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Old May 6th, 2011, 07:50 AM   #1
Jeff23spl
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AB redondancy with small plc

What it is the smaller entry level for approved redundancy using AB stuff

I read about Controllogix redundancy but i'm wondering if it can be done with 2 compact L32E ?

I have seen a panel using 2 L32E but i believe it is done manually with 2 separate communication port, 1 for i/0 and 1 for data exchange between CPU. I'm i right ?
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Old May 6th, 2011, 12:11 PM   #2
Ken Roach
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redux

The principal Redundant control system offered by Rockwell Automation is ControlLogix Redundancy. It uses ControlLogix (1756-L6x) controllers and ControlNet for I/O networking.

There is a DeviceNet-based "Easy Backup" mechanism for CompactLogix that provides a "hot standby" controller. It lacks many of the redundancy features that ControlLogix Redundancy provides, such as media redundancy, scan synchronization, data table cross-loading, and program edit cross-loading.

ControlLogix Redundancy is described in the ControlLogix Redundancy System User Manual, 1756-UM523 (http://literature.rockwellautomation...m523_-en-p.pdf). You must read and understand that manual and the Redundancy firmware release notes before attempting to implement ControlLogix redundancy.

CompactLogix DeviceNet "Easy Backup" is described in several Release Notes and application notes for the 1769-SDN module, including RA Knowledgebase document 50690.
http://rockwellautomation.custhelp.c...ail/a_id/50690
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Old May 6th, 2011, 08:27 PM   #3
nwboson
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I'm not sure what you mean by 'approved redundancy': what approval body are you referring to?

Its possible to roll-you-own full hot standby PLC redundancy with milliseconds-speed switchover using an Ethernet-based PLC like the Micrologix 1100/1400 or Siemens s7-1200. But this is not an off-the-shelf approach like the redundancy-designed off-the-shelf devices - and it requires careful design and testing.
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Old May 6th, 2011, 09:16 PM   #4
JeffKiper
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how do you do the switch over in the MLX ?
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Old May 6th, 2011, 10:29 PM   #5
Ken Roach
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What I would consider an "approved" redundant PLC system is one that is "advertised, marketed, and intended by the vendor to function as a Redundant controller and network system", but maybe I'm putting words in the OP's mouth.

Just having a redundant controller doesn't make your control system "approved" for a specific application by any controlling or regulating authority.

If you need a specific Safety Integrity Level (SIL) to satisfy an insurance requirement, you have to follow ISA S84.1 or the IEC 61508 standard or other standard for your industry and application.

In my experience, most projects that lack the budget for purpose-built redundant controllers also lack the budget or manpower for careful design and testing. I've certainly seen some good "homemade" backup systems but all of them cost more in engineering time than a purpose-built redundant controller would. I've only seen one of them used more than once, by a switchgear manufacturer who put a lot of time into planning and testing. Even they didn't advertise the control system as "redundant", but rather "fault resistant".

I've been working with a ControlLogix redundancy system for the past month and the presence of the redundant controllers and redundant ControlNet network is mostly invisible to us. The parts we spend time on are the double I/O and the voting inputs.
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Old May 6th, 2011, 11:14 PM   #6
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Use comm and IO based watchdogs to perform the switchover. Master always writes state changes to slave via Ethernet so handoff can be seamless. IO semaphore to hand control back and forth: all pretty standard computer science stuff. Run identical code in both PLCs.

Analog IO follows master and is not bumpless. Digital IO can be bumpless. Due to the lack of bumpless analog IO, application and state machines need to be able to handle bumps on analog handoff.

Ken is right: this is no cakewalk. Its faster, simpler and more reliable to use off-the-shelf for low volumes (it seems to work that way with everything, doesn't it). However, as an analogy, in spite of the fact that I can buy a fabulous new car off the showroom floor, that doesn't stop me from taking the time to enjoy building my own. To complete the analogy, the chance that I'm going to do as good a job building that car as a few dozen engineers and millions of dollars is about zero - but I still enjoy doing it.

You need enough volume to justify the roll-your-own approach and be prepared to test thoroughly. And because you're not going to easily handle bumpless analog transfer, this low-end approach only works for apps that can handle that analog 'reset' on transition - or you have to design your own analog transition circuit.
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Old May 8th, 2011, 03:28 AM   #7
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Quote:
Its possible to roll-you-own full hot standby PLC redundancy with milliseconds-speed switchover using an Ethernet-based PLC like the Micrologix 1100/1400 or Siemens s7-1200. But this is not an off-the-shelf approach like the redundancy-designed off-the-shelf devices - and it requires careful design and testing.
That is 'warm standby' not hot standby.
Hot standby has no interrupions/delays at all. Have many discussions over this one from time to time.
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Old May 8th, 2011, 09:05 AM   #8
Hydro Power Guy
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Here is the Rockwell pdf that details your choices for redundancy.

http://literature.rockwellautomation...p014_-en-p.pdf

In addition to the compactlogix devicenet based option, there is a "soft" redundancy available. Note that you need to look at your control system details to see if it is even advisable to do this. There are pros and cons to all of these systems.
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