Dual CLX Processors in the same Chassis

[Bullzi]

Hi everyone,
Going to be starting a new project soon. This one will be taking me out of my comfort zone but I am looking forward to the challenge.

The design I got from the engineering company calls for dual L73 processors in the same chassis. I have done some research on Redundancy on the AB web site. Everything I have read talks about using redundant chassis with 1756-RM modules in each.

So my question is if I have 2 processors in the same chassis how does that work? Do I select one as the primary and one as the secondary? Will they exchange data between them so the program and data are the same in each. Just trying to get my head square on this.

If any of you have worked with a set up like this please fill me in on how that works. Everything I have done to this point used only one processor per chassis.

Thanks for any help you can provide!!

 

[ASF]

I haven't had any direct experience with this sort of setup, so I'm probably not the most qualified to answer, but...

If they are in the same chassis, I don't think they are redundant - I think to get redundancy you have to duplicate the whole chassis. Generally if there's two CPU's in the rack, it's for processing power - maybe one PLC on it's own didn't have enough memory, or couldn't execute the code fast enough. I have seen racks with two CPU's before where one was a standard CPU and the other a high speed CPU, which had very little code - just enough to drive the high speed components of the machine while the main CPU took care of all the motor controls, visualization, comms, etc.

 

[gmferg]

Are you trying to do redundancy or two independent processors in the same chassis?

Redundancy:
As your research has shown, redundancy requires two chassis's with identical modules which consist of only processor(s) and comms modules.

2 processors in one chassis:
You can install multiple processors in one chassis. This configuration does not allow for redundancy. Each processor has it's own unique program. Within the chassis, each output module can be "owned" by one processor. Each input module can be "owned" by one processor but multiple processors can "listen" to an input module.

 

[Operaghost]

Redundancy requires you to eliminate any single point of failure. Two controllers in the same chassis is not and can not be for redundancy.

Two controllers in the same chassis operate as completely independent controllers. They each have their own project and may or may not even know that the other controller exists. Two controllers in the same chassis is a convenience and a cost savings. They can share the chassis, the power supply, and the communications modules. They also save cabinet space since only once chassis must be mounted.

The controllers can share their input modules but they would control separate output modules. They could potentially communicate with each other across the chassis backplane using Produce and Consume tags.

Someone could possibly rig up a sort of home-brew redundancy to have one controller "take over" the outputs from the other controller. But at that point you are reinventing the wheel, only worse, and it won't be round, and you have a single point of failure so it isn't really redundant.

Hope that sheds some light.

OG

 

[BobB]

There are different levels of redundancy - and then there is hot and warm standby/changeover.
I prefer dual redundant PLCs and power supplies in the same rack and then a rack system off that with the I/O in it.
The issue with full dual redundant systems is you also have to duplicate the I/O - heaps of relays, signal splitters - you name it.
All of a sudden there are thousands of single points of failure added to the system.
Backward step and a night mare to troubleshoot!

 

[orlyjs]

Basically, redundant controllers requires to be in a separate chassis with comms interface module.
If you install two controllers on a single chassis, each controller will own (or control) its assigned output module, means that the second controller cannot own an output module if already assigned to the first controller.
I did try to make it work with two controllers on a single chassis and redundancy ONLY work ONCE. The steps are tricky and I will not recommend you to implement it.

 

[BobB]

The Omron system has a monitoring and changeover unit - no conflict at all - just works. I have even seen 2 PLCs used with an output from the first to the second to stop the outputs working - geez - redundant?

 

[Bullzi]

Thanks Everyone for the info. That is what I was thinking too. I will have to go back to the Engineer and see what he had in mind when he designed the system.

If they decide to go with true redundancy I have a couple questions:
1. The hardware setup up that I have seen in the manuals shows 2 small chassis with a Power Supply, ENET card, Processor and 1756-RM or RM2. All the I/O reside on a separate chassis connected via Ethernet/IP. Is this how it has to be done? Just want to make sure I recommend the proper set up to the Engineering Company.
2. I am confused on the Firmware version. It looks like version 20.055 is the highest version they show with Redundancy. So I will have to flash both processors to v20.055 and use RSLogix 5000 v20 for programming instead of Studio 5000. Correct? Have they updated the firmware for redundancy to v21 yet?

Thanks for the help!!! I am sure I will have more questions as this project moves along as it has some new (to me) hardware.

 

[ASF]

1. Yes, that's the only real way to get "true" redundancy
2. I can't speak in specifics about redundancy firmware revisions, but yes, redundancy is not supported on all firmware revisions and AFAIK hasn't made it past v20 at all yet. If it IS redundancy you're after, then (aside from duplicating the chassis to do it properly) you need to find a firmware revision that supports redundancy, and make sure both CPU's have the same version.

 

[Bullzi]

Well just found out that they are going to remove the second processor and just go with a single processor. That should make my life much easier Thanks everyone for the help and good info.