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robh

MicroLogix 1500 fault

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So this morning around 4 am I get a call to one of our production machines because it won't work. I look around for a few and eliminate the obvious possibilities. I then open up the electrical box that holds my MicroLogix 1500. What do ya know the fault light is blinking! I hook up mr. notebook and find that the fault description is something I don't quite understand. Reset fault, back to run mode, machine is working again. Fault description is as follows..."The controller cannot communicate with an expansion I/O module. Exact module cannot be determined." HMMMMMM.... This is a MicroLogix 1500 LRP with a 1769-OV16 connected to the side of the controller, then a 1769-CRL3 connecting to a 1769-PA4 power supply and a 1769-IQ6XOW4. This has been working fine for the last 2 months. Any ideas? Rob

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Contacted Rockwell via live chat and the tech could only think that maybe there was a loose connection. Have to double check it. F.Y.I. When using the Rockwell live chat you are required to select a "ChatQue" before you can connect. I have noticed that the list is fairly small, with MicroLogix and RSLogix500 not being a choice. I selected "ControlLogix Hardware/Software" because it was the closest match. I asked the tech what would be the best selection and he told me that the MicroLogix and RSLogix500 are not currently supported by live chat, but should be sometime in the future and he would do the best that he could to help me out. Not sure how many people use this but I have found Live Chat to be a nice tool to have in my bag.

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I have had a similar situation with a ML1200 with expansion cards, the ribbon cable connection was loose. I reseated it and never had another problem.

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Part 2...the saga continues.... So about 30 minutes ago another one of my MicroLogix 1500 with 1769 I/O faulted just like the first. This particular machine has been in service 6 months after a complete rebuild (everything electrical was replaced). I personally checked every I/O cad connection and found nothing that looked problematic. I am waiting for my distributor dude to call me back. Other than the connection theory, what could be causing this? Both units have Islatrol line filters feeding any power connected to the PLC and I/O cards, also all outputs are isolated with relays. Power bump?

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I have had this problem when a plant "electrician" gets called out to a machine before the controls tech. The "electrician" trys to reseat connectors while the processor is in run mode. When it faults out they call me, and low and behold the "electrician" is nowhere to be found.

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If this strange event is occurring on another process, could you be having a power problem in the plant?

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Very possible. My little gremlin has made itself disappear as of recent.

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I am having this exact problem on a machine that I built custom, it has a ML1200 with two expansion modules: 1762-OW16 and 1762-IF20F2, It has been running for about 1 year with no problem and all of a over the past month I have been getting "the controller cannot communicate with an expansion i/o module. exact module cannot be determined". I changed the ML1200 and output module with a new spare that I had, and I am having the same problem. Was anyone ever able to fix this problem. Help

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Not a fix but troubleshoot tool...when the expansion card loses power on the bus connection it causes a communication fault to the processor and locks the bit in the registry. if you can confirm that there is a Stop Processor bit 1 and a comm fault 1 then you know your connection may be the problem. if you have power problems....check the ground resistance at the panel, noise/spiking are only effectively reduced if there is a good path nearby. If your distribution is at the other end of the plant, your line filters can only do so much.

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With regard to the MicroLogix 1200 and the expansion I/O, that OW16 module would be my prime suspect. Look carefully at any inductive loads and check them to be sure the snubber circuits are in place and appropriately connected. A big motor starter or solenoid valve can snap a couple hundred volts of back EMF when it opens, which doesn't necessarily cross the backplane but can induce disruption in the 1762 bus. On the MicroLogix 1500, the thing I'd look at is the expansion power supply. The power supply itself, or a dip in the power feeding it, would cause that expansion bank of modules to fault.

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There are 10 motor starters running on this module, all of these are 110 vac coil; controlling 460v 3ph motors 2 hp each. There are no snubbers on the coils, what would you recommend? Thanks,

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We either use a small interposing relay sized for the output of the card, or if you check the cut sheets for the contactors they should give you data for installing the proper size component. When the coil loses its power, the collapse of the flux field can send a DC surge back to the card.

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Here is AB's guidelines. Recommended_Surge_Protection.pdf

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