Scan Enet I/O from Multiple PLCs

scott.maddox

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Join Date
Aug 2013
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Does anyone know if you can scan a 1734-AENT & IV8 from multiple PLCs? I can see how outputs would possible cause a problem, but i was hoping this was possible with the inputs. I realize i can do the produce and consume, but i just think it would be cleaner if this was a legit was to do it.
 
I don't know those Rockwell parts in particular, but EtherNet/IP has this capability through their Listen Only connection type - it listens to the input data (input as viewed from the PLC) and makes it available. This is, in my opinion, exactly what multicast is good at, but I don't think using multicast is a requirement (depends on specific devices). It's somewhat unusual to use this, but it was designed for what you are asking. I would suspect it is supported, but one of the Rockwell guys on here will likely know for sure.

Realize that there will be a primary owner regardless, and design behavior for EtherNet/IP class 1 is that if the primary owner goes away, so do the listen only connections. From this standpoint, it really provides no additional protection from exposing the data over produced/consumed tags from the primary owning PLC. Either way, if the PLC goes away, so does your data.

There is another type which can handle multiple PLCs at the same time and both inputs and outputs - this is a Redundant Owner connection. The only system I know of that supports this is Schneider, but it's possible Rockwell, Omron, or others may support it as well. This would likely require special hardware all around on the system, and I know Schneider uses it exclusively on their hot standby systems.
 
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You can definitely make multiple Listen-Only connections to a 1734-AENT (and the analog modules connected to it) from ControlLogix and CompactLogix controllers over EtherNet/IP.

As described, there will be one Owner of the module. The connection establishment (the RPI and identity information) and the Configuration connection will be made only with the Owner.

Using Multicast is the most efficient and fastest way to do this.

I've never tried to do Listen-Only with Unicast connections; I'd have to do some research to advise you on this.

Your friendly neighborhood RA office should have somebody who can explain this to you in detail.
 

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