Hello everyone, I have noticed a peculiar pattern with an integer file (N46:33) containing the binary value 11110, which equates to 30 in decimal. Upon conducting a search (cross-reference), I discovered that the only instance referencing N46:33 is within a Compute instruction. Despite this, after downloading to the PLC, N46:33 occasionally reverts to 1111 without any other logic instructions that could write to it. Is there a method to set N46:33 as read-only to prevent this issue?
I was considering inserting a MOV command to effectively transfer the data '11110' into memory location N46:33.
Does the compute instruction lead to the destination N46:33?
Cross-references may not display usage if they are referenced indirectly through an index or written using HMI, Scada, or similar systems.
What is the CPT instruction in PLC programming - is it considered an expression? If so, what components make up that expression? Are the surrounding N46 words comparable in function? Is there a power up or first scan routine that initializes the integer word(s)? While you cannot restrict it to read-only, you have the ability to input a specific value. Another option is to monitor for values being written by other PLCs or SCADA/HMI systems.
In order to find indirect references in a program, you should search for the symbols [ or ]. Unfortunately, in rslogix500, it can be challenging to locate items that are indirectly addressed. For example, you may need to search for something like N46[XX], where XX represents another address that may sometimes be 33. Look for the value being inserted into it or search for a file that matches the binary if a masked move was used to input a specific value. If you're having trouble locating the reference, feel free to upload the file here for assistance in identifying where the changes are being made.
Hi there! It sounds unusual for the value of N46:33 to revert without any logical instructions writing to it. As far as I know, most PLCs do not support read-only functionality, but we can seek an alternative solution. Have you tried isolating the issue by monitoring your program's execution? It's possible that an unexpected logic path is affecting the value. One more thing to consider - if you are using any HMI or SCADA system associated with your PLC, make sure they don’t have write access to the N46:33 register that could unintentionally modify the value. Additionally, ensuring that your PLC programming software is up-to-date might also assist in solving odd glitches.
It sounds frustrating to deal with that unexpected behavior! While you can't technically set a tag as "read-only" in most PLC environments, you might try using a watchdog timer to periodically check and reset the value of N46:33 to 30 if it changes. Another option is to review your program logic more closely for any unintentional writes that may not be immediately visible, like system functions or other tasks that could impact it. Have you also considered any possible external influences, like HMI or SCADA systems writing to that variable?
It sounds like you're dealing with a classic case of unintended data modification in your PLC! Unfortunately, most PLCs don't allow you to set a specific memory address like N46:33 to be read-only through programming alone. However, you might want to check for any hidden logic or background tasks that could be inadvertently writing to that address. Also, consider adding a watchdog timer that resets N46:33 to your desired value if it ever falls below that – that way, even if something tries to change it, you can ensure it maintains the correct state.
✅ Work Order Management
✅ Asset Tracking
✅ Preventive Maintenance
✅ Inspection Report
We have received your information. We will share Schedule Demo details on your Mail Id.
Answer: 1. Why does PLC5 File N46:33 occasionally revert to 1111 from 11110? - The issue may be due to unintended write operations in the program logic or external factors impacting the file's value.
Answer: - To prevent accidental changes to N46:33, you can consider setting it as read-only using appropriate programming or configuration settings within the PLC software.
Answer: - You can implement monitoring tools or add additional logic checks in the program to detect any unauthorized modifications to the file's value.
Answer: - It is possible that communication errors during the download process could affect the integrity of the file. Checking the download process and ensuring a stable connection may help address this issue.
Join hundreds of satisfied customers who have transformed their maintenance processes.
Sign up today and start optimizing your workflow.