Redundant ControlNet Application

[BIG_K 0]

I have a proposal for an application that has 18 remote flex I/O racks, each rack includes standard digital and analog I/O along with a ProLinx MCM modbus module to read from three modbus slaves. 54 modbus slaves total and there are also 7 HMI's. All of the remote racks and HMI's are on redundant ControlNet. My question is; Can this application function correctly with 18 racks, the amount of modbus traffic, and 7 HMI's on redundant ControlNet?? Thanks in advance, Big K

[BobLfoot 302]

I will answer your question with the unfortunate next question. Can this application function correctly is a very subjective question. You can certainly get this system to communicate and return data to your PLC, but the question will be at what refresh or update rate. I have a suspicion that unless your process is a slow batch process you will not be pleased with the response rate as you describe it. Were it my system I'd plan as follows: {note this is a "kentucky Windage" guess after working on several systems - I did not run the design number. 1 Controlnet Network dedicated to the 7 HMI units. 2. Three Controlnet Networks with 6 remote racks and 18 modbus slaves per network. I am sure that some more skilled members can point you to the design tools to figure to the exact limit what design would be best. You might also find you AB rep a useful resouce in reviewing your design. Good Luck and let us know how it goes.

[BIG_K 0]

Thanks for the reply. This application covers a large cooling water plant and the hardware layout I listed below is what our integrator is proposing to our company. I wasn't sold on this plan and needed a second opinion. Update times are not critical but are important in that there will be several P&ID loops for the various instruments. Your recomendation looks interesting. I only have experience with single non redundant networks so I will do more research on ControlNet architectures. Thanks, Big K

[Money4Nothing 0]

You will not be able to implement a system like this on one ControlNet Network. A 1756-CNBR module only support 64 connections. If you are using RSLinx enterprise, you will have 5 connections per HMI. Each of your analog I/O modules will use a connection. If you rack optimize all of your Digital I/O, then each Flex Chassis will use a connection for digital I/O. I didn't know that Prolinx made a Flex I/O Modbus module, but if they do it will use at least one connection. All of this works out to well over 64 connections. Keep in mind also that a Logix5000 controller supoorts 250 connections. If you end up with more than 250, you'll need multiple controllers. I would go with BobLFoot's suggestion of using multiple networks. Put 4 (or 3) CNBR modules in your controller chassis, one for your HMI, and divide the rest of your network among the other 3 (or 2). That should work with pretty good response time. Good Luck! $

[Contr_Conn 4]

Not really, in most of cases you limited within 40s see 1756-RM094 for details. 1756-CN2R supports 100 connections.

[BobLfoot 302]

Thanks for the design math M4N - I knew I was on the right track from experience, but could not lay hands on the design specs to justify my hunch. Heres a rep point or two for you. A related question and observation. Are you using redundant PLC's or just redundant ControlNet. If your PLC's are redundant be sure and visit the rockwell knowledgebase and get the latest tech notes on designing redundant systems. The programs you write and work well with a single PLC often bomb when moved to a redundant platform and need a total restructure to get a usable scan time. BobLfoot

[BIG_K 0]

There will be redundant processors along with the dual Cnet. Also included are redundant data servers running RS View 32 SE so I'm considering moving all of the HMI's to EtherNet as clients to the servers. I wasn't sure if Cnet could handle 7 HMI's even on its own CNBR. The literature for the CNBR states up to 64 connections for redundant mode, but I've heard that in reality you only want to have 40 to 50 connections for optimum speed??? When I first saw the proposal from our integrator, I thought that the CNet looked overloaded, I'm now learning that indeed it is. Thanks for the replies, Big K Edited 4 May 2006 by BIG_K

[Money4Nothing 0]

^^ This is correct. Its not a terribly good idea to design a ControlNet Network to use more than 40 connections. Although I have done it! Try to keep your RPIs as high as possible as well. I'm not sure what moving all the HMIs to Ethernet will do, I've never used RSView SE like this before. I don't know if it will simply bridge all the connections, or if the ENBT will negotiate a new set of connections with ControlNet....I'll try to look it up if I have time, but maybe someone knows off the top of their head. Its also a good point about redundant controllers. It will really chew up your scan times if you're not careful. Be sure to read all the documentation and ask questions $

[BobLfoot 302]

Big K just realized you're location this morning while browsing. Next time you fly out of terminals C or E at Bush you baggage will be inspected and sorted by a version 13 Redundant ControlLogix PLC controlled system. Spent the better part of 8 months in 2005 in Humble Houston working on the bugs. Good Luck with your app.