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Old July 22nd, 2014, 11:54 AM   #1
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kingnutin is offline
Join Date: Oct 2008
Location: Abilene
Posts: 6
Multiple Controllers Read Same Tag

I have twelve ControlLogix controllers spread throughout an oil production facility with local Panelviews on each unit. All PLCs are connected together wirelessly. There also exists a central control room running FactoryTalk View SE.

There is no actual control being done over wireless. Some tags are already being passed from the PLCs to the HMI in the control room for information only.

My problem exists that I need to pass ten or so tags from one location to all the PLCs. These tags are used in flow calculations at each PLC and don't need to be read but every ten minutes or so. To me the best way would be to create tags in the SE HMI and have the PLCs read that tag. Can a PLC read a tag from a SE HMI?

I have heard of produced consumed tags, but have never used them. The problem I have with that idea is that I could write from the SE HMI to one PLC using a reference tag to that PLC, but what if that PLC goes down? How can the other PLCs read that produced tag? The wireless signal out here can be very flakey at times and PLC can lose power, but if they can all read from the SE HMI it seems as though my chances for the HMI to go down would be minimal.

All thoughts and ideas are very welcomed and appreciated.
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Old July 22nd, 2014, 01:35 PM   #2
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diat150 is offline
Join Date: Mar 2006
Location: Louisiana
Posts: 1,260
you could write a script in ftview that writes the values to all plc's every 10 minutes.
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Old July 22nd, 2014, 01:57 PM   #3
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FluxCap is offline
Join Date: Jan 2013
Location: Louisiana
Posts: 111
Hmmm. You say that all PLC's are connected together wirelessly already. Is this how SE in the control room gets its data? Where does the data originate that you need to push out to all of the PLC's?
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Old July 22nd, 2014, 02:14 PM   #4
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ajmist is offline
Join Date: Jun 2014
Location: Tucson
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I'm not familar with the ControlLogix, but with the MicroLogix it was easy to have table data shared from one PLC to several more. With an "age" register I was able to continuosly synchronise 25 PLCs recipe databases with no single point of failure, by having each PLC read from a few others, updating when the received age was newer than the local copy.

Probably overkill in your case, but depending how important the data is ...
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Old July 23rd, 2014, 05:47 AM   #5

MichaelG is offline
Join Date: Nov 2006
Location: Brisbane
Posts: 685
Does this work for you?

HMI sends flow stuff to the closest PLC (lets call it the PLC source)
Remote PLC's "Read" from the PLC Source
If the "read" fails then try again until it works
Wait 10 seconds -then read again
Bonus is this is "safer" as the PLC reads and is control of where the data goes and the communication code is in this PLC, however, this is "polling" and unnecessary communications occurs as the data has not changed.

An alternate - less communications on the wireless
HMI sends flow stuff to the closest PLC
PLC Source monitors the values to see if they have changed
if changed then send to all other PLC's

Every 10 minutes or longer just send anyway (overwrites the remote PLC's data just in case it changed)

to send and receive use the MSG function - lots of help here on how to program
Also make sure that you are sending a UDT tag

Last edited by MichaelG; July 23rd, 2014 at 05:52 AM.
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Old July 23rd, 2014, 01:56 PM   #6
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kingnutin is offline
Join Date: Oct 2008
Location: Abilene
Posts: 6
I haven't done any scripting in FT before, but it seems like that may be the best plan for my situation.

If I can't get that to work I will try Michael's idea.

Thank you for all the thoughts, I really appreciate them!
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