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robh

ControlLogix I/O

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I was wondering if anybody had any advice when using distributed I/O with ControlLogix. I have been looking at using 1794 Flex I/O or 1734 Point I/O with Ethernet IP for communications. I still have a lot of reading and planning to do, just looking for things to trip over.

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With Flex I/O on ethernet, make sure you follow all the recommended wiring practices. Proper grounding, etc. (and use steel din rail, not aluminum). If possible, avoid 32-point modules. Each one requires a separate connection compared to only one connection for a whole "rack" of 16-point modules. Default RPI is 5 msec. You can run out of bandwidth on the ENBT very quickly if you don't change it. And, of course, install the right kind of ethernet switch - you're dealing with multicast traffic here.

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Interesting, I had never thought of that. So aluminum DIN rail can cause grounding issues? Would that be time causing corrosion?

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Good stuff

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Use 1734 Point I/o modules they were very cheap & Rouged. they withstand at high temperature

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Just my $0.02 but have you looked at ControlNet with CLGX. The same 1734 IO will work ControlNet as with Enet and the IT gurus won't muck it up on ya. Plus CNET is scheduled I/O not if I've got bandwidth I/O like Enet.

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A little. The main difference I understand between ControlNet and Enet I/P is that Cnet data is scheduled. What is the benefit to this? Most all of the hardware is the same? This is the first time I have worked on a large scale distributed I/O project. I am still reading a lot and trying to understand what is going to be the best fit for our operation. Fun stuff!!! Thanks Edited by robh

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Here is the worst case comparison between Ethernet IO and ControlNet as I understand it. Others may add or disagree. ControlNet System once set up with a NUT of 25ms will provide a fresh read of each input and a fresh send of each output every 25ms guaranteed. The available bandwidth will be approipriated to guarantee this. A program Upload or Network Scan using the remaining unscheduled Network bandwidth will take a varying amount of time but never interfere with the I/O updates. Ethernet System once set up will try and update the I/o every 25ms. However when Jane in Payroll dumps the Checks for the week to the printer and that data has to pass thru the same switch as your I/O the I/O gets delayed. No guarantee as to when you get information. There is talk of deteministic I/O which will eliminate this problem, but I am not aware of it's coming to fruition at this time. TW, Ron, Ken, Panic and others probably have more recent experience and "inside" information to add.

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I don't have any real world experience with Controlnet. None of my networked I/O is time critical. But if you must depend on it on a timely basis definately use Controlnet in place of Ethernet. All of mine is on Ethernet but they are HMIs and Produced/Consumed tags sending status information back and forth. But also don't forget about Devicenet. I have used it quite at bit with no problems

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Obviously, don't put them on the same switch. In fact, don't put them on the same subnet. If possible, have a physically separate network. The determinism issue is pretty much a red herring. Is a variation of some microseconds on a 25 millisecond rpi going to have disastrous consequences for your system? There are pros & cons to both networks, but one of the big differences is the up-front costs associated with ControlNet. You need RSNetworx s/w to set up and maintain a Controlnet network, whereas ethernet I/O can be set up entirely from RSLogix5000. Most modern PC's have a built-in ethernet port whereas a special interface card is required for CNet. There's additional hardware required for enet (the switch(es)) and the network is a star topology compared to Cnet's trunk-line/drop-line.

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I have four dedicated subnets in our facility for PLC's and HMI's {Wonderware PC's}. All PLC's are Ethernet PLC5's, SLC's and CLGX's. It still takes over 2 seconds in most cases to Pass an Integer from PLC 1 to PLC 2 and back to PLC 1. I know that part of this is scan time of the PLC's but most of the lag is network propagation. If we were using Ethernet I/O I worry that we'd overshoot locations on transfers and such. And yes all Data processing and Office PC's are on seperate hardware and subnet from PLC's.

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I have experienced abnormal delays and drop-outs in a couple of situations. In one instance, sparkiy discovered a crossover cable on one port of a central switch - changed it for straight and problem disappeared. In another instance, it seemed that the traffic was going through too many switches - re-routed the cable to another switch and problem disappeared. The delays you're having are uncharacteristic from my perspective. Random thoughts.....if your PLC1 and PLC2 are on separate subnets, the delays are likely in the router/gateway. If you can, try changing their subnet masks so they can "see" each other and bypass the router.

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As far as real numbers with Ethernet I can offer this 9 Controllogix processors communicating to 1 Controllogix processor between ENBTs and ENETs. All producing and consuming 80 bytes at a 5ms RPI with no problems Standard Panelview with 448 bytes In and 444 bytes out, 100ms minimum which is RAs suggestion Another thing about Controlnet is redundancy. Just a random thought for weighing Controlnet compared to Ethernet. Obviously I don't need scheduled I/O so I don't do redundany either but redundancy is only supported by Controlnet for now Gerry, can you explain the problems with AL DIN? Thanks TW

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I don't have a technical explanation, other than to say that ali apparently provides a poorer quality ground. I say apparently because it's a recommendation from the experts at Rockwell to use steel rather than ali. I suppose copper would be even better. It may be something similar to the problem with aluminum house wiring where the connections deteriorate over time.

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Ok, it looks like I need to use up my AL DIN and switch to steel. Thanks for the tip

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Reasons to use Ethernet vs. Controlnet? Cost...You'll need RSNetworx for Controlnet, a PC card for C-Net, new tools to deal with the coaxial cable, and the hardware including taps is not cheap. Ethernet can work just as well unless you need redundancy, but you need to plan for what might happen if your packets vanish into cyberspace. C-Net takes care of that for you. Ethernet I/P does also, to a point. We used ethernet I/O in a time critical application controlling a very large rubber calendaring line (lots of big dangerous moving parts!). With careful planning and even some additional watchdog code in the PLC, I am very confident in it's safety. If I recall, we also reduced the comms timeout settings for the ENET cards too, to minimize any delay that might allow machinery to run after a fault occurs. ALWAYS ABSOLUTELY ISOLATE YOUR CONTROL NETWORK FROM ALL OTHER TRAFFIC! Use the best practices and quality components to guard against interference too. On our system, we tested things by unplugging cables, and turning off the power to our switches just to make certain that the system responded appropriately and shut down safely. my $.02

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Fine as long as you control the hardware, but if it is ethernet it belongs to IT Department at our company and they set the rules to meet their requirements not ours. Ethernet probably will have a place, but it is still not install and forget like CNET --- YET. my$0.02

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This argument from IT really irks me and is an accident waiting to happen. I am not seeing the difference between Ethernet and Controlnet here. They serve the same purpose, they are control networks that are part of a machine. Not part of an information network Just some thoughts of comparison A serial cable can be used on a computer, shouldn't IT set the rules for the connection of PLC serial ports? Both Controlnet and Ethernet can run over fiber, doesn't this mean that IT should set the rules for the Controlnet since the media is something that they set requirements for?

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Hush your heresy Brother TW. It is bad enough IT must be dealt with when we Ethernet, don't give em the entire farm.

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I tend to be lucky in this area. We only have one IT guy and he is more than willing to allow us to do what needs to be done. Heck he will do anything to keep us happy and out of his way. I do agree that there needs to be a defined line in the sand between machine control and happy fingers in accounting. After discussing it here at work we are leaning twards Ethernet.

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O.K. Besides 1794 Flex I/O or 1734 Point I/O, has anybody used other brands with ControlLogix? Like ADs Terminator I/O? Are there any other I/Os out there that are worth looking at?

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The big thing to look for is the product should be Ethernet/IP compliant and support Class 1 or I/O messaging. If the product does this then in theory it will communicate with the Controllogix

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If you are not controlling carbon rods to your reactor and you don't mind your control gear doing a bill gates on you then enet is fine. I would not touch it until it becomes scheduled and deterministic. Which is not likely. I guess eventually fast enet will be the norm, higher speed for enet is the only way around this. speed will prevail. Presently as soon as you mix with IT equipment you lose all reliablity and accountability for your control system. Losing your printer for half an hour is not the same as losing your process!!! The undeterministic nature of enet makes it a dog when comms errors begin to accumulate on your network. I presently ony use enet for non important stuff. It is a common trend to head enet way and also you see this with software, Revision after revision to fix bugs the IT way. Can you aford your control system to be like that. I think not.

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Gentlemen, as an example visit plccenter.com and look at the vast ControlNet items. Enet items.....they are rare at that site. Why? Tech. trend. Give it up die hards.....controlnet is dead!!! (Controlnet is like an old carburetor and enet is the new fuel injection system.) I have MANY machines in Chicago, NC, Vermont and Juarez, Mexico. I can easily connect thru a VPN to ANY of these controllers via the net and make changes quickly...NO TRAVEL!. This is secured and the IT department cannot find any valid excuse to limit my access. (There is no computer needed like the old dial-up and PCAnywhere days.) I can monitor and alter Cognex cameras, update Motoman robots with the enet Anybus installed and test or alter the PanelView Plus screens. I can troubleshoot the 1734-AENTs (located at several locations on the machines...cheaper per point than plc I/O.) You can sure tell that the group here is getting older.....old folks get set in their ways and do not like change or new technology!!!!

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Counterpoint - Are you advocating using new technology, just because it is new. I can still rewire a relay rack and change the way our Dry Mix Operation runs or I can hook up my laptop and connect across the Internet and change the programming of our Cognex inspection system. You will learn with age that the technical spec if only a fraction of chosing a control system. If it can't hold up to financial, human and environmental factors then it is worthless. The fanciest computer hardware that can't be replaced easily or afforded by your company doesn't help you much. With age you'll come to appreciate more than the newest shineiest toy. I hope.

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