced Flexibility- Any code changes to the parent FB11 will impact on all the valve FB's. This could cause problems when commissioning a live plant. 3. Program Structure - Any variables declared in valve FB's will be part of the static area of the parent FB instance DB11. According to 6-1131 standards the static variable area should not be addressed outside the respective FB (encapsulation). I know that S7 does allow addressing of static variables from anywhere within the project, but is this reliable / good practice? I would welcome anyopinion on this.
FatRich Posts: 24 Rating: (2)	Last edited by: FatRich at: 10/9/2008 3:47 PM Last edited by: FatRich at: 10/9/2008 3:00 PM
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10/10/2008 9:24 AM	Rate (0)
Wizard Diamond Member Joined: 4/24/2006 Last visit: 1/24/2025 Posts: 8808 Rating: (1177)	Dear FatRich, Have you seen these faq's: http://support.automation.siemens.com/WW/view/en/18723826 http://support.automation.siemens.com/WW/view/en/21671563 Perhaps they might help. Best regards, Wizard

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10/10/2008 10:20 AM	Rate (0)
FatRich Posts: 24 Rating: (2)	Thanks
FatRich Posts: 24 Rating: (2)
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