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sadlerej

remote i/o

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I have a proposed project that requires me to monitor 3 areas of 110 different machines. The physical space is about 120 feet across and 600 feet deep. The sensors that I need are 1 proximity switch 1 limit switch and 1 4-20ma analog signal. My question is what is the best and most inexpensive way to get to each machine (modbus, profibus, remote I/O, Ethernet or ect.)? The PLC’s that I have available are a SLC 5/04 or micrologix 1100. Thanks for your help Ed

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The question I would ask is what I/O platforms do you already have in place? If you have RIO ,D-NET, ect . and are comfortable with it I would stick with what your support personel are OK with. This could be a selling point if your a SI you may be able to pitch the idea that you can stay with current hardware and not have to increase the spare part qty. I have mostly RIO so I would have a tendency to lean toward that. I don't like any in house guy that changes his baseline just because he wants to PLAY with something new. If the new system is truly better for the application then run with what is better.

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My first instinct is to recommend a DeviceNet-based system because I think you could get the small I/O counts most efficiently, and the cabling is fairly flexible. The SLC-5/04 supports DeviceNet connectivity via the 1747-SDN, but the MicroLogix 1100 has no remote I/O options. Let's discuss more about your requirements and your installation. Do you need this I/O fast for fault detection, or can you get it slowly, like once or twice a second ? Do the machines move ? I presume there is already power to the machines, but is there any other existing network infrastructure, like Ethernet or DH+ ? Are there already controllers on the machines that have a common network interface ? Reading your post, I am not sure if the "available controllers" are the ones controlling the machines or the ones you have outside the machine area.

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Look, you're looking for two different problems. The same two that always have to be considered. The digital inputs are going to be cheap and easy to deal with. The analog inputs tend to be very expensive. So work your way around the cost of those. The least costly wiring in terms of installation and copper is going to be putting input cards directly at/on each machine and wiring only network cable (whatever the type) to each machine. However, the I/O density is so pathetically low that it will be very costly to do it. A single analog input card will run at least a few hundred dollars. The least costly in terms of I/O would be to put in a single large ControlLogix PLC with multiple racks connected by ControlNet. In-rack I/O is always the cheapest. However, have you seen any of these massive I/O rack behemoths lately? The reason is because the cost of all that wiring (labor and copper costs) is far more expensive than distributed I/O. The costs are going to be less than $100 per machine for the I/O, but the wiring costs are going to be substantial. The actual I/O cards may not be what you are expecting either. For instance, a Micrologix is really a programmable I/O card that comes with 2 analog inputs (use a resistor to convert 4-20mA currents to voltage) and 10 digital inputs. Add a 4-input analog input card and you can read 5 machines with a single Micrologix 1100, for a total cost of about $170/machine for I/O. Running through the same scenario with Acromag cards (Ethernet/IP or Modbus/TCP, your choice), you can get a 12 input digital card and a 6 input analog card for about the same money, dropping your cost per machine down to about $150/machine. What you need to do is exactly what I'm descrbing...lay things out and figure some costs with various configurations varying from high to low density. You'll find the sweet spot, wherever it may lie. Don't get too hung up on the specific network. There are reasons to avoid certain ones. Remote I/O pricing has just gotten out of sight (AB is trying to get away from it). Profitbus is a pain in the rear to deal with all the way around. Modbus/TCP and Modbus are not supported by AB but for instance you can read up to 16 of them through a Digi One IAP card acting as a protocol bridge. Although you have to pay for switches with Ethernet, I just bought a 1000' reel the other day for $90. I'd pay 5 times that for DeviceNet cable. But I also wouldn't have to pay for Ethernet switches. Again...there are tradeoff's and the only way to figure this one out is to price out a few different combinations and see which one looks the best.

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