1794-OA16 Modules, Outputs passing voltage

[Funkhauser 0]

Hello, I have a very strange issue with a rack of remote IO on my Ethernet setup. There is one rack which contains 3 1794-OA16 (120 VAC output) modules connected to 1794-AENT module. I have several other IO racks on this network without any issue. The issue is that I'm measuring 100-115 VAC on random outputs (they change after cycling power or replacing parts). I also will see output lights on for a couple of these points which also can be seen in the output word while online with the compactlogix PLC (1 on the output point while monitoring the word). These outputs are untouched in the PLC program, verified through cross checking in RSLogix 5000. Also, unplugging the Ethernet cord and separating the TB3 bases so each output module is isolated I still see voltage on the outputs. I have also replaced the AENT, TB3, and OA16 devices and still have this issue. I isolated the modules from the field wiring and 100% certain there is no wiring issue causing this. I'm wondering if this is a case of a corrupt PLC file screwing up the configuration sent to this specific AENT? Has anyone encountered this problem before? Not all outputs have this voltage so I'm led to believe this is not normal situation of seeing voltage but no current. I appreciate any feedback, thank you. EDIT: Realized title error, please excuse it. EDIT 2: Spoke to AB tech support, they said it is a possible bus issue but I'm not convinced since all the parts have been replaced. Edited 15 Sep 2014 by Funkhauser

[TConnolly 13]

Solid state IO modules have leakage currents. If the output is unwired or wired to an open circuit or very high impedance device then you may measure line voltage at the terminal even when the output is off. The manual for this module states that it has a 2.25mA leakage current when the output is off, which can be enough to marginally illuminate low power indicators like LEDs and even enough to hold in a mini din rail type relay. A high impedance digital multimeter will register line voltage when the output is off unless something like a contactor or solenoid coil offers a drain path for the leakage current. If you do not have the manual which discuses this then go to the literature library at the AB website and search for your part number. The manual will also link to this document which I recommend you download and read. Edited 15 Sep 2014 by TConnolly

[Gerry 13]

Are they truly random? That you can see the bits on in the PLC data table suggests that they may be a remnant from previous program editing. What happens if you reset the bits in the data table? Are there other PLC's on the same ethernet network? i.e. maybe a different processor is writing to the ip address of your flex adapter. Leakage has always been an issue with triac outputs, but that won't affect the led indicators - they are driven from the backplane (bus) signals, not the triac output. Much older generation triac modules (1771 & older) had neon indicators driven from the triacs, which often gave false indication for light/no loads.

[Funkhauser 0]

Hmm very good replies thank you. I wasn't aware of leakage current, or most likely forgot from my semester of studying solid state switches, which does explain reading a voltage. I'm surprised AB tech support didn't bring this up while speaking with them today! Most of these outputs are firing solenoids but we had wiring issues regarding the field neutral so no proper drain path. As for the outputs which turned themselves on, they were not random so they must be caused by a separate problem. I was reading a voltage (drain current) on what appeared to be random points but is most likely due to the multimeter measuring 100 vac at 2.5 mA. When I turned off the outputs the light indicators turned off so I'll have to recheck my logic to see if anything is writing to the output word. Very happy my own ignorance may be the culprit rather than a corrupt PLC file or an evil bus bug that corrupts anything hooked up to it. Thanks very much for the replies.

[Ken Roach 194]

You are probably experiencing two or three separate problems. The leakage current issue from triacs gets everybody the first time, especially with low-power output devices like neon indicators or small valves. From reading your post, I think you are seeing "leftover" data in the Output data table. To confirm: you are seeing 1's in the output data table on points that are not referenced in the controller program, and you are also seeing the channel status indicators on the FLEX output module turn on for those points. It's easy for data to be "left over" in bits that are not commanded to be true or false by the controller program. The simple solution is to just poke 0 values into the data table. The certain solution is use OTU instructions to command them to be false all the time. It's possible that you have a MOV or COP instruction someplace, or that another controller is writing to your controller's tags. You should example the program closely for possible indirect address problems.

[Ken Roach 194]

A firmware problem is unlikely, but not impossible. I remember a service advisory several years ago where certain FLEX modules would receive the wrong data over EtherNet/IP and turn on the incorrect output points. It was in 2004, and the details are here in the Knowledgebase. It affected a narrow range of 1794-AENT production firmware. https://rockwellautomation.custhelp.com/app/answers/detail/a_id/53304 I think it's unlikely that this is the problem, because you are seeing data in the output table that corresponds to the output bits that are turned on. That was not the problem in the service advisory.