ControlLogix Redundancy (Same Rack) with Point I/O

[giantjean]

Hi Experts!!!

I would like to know if there is a possibility to have redundant processors in the same rack and talking to 3 racks of Point I/O. Or how would I be able to create a redundant PLC using Point I/O.

Thanks!

 

[DBLD99]

No - Redundant processors need to be in separate rack. With all modules being the same ie... redundancy module and communication modules.

If you put 2 controllers in one rack only 1 of those controllers will be the "owner" of that IO.

 

[Firejo]

My view on redundancy is that unless you duplicate everything in separate enclosures you’re wasting your time and money. For example, in the scenario you’re asking about, you might be protected if the processor fails but is the processor the likely failure? Also, I say “you might be protected” because if the processor does fail what are the chances that it will take the rack down at the same time (I.E. is causes a short on the backplane, etc…)? What if one of the Point I/O racks fails? What I would do if find out what the weakest link is and figure out how to deal with that.

Several years’ ago redundancy was the hot topic and everybody was clamoring for it. Somewhere along the line people started realizing that dual processors (even two racks but in the same cabinet) did little to protect them from system failure but did put more money in the suppliers pockets.

My personal experience comes from a GE Faunac application I was involved in about 12 years ago. The customer insisted on dual radio networks believing that the radios were the weak link. A lot of time and money went into designing and building the system which wound up with a 90-30 master with a dual serial port communications card connected to two master radios (Serial SNP) connected to four (I think) remote stations with 90-30’s each with the same dual communications cards and two remote radios. They wound up having a discrete output card controlling relays that powered the radios up so if communications failed for a period of time the controller would open one relay and close another (powering down one master and powering up the other). The remote stations had similar setups with relays controlling which radio was powered up. Not only was the system expensive because there were two radios per PLC (as well as coax and antennas) but they also had to use the 90-30 instead of a Micro because the Micro’s didn’t support two communications ports. They tested the process of switching over from one radio to another and it worked just fine but the realization that way too much money was spent to buy a false sense of system reliability when a welder used the steel post the panel was mounted to as a ground for his welder and fried both PLC’s. To add insult to injury the radios were untouched.

Just my two cents.

 

[Paullys50]

Several years’ ago redundancy was the hot topic and everybody was clamoring for it. Somewhere along the line people started realizing that dual processors (even two racks but in the same cabinet) did little to protect them from system failure but did put more money in the suppliers pockets.

This has been my feelings as well, something else will bring the plant down long before a processor will. So redundancy as a means to protect from a system failure doesn't seem to be justified. My justification for redundancy has more to do with keeping downtime to a minimum while implementing changes, testing those changes and qualifying those changes. Redundancy at the PLC and SCADA level allow controlled system updates/changes with easy recovery in case testing fails and your downtime window has closed. Easier to drop back and fight another day.

 

[Ken Roach]

The "redundant processors in the same rack" technique is referred to as "software switching backup" at Rockwell.

It's supported grudgingly, and discouraged by a lot of the guys like me who have worked both on that sort of thing and on real redundancy systems.

Because you're describing a small number of POINT I/O modules as the I/O structure, it suggests to me that you're trying to meet a system specification that requires Redundancy with the minimum capital outlay. In my experience the control hardware should not be a major element of cost savings in a process control system; underpowered or mis-applied controls cost more in the long run.

There's also the "DeviceNet Hot Backup" method that uses two CompactLogix and I/O on DeviceNet (like POINT), but that also has two separate CPUs.