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.