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BobLfoot

1794 Flex I/O Oddity

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Interesting problem with a rack of 1794 Flex I/O communicating to a PLC 5 via RIO this weekend. Moisture penetration into 1794-TB3 Modules appears to have hampered operation of the rack. The rack in question is defined as Rack 3 Group 0 for a 1/2 rack. It stopped communicating with the PLC 5 and gave us only a flashing Adapter Active light. Per the user manual this means the adapter is in Program / Test Mode. Our PLC, however was in run mode and communicating with several other racks of Flex as well as two DL40' and a 1771-ASB Rack without issue. Thinking it was a cabling or device issue we first recued the network to no devices and checked for the 75 ohm reading which should come from our EOL resistors. After passing the EOl Resistor test we ran with only the faulty rack and the PLC connected and the faulty rack still showed Program/Test mode. We then disconnected the 1794-TB3's from the 1794-ASB and performed an Auto-Configuration which returned a 1/4 rack and solid Adapter Status Light. We then added the 1794-TB3's and I/O Modules for Groups 0 and 1, ran another Auto-Configuration and all was well. We then added the 1794-TB3's and I/O Modules for Groups 2 and 3, ran another Auto-Configuration and the adapter was back in Program/Test Mode. Next we repalced the 1794-TB3 for Group 2 with a new one from our storeroom and ran another Auto-Configuration and all appeared well as the Adapter Status Light was solid green. This was misleading, however, as no Inputs from Group 2 were reaching the PLC Data Table. We then repalced the Group 2 Module an 1794-IA8 and still not Inputs were making it to the PLC5 Data Table. We then repalced the remaining 1794-TB3 mnodules for Groups 0,1 & 3 and the rack returned to normal function. Things I found strange about this. 1. A Terminal Base failure not being reported as a rack/module failure just the falshing adapter status light. 2. How a failure of a TB3 in Slot 0 ior 1 would prevent slot 2 from registering data without a red led to be seen,

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I have seen numerous weird problems with Flex I/O and they usually were traced back to a connection issue with the 1794-TBx modules. I have seen adjacent modules lose all or part of their functionality due to a bent pin or corroded connector, and have found that the connector could be anywhere upstream of the module exhibiting the symptom. I made it a habit when I worked with lots of Flex I/O, to fully disassemble and very carefully reassemble racks that gave repeated problems. Sometimes a little plastic mold flash or a burr on the DIN rail has caused uneven alignment. Combined with vibration, this can cause seemingly random problems. Also, be very meticulous in your reading of the settings for the adapter and how that affects the status bits received by the PLC-5. The 1794-ASB has evolved to include support for Logix PAC access, and RIUP, so you can choose whether or not to report a missing or damaged module as a rack fault IIRC. Paul

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One confirmed oddity to watch out for: All analog modules are OK with an ASB. All digital modules are OK, too. If you mix analog and digital, watch out. The first slow next to the ASB can be digital or it can be empty. It CANNOT be analog. There is some sort of timing issue that then causes the ASB module to go goofy and the config lights to flash.

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