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mhowasso

Multiple Analog Signals

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Using a CompactLogix System, what is the best way to get approx 24 (4-20mA) Analog Inputs using the least amount of hardwiring and modules..??? Preferably not just bringing each as a single device. Thanks

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How fast do you need the data ? A multiplexer will allow you to use a single PLC channel for several analog inputs but it takes time to switch and you need to carefully wait for the signals to settle. Are these devices all driven by the same DC power supply ? If you need a differential input or isolation, there's really no substitute for individual input module channels.

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Here are a couple of devices one with a analog output and another with modbus output. http://www.phoenixcontact.com/global/news/248_59284.htm http://www.miinet.com/products/sg_distributed.shtml

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Without knowing more about the system, I would suggest you look at POINT I/O, such as the 1734-IE8C. If you already have a compatible communication network you could put three of these modules out at the field and all you have to bring back is the communications wiring. If you turn out to need 25 instead of 24 it's easily expandable. Just an alternative to multiplexing, may not be what your interested in.

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Why do companies spend fortunes on equipment and go cheap on controls? Who would ever design a system with 24 analog points and then try and go cheap on the PLC that monitors/controls them. I do not understand the logic. If there was a design need for 24 analog values, why would you not design you control system to handle the same number of values?

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When you say reduce the wiring, do you mean the wiring in a control cabinet or the wiring that goes out to a remote location? If you are looking to reduce the wiring out to a remote location, is there any existing infrastructure such as buried phone lines? For each of these instances there is a different solution that might be applicable (i.e, wireless or remote I/O).

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Agreed! Plus the more complicated you make the system via multiplexing or otherwise, the more difficult to startup and troubleshoot. However, if the primary purpose is to decrease hard wiring then the point I/O recommended by Tim or flex I/O can be dropped local to your transmitters and brought back via an Ethernet cable (or some other comm cable).

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The Point I/O or Flex I/O seems more of what I am looking for. I just would like to bring back from the remote devices the least amount of hardwiring to the main control cabinet which houses the PLC. One EtherNET cable would be ideal. Thanks

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The mass of hardwiring was spec'd out by Integrator on this project. This seemed very complex and he has used the largest rack and used every slot for I/O. And he still needs more room and is recommending another PLC. It seemed rather stupid to me, but I am just a lowly employee.

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I agree. I tend to like to drop my I/O close to the equipment to minimize hard wiring. In fact on many projects, we can use a similar panel design for our remote drops lessening the design costs. The flex I/O and point I/O are a little more expensive than rack mounted I/O, but if the rack I/O is installed properly then they wire to interposing terminals and between the terminals, wire and install time the I/O drops are cheaper. You may need to check the PLC and see how much memory there is and CIP counts are important when dropping I/O so read up on that. If this is a new application that has not been installed, I would not just add a second PLC for no reason. If there is a logical divide between two systems or processes then it may make sense. In other words if you can bring a line down and the other line can run without it then a second PLC would make sense. If the two lines depend on each other such that one cannot run without the other, then I would try to put everything on one PLC. All that said (or written) we are trying to move away from rack mounted I/O for most of our I/O needs.

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