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Why Can't I use GD communication in S7 400 H CPU?

Created by: KayBee at: 6/11/2011 4:55 PM (9 Replies)

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6/11/2011 4:55 PM	Rate (0)
KayBee Posts: 241 Rating: (7)	Hi, Why there is restriction to use GD (Global Data) over MPI in S7 400 H CPU with any other S7 CPUs? Any Idea?
KayBee Posts: 241 Rating: (7)
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6/11/2011 5:17 PM	Rate (1)
Ayman Elotaify Platinum Member Joined: 5/28/2008 Last visit: 3/23/2025 Posts: 4518 Rating: (843)	PowerTags Hi, Why there is restriction to use GD (Global Data) over MPI in S7 400 H CPU with any other S7 CPUs? Any Idea? Hello, GD-Communication is not suppoerted for S7-400H CPUs. It's supported for S7-300 & S7-400 CPUs. In the following FAQ, you will found how it works. Global data communication of two S7-400 CPUs via the K bus Hope this helps.
	Ayman Elotaify
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6/12/2011 7:40 PM
gre_m Posts: 947 Rating: (45)	well done
gre_m Posts: 947 Rating: (45)
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6/12/2011 8:27 PM	Rate (0)
hdhosseini Diamond Member Joined: 1/28/2009 Last visit: 3/8/2025 Posts: 6860 Rating: (1366)	PowerTags Hi, Why there is restriction to use GD (Global Data) over MPI in S7 400 H CPU with any other S7 CPUs? Any Idea? The original question exists till now "Why GD communication is not possible in H CPUs" As a gigantic digital circuits , there should be some limitation for GD communication but somebody inside job should give us the reason. Regards
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6/12/2011 11:59 PM	Rate (0)
Ayman Elotaify Platinum Member Joined: 5/28/2008 Last visit: 3/23/2025 Posts: 4518 Rating: (843)	hdhosseini PowerTags Hi, Why there is restriction to use GD (Global Data) over MPI in S7 400 H CPU with any other S7 CPUs? Any Idea? The original question exists till now "Why GD communication is not possible in H CPUs" As a gigantic digital circuits , there should be some limitation for GD communication but somebody inside job should give us the reason. Regards Hello again, I hope I will success this time to explain the reason. So as the GD-Communication needs a unique MPI address between the two CPUs that should be communicated with each other, so it's not possible with H-CPUs. In case of communication between 2 Fault tollerant CPUs, you requirements to to treat each system (H-CPUs) as one CPU. It's not one CPU. It's two and each CPU have a different MPI address. In GD-Cpmmunication, you have to define which CPU is the sender and the destination for each bucket of data for the other CPUs. What will be the case with H-CPUs if you have two different fault tollerent systems? If you select one CPU to be a sender, then what if this CPU became down for any reason, the redundant CPU should be master in this case, so for GD-communication, there will not be a sender CPU! what will happen?!!!! For this reason, I think Siemens guys prefer not to allow GD-communication with S7-400H. Hope this helps.
	Last edited by: Ayman Elotaify at: 6/14/2011 12:54 AM Ayman Elotaify
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6/14/2011 12:20 AM	Rate (0)
Krish Silver Member Joined: 5/24/2007 Last visit: 4/24/2024 Posts: 284 Rating: (27)	Hi My understanding of the H-PLC is that, the H_PLCsare always trying to synchronise all their data through the sync module which you need for a redundant function. This synchronisation includes also the Global Data. Only thing excluded in the synchronisation is the data related to the individual status of the hardware (CPU, CP etc). So if that is the case how can you have a GD communication? And since GD is same on both sides why would you need one?. Krish

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6/14/2011 1:07 AM	Rate (0)
Ayman Elotaify Platinum Member Joined: 5/28/2008 Last visit: 3/23/2025 Posts: 4518 Rating: (843)	Krish Hi My understanding of the H-PLC is that, the H_PLCsare always trying to synchronise all their data through the sync module which you need for a redundant function. This synchronisation includes also the Global Data. Only thing excluded in the synchronisation is the data related to the individual status of the hardware (CPU, CP etc). So if that is the case how can you have a GD communication? And since GD is same on both sides why would you need one?. Krish GD communication isnotsupported by H-System. Please read section 11.4 of this manual: SIMATIC Fault-tolerant Systems S7-400H
	Ayman Elotaify
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6/14/2011 6:24 AM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 3/20/2025 Posts: 3042 Rating: (1061)	Dear all I think by now we can all agree that GD communication between a H-system and a"single" S7 CPU is NOT supported. As forthe original question ("Why there is restriction to use GD (Global Data) over MPI in S7 400 H CPU with any other S7 CPUs"), here's my take on things: To begin with, I need to disagree with Ayman Elotaify's assertion that each H-CPU has a unique MPI address (they can have the same), so that reason for not supporting GD in a H-system does not exist as far a I'm concerned. I'd go as far as saying that Siemens could implement/suppport GD commsfor a H-system if they really wanted too. Thequestion is: Why would they bother? GD comms have somedisadvantages, most noteably: - Only atinyamount of data can be transferred (22 Bytes for 300's, 54 Bytes for 400's) - The data exchange isNOT acknowledged GD comms really have no place in a H-system, and that - in my opinion - would be the main reason for Siemens to not bother implementing/supporting it. For completeness, the link below has more on this and I hope this helps: What properties, advantages and special features does the global data communication offer?
	Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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6/14/2011 9:34 PM
gre_m Posts: 947 Rating: (45)	Hi H-system using an event-driven synchronization. For GD-communication is difficult to ensure, so I believe that Gd - Communication disabled in H-system
gre_m Posts: 947 Rating: (45)
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6/15/2011 7:10 AM	Rate (0)
umencho Diamond Expert Joined: 3/21/2006 Last visit: 3/19/2025 Posts: 10239 Rating: (1183)	gre_m Hi ...so I believe that Gd - Communication disabled in H-system Fromfritz's summary Notes: Global data communication is not available in the S7-400H.

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