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Old March 20th, 2017, 01:27 AM   #1
epicnote
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ControlLogix Redundant IO System

Hi All,

Is it possible to create redundant remote I/O system (2 unit of 1756-CN2R in remote I/O chassis) as per attached architecture? If one of the 1756-CN2R module in remote I/O chassis faulty, the 1756-CN2R will take over to control the input and output
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Old March 20th, 2017, 02:27 AM   #2
Ken Roach
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No.

ControlLogix Redundancy is built to survive the failure of one processor or network module in the redundant pair chassis, and can be built to survive the failure of one network cable on the dual-media ControlNet.

But it is not designed or intended to switch from one ControlNet module to another within a single I/O chassis.

That would not add significant robustness to that chassis, since the chassis still has a single power supply and individual I/O modules.

This may have changed in the recent past, but that's how I understand the ControlLogix Redundancy architecture, and I know it well.

Where did you get this diagram ? In my opinion it's a poor diagram: it doesn't show the Ring architecture for Ethernet or the dual-media architecture for ControlNet, and the presence of multiple 1756-CN2R modules in the remote I/O chassis can evidently be misleading.

It is possible to put multiple ControlNet modules into a remote chassis like that, but it would be to split up the I/O and network bridging functions, not to provide a network module failover feature.
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Old March 20th, 2017, 02:43 AM   #3
epicnote
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What about if we configure the logic to inhibit the controlnet module that failed?

Quote:
Originally Posted by Ken Roach View Post
No.

ControlLogix Redundancy is built to survive the failure of one processor or network module in the redundant pair chassis, and can be built to survive the failure of one network cable on the dual-media ControlNet.

But it is not designed or intended to switch from one ControlNet module to another within a single I/O chassis.

That would not add significant robustness to that chassis, since the chassis still has a single power supply and individual I/O modules.

This may have changed in the recent past, but that's how I understand the ControlLogix Redundancy architecture, and I know it well.

Where did you get this diagram ? In my opinion it's a poor diagram: it doesn't show the Ring architecture for Ethernet or the dual-media architecture for ControlNet, and the presence of multiple 1756-CN2R modules in the remote I/O chassis can evidently be misleading.

It is possible to put multiple ControlNet modules into a remote chassis like that, but it would be to split up the I/O and network bridging functions, not to provide a network module failover feature.
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Old March 20th, 2017, 11:06 AM   #4
mellis
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I don't believe you can schedule the network to include two paths to the same output modules.

This is the typical wrong-headed attempt at improving the reliability of a system by taking the part that is more reliable and trying to make it redundant while ignoring more failure prone parts.

You can't make the DNB or the DHRIO redundant, so what does making the CN2R redundant get you?

Last edited by mellis; March 20th, 2017 at 11:11 AM.
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Old March 20th, 2017, 12:46 PM   #5
AGill
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Quote:
Originally Posted by epicnote View Post
What about if we configure the logic to inhibit the controlnet module that failed?
Or logic to disregard inputs values based on a faulted module state...

But similar to the previous posters, you'd be moving out of the 'typical' redundant setup, and perhaps it's worth stepping back and correctly evaluating what you're trying to achieve and what event(s) you're trying to safeguard against. The solution may be simpler (and cheaper and more reliable) than you'd expect.
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Old March 20th, 2017, 12:59 PM   #6
Ken Roach
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Quote:
What about if we configure the logic to inhibit the controlnet module that failed?
No.

In theory you could set up such a "software switchover" on top of the Redundancy system, but in the best case it will be very slow, result in a fault-mode state for all of your I/O, and you will get no support for it if it doesn't work.

If you're in a commercial situation where the customer has taken a DeltaV spec and is using their redundant ethernet networking to block you out, either stick with the normal ControlLogix Redundancy or walk away from the project.

ControlLogix Redundancy is very good at doing what it does. Don't try to make it more complex, or try to make it have features it doesn't have.
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