Primary and secondary PLC on same rack.....

Just two questions guys,

- How hard would it be to have a ControlLogix rack with two CPU's running the exact same program where one is the Primary PLC running your process and the other one is your secondary sharing all the I/O, where if the primary fails I can switch to the secondary and get my system running again.

- If this can be done where do I start.

While this is possible, this is not "redundancy". Redundancy requires among other things: separate chassis, power supply, and hopefully a separate power source.

You are protecting against a failed controller (one of the least likely components to fail), but power is a far more likely point of failure. You would be far better served by installing redundant power supplies rather than setting up a Rube Goldberg controller scheme.

OG

But if you insist on going down that path, keep in mind that the two controllers would be completely independent of each other. In other words, if you make a change to one, the other will not see that change until you repeat the procedure on the second controller. You also cannot just download the same project into both controllers. They would require different projects.

Basically, you just need to add all the I/O into both controllers. Setup some sort of communications between the two controllers where they monitor each other's status. This could be done using a MSG instruction, Produce and Consume tags. I don't think the GSV can be used to retrieve the status of the other controller.

You will also need to include logic to inhibit all of the I/O modules in the backup controller when the primary is up and running. Likewise, you will need to include logic in the primary to inhibit the I/O modules when the secondary has control. Look at the SSV instruction for the MODULE Objet and you will see a property for inhibiting. The modules must be inhibited (outputs in particular) in one controller because two controllers cannot controller the same output module at the same time. Inhibiting will tell the controller to ignore that module.

OG

We done a few redundant contrologix. As OG pointed out already, the setup you are proposing doesn't really do anything to improve the reliability of a system. Other things are more likely to fail than a processor and when it's a software error, having redundant processor doesn't protect from that. This is before even delving into the question of whether it's possible or not and I suspect it's possible but with the caveats already pointed out, that you need a slightly different program in the two PLC that cross-monitor each other's health but I don't know how you going to work out the output control issue since only one processor can own an output card.

I have a small training cabinet with some devices that I'm using for testing. I have two PLC's running the same program on one rack. Only one is running the motors, while the other one is on stand-by.

I tested if the main PLC stops working by pulling it out from the rack. I can then go to the operation panel and restart the operation of the motors etc. this time using the PLC that was on stand by.

Does this makes any sense?

It's like having a spare car just in case you get a flat tire. Sure, it'll work. But you are solving the wrong problem.

Oh, and while your PLCs might have the same logic, you must have two separate "projects". A project is tied to a slot #. So like I said before, you cannot take the project from the slot #0 controller and download it to the controller in slot #4 (I'm just picking slot #s here). It will not allow this. What you have is really just two independent controllers that happen to reside in the same physical chassis.

So back to your original point, yes it is certainly possible. But don't confuse this with redundancy. Those two controllers are most likely fighting each other to control your outputs. When one has control, the other is probably showing faults with those modules.

I know you never said redundancy, I did. I just want to be sure we are clear on that.

OG

I have done a few redundant system with compact logix controlers, where I have had exactly the same program in each processor and has worked perfectly.

Feeding both PLC's the same inputs and blocking the outputs from the backup PLC, it's very nice when you want to do changes, you can work in the backup PLC until your satisfied with the result and dump a copy into the main PLC