Hello everyone, I recently acquired a surplus 1756-L71 ControlLogix system, which includes two racks equipped with several digital input (DI), digital output (DO), and analog input (AI) redundant modules from the 1715 series. These racks are interconnected via 1715-AENTR adapters. To clarify, I am not an expert in these types of control systems, and I am reaching out for assistance because I couldn't get the answers I needed from technical support. My primary concern is that both my Human-Machine Interface (HMI) and the RSLogix5000 software are showing abnormal behavior—specifically, the analog readings are intermittently "flickering" between 0.00 and their accurate values. To illustrate, one moment the reading is 0.00, and then it jumps to the expected value the next second, before reverting to 0.00 again. This troubling pattern continues. Here are some details that may help diagnose the issue: - The modules in use are 1715-IF16. - We are utilizing the 1715-TADIF16 termination assemblies. - The modules are configured in 'redundant mode' but not in 'SIL2 mode'. - I have verified that the incoming current is correct, both through multimeter measurements and by simulating current with a generator at the termination assembly pins. Any insights on what might be causing this flickering issue would be greatly appreciated. Thank you for your assistance!
Is the value dropping to zero in the PLC? I recommend tracking the trends to verify if the values within the PLC are genuinely fluctuating. I've encountered a similar issue when entering a zero value to reset a register before inputting "clean" data. The effective solution in this case was to avoid that practice or to use specific values tailored for screen display purposes.
Hello VAN, indeed, even in the Programmable Logic Controller (PLC), the values can drop to zero. Additionally, I want to clarify that I am not assigning any values within those input tags.
The issue may stem from the wiring or setup of your analog card. Ensure that your channels are set up correctly for either single-ended or differential connections. Review the wiring diagrams for your analog instruments to verify their configurations. To troubleshoot, disconnect all channels and reconnect them individually. Based on your description, it appears that your analog signals may be sharing a common ground, which could be contributing to the problems you’re experiencing.
Begin by disabling the module and inputting a value into the data tags. This process will help you identify whether the zero value originates from the module itself or if it's being generated by the controller's code. If the values revert to zero, it indicates that some component within the controller is erroneously writing to the tags. To troubleshoot, perform a cross-reference of your input data tags using the "By Logic" method to identify any undesirable, “destructive” write operations. Additionally, conduct a cross-reference of your input data tags using the "By Tag" approach to check for any instances of double-aliasing that might be contributing to the issue.
Thank you all for your help! The issue I faced stemmed from using a COP instruction to map AI to Modbus registers, but I mistakenly inverted the SOURCE and DESTINATION parameters. As I mentioned earlier, my experience with RSLogix 5000 is limited, and investigating "destructive" writes, as suggested by Daba, helped me identify the problem. I truly appreciate your assistance!
Hey there! That flickering issue sounds frustrating, especially with analog inputs. One thing to check is if there are any grounding or noise issues in your setup since that can cause analog readings to fluctuate like that. Also, ensure that the termination assemblies are properly seated and that all connections are secure. Sometimes, issues with the HMI or the configuration settings in RSLogix5000 can lead to odd behaviors, so double-check those parameters, especially for the redundant setup. If you haven't already, try isolating the modules to see if the flickering persists with just one module connected. It could help pinpoint whether it's a specific module or more systemic. Hope you get this sorted out soon!
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