Redundant PLC’s

Rockwell Automation has a full-scale Redundancy system for ControlLogix that includes two controllers, a redundancy manager module, and ControlNet or EtherNet/IP networking for I/O. It's a major undertaking and costs more than double what an ordinary simplex PLC would.

There is also a DeviceNet standby mechanism for CompactLogix that is less functional than full-bore Redundancy but allows for two controllers.

RA does not support, recommend, advertise, or warranty any other Redundancy method.

I don't know that you can accomplish the exact scenario you have described with AB product, at least not easily. However, have a look at the Controllogix redundancy module 1757-RM2. http://ab.rockwellautomation.com/Programmable-Controllers/ControlLogix-Redundancy-Modules#/tab1

I haven't used this exact module, but I used the previous version, known as the 1757-RM. This is a pretty impressive and easy to use module, but does require planning on the front end. Notable requirements include: Must have two separate racks - 1 for primary and 1 for redundant. These racks must match exactly (rack type, module type, module number, firmware revision down to minor rev). Typically, you locate only comm modules in the redundant racks, because you have to have two of everything. I have set up several of these, although they are a little involved to configure, they work great once successfully deployed. Also, as you can imagine, it is a bit costly. Good luck!

Not sure if the one PLC can be offline and accomplish what you want but I know you can certainly set up a redundant PLC. We were doing it at my last job with old PLC-3s. If one faulted, the redundant picked up the ball without even a hiccup. I don't know how we did it as I wasn't there when it was set up. I'm just saying it can be done.

Just go with Contrologix redundancy. However, it doesn't protect against logic fault (I learned the hard way).

what you are looking for is called redundant PLC setup with 'bumpless transfer'.

Iv'e also see the AB -RM module do it, and its nice (so long as the setup/config have been done, and ALL the card firmwares are the exact same...it makes flash-upgrading the firmware a big pain if you have to do that.
The AB module even handles ethernet comms - each redundant rack has its own ENBT card. You give the primary rack the 'normal' ethernet address, and the secondary rack gets the primary rack address+1. If you have a transfer of control to the secondary rack, the -RM card *changes the IP address* of the secondary ENBT card down one number to the same address of the primary rack - makes redundant processors a lot easier to manage
...(this also lets you make changes to each processor seperately...if you ever had to (but this is not advised- the -RM module automatically scans the primary controller for changes and updates the secondary so you only need to ever work with the primary.

I have no idea how other manuf's handle this (good question to ask though)

I'd expect that all of the larger PLC manufactuers have a similar offering (Siemens, Modicon, GE...). Large DCS manufacturers (ABB, Emerson, Honeywell(?)) routinely ship redundant/bumpless by default, but DCS prices get up there pretty quick (how much I/O do you have?).

You should look at the company who you have the most experience with first.

-John

all high end of major brand is having redundancy package.

What you wanna do at lower cost would not switch fast enought for the majority of application VS a real redundant system using a redundant communication media and a third communication point deciding which plc is healty or not.

You also need to duplicate everything around it because the weaker chain can stop it...maybe actually the weakness is your AB plc but tomorrow it could be a cheap sensor doing almost nothing important but that could kill the line etc...So usually a full redundant system will use a voting system and 3 device reading the same thing and if at least 2 out of 3 see it into problem, it is time to shutdown the line.