Modbus in ControlLogix

[robert_ 1]

Greetings! I'm working on a project of a crushing and conveying system of mineral, which is controlled by PLC's controlLogix. All I see is decentralized and that the system is branched RIO panels, where there racks without cpu, only with analog input / outputs, and modules for RTD `s counters for encoders which I believe are just units of I / O remote, but also features modbus (I think that's why you use a module that reads: "Prosoft") and also features interface drives "Dupline" which I am told are one such RTU's (not much of this alone commented what I'm told), but I know this has to do with the field devices to be connected in just two wires for miles MTU, but if so ... Why this decentralized system RIO panels?, Could anyone tell me roughly how it is that the system is working? (hopefully understand my ignorance in decentralized systems and networks that is my first experiments, only worked with input / output parallel wired or...

[PLCMentor.com 60]

So I'm not completely sure I understand all you are describing and I am not even sure I completely understand your question so here is how I understand it. You have a controlLogix with remote I/O. The remote I/O uses modbus to communicate. I think there is another comm network but I am completely lost on that one. Your question is, why distribute I/O in this manner? I would assume that in the past your I/O has been all local and possibly located in the rack with the processor. The simplest reason to use distributed I/O is to cut down on wiring and the installation costs associated with such. You place I/O locally close to the field devices and limit the distance that wire needs to be pulled. The information gathered by the rack is then communicated, in your case, by modbus back to the CPU. There are other advantages such as modular designs allowing for organized expansion, but generally the costs savings by locating your I/O locally is enough.

[robert_ 1]

Thank you very much sparky. Look, waht happens: I don´t understand why the system uses modbus, "dupline" and ethernet too. My questions are: Which is the fieldbus in this case?. why using ethernet?, why using modbus?, why using dupline? perhaps be that modbus is to communicate between RIO racks??, and dupline is to communicate devices like pull cords installed in the conveyor through modbus?, and ethernet is to communicate between different existing stations ?

[PLCMentor.com 60]

Ok, so I would have to see your network topology to be able to answer this question. I can make some guesses. I had to do a little reading into dupline as I have never heard of it before. Dupline is a fieldbus type network that can transmit up to 10km. Maybe that is the reason they used it. Ethernet is only good for about 300 feet or less. You will have to look at how things are connected to determine the actual functionality of what each network is doing. I suspect now that the modbus cards are used to communicate with the Dupline master and then it communicates to remote dupline modules. The ethernet is the best network for communicating with an HMI or PC and is generally very good for connecting Remote I/O cabinets as long as the distance is close enough. All of that is a guess. I have seen people do weird things with network installations. I have even seen where engineers used as many networks as they could to learn how to use them. Not good for the end user, but I have seen it done. Hope some of this helps shed light. You need to have some sort of network diagram for such a system and if you dont then you need to start investigating what is connected to what to better understand your system. The Logix platform will generally shed some light into all this as many of the network connections will show up in the program configuration. I am not sure about this Dupline setup though. It will probably not show up in the Logix configuration.

[robert_ 1]

ok sparky thanks again...I think that the prosoft card in ControlLogix is to communicate with dupline fieldbus and this in turn serves to communicate via Ethernet with remote rack expansions, because the prosoft card has IP adress. In any way should be investigated further, since this is a new project in the company, But I wonder if you can give me some examples of configuration of ethernet and modbus networks in Logix. Thank you!!

[dmargineau 6]

Usually the communications protocols are chosen depending of the system components' capabilities, origin (manufacturer), the required functionality and the overall physical layout of the system Modbus, with its various "flavors"(RTU, ASCII, TCP/IP, UDP, Plus, etc.) has been published developed by Modicon since 1979 and it is being used to communicate with (or between) Modicon manufactured products (CPUs, HMIs, etc.). Again, Modbus is the "application" layer, which could be deployed on RS-232 (Serial) or Ethernet "electrical" layers depending of the system's layout or functionality requirements. If you need to create a communications path between an Allen-Bradley controller and a Modicon CPU you will have to use a protocol "translation" bridge between the two PLCs using a dedicated module such as the ProSoft MVI56E-MNETR module. The ControLogix's "native" communications language, EtherNet/IP, will be "translated" to Modbus TCP/IP for example and the communications will be carried by the Ethernet's "electrical" layer. Locally, between the same manufacturer components, you could choose any supported protocol. For newer A-B products, EtherNet/IP is the most common and modern choice; if not supported, using ControNet, DH+, DH-485, Remote I/O or DF-1 are other solutions. For Modicon manufactured products, Modbus TCP/IP or UDP are the two most commonly used protocols deployable on the Ethernet layer; Modbus RTU and Modbus ASCII are serially deployable, while Modbus Plus is using Schneider Electric's Fieldbus electrical layer (using a dedicated co-processor). Edited 16 Oct 2012 by dmargineau

[robert_ 1]

ok, but may communicate remote extensions (RIO) by ethernet?,this is using modbus RTU. And, can field devices such as pull cords switches of conveyors by "dupline" (which is a fieldbus) while by modbus too?... then Dupline fieldbus could be a kind of intermediary for the issue in scope distance?

[dmargineau 6]

Yes, you could control remotely located I/O on Ethernet, however, there is a distance restriction, 328 feet (100 meters) for Cat.5/6 media and up to 1.3 miles (2 km) for fiber optic media. Modbus RTU is running on RS-232 (Serial) physical layer and its distance restrictions are the most acute of all, 50 feet at the most. Probably this is why the original developer used Modbus RTU to connect I/O to the distant communications controller/hub and then "translated" the I/O info into Dupline Fieldbus in order to transmit it to a miles away device. Again, communications topology depends of the system components' capabilities, application's requirements and physical layout of the system.