An easier way to use cross-reference
- Open the project
- Right-click in a blank section on the ladder display
- Select Properties
- Click on the Address Display tab along the top of that window
- Check the box for "Display Cross Reference for Outputs on Rung" and click OK
This is a computer setting so it will continue to use this for all projects until you reset it.
Now go to the location where you were trying to change the value. Underneath the instruction you should see in yellow the address, the type of instruction, and the file:rung number where it is being used (ie..N7:0 - MOV - 2:5). If you click on the file:rung it will jump to that location. And from that location you can jump back to where you were.
At that location check to see if that address is being used as the "destination". If so, then that instruction is writing to your address. If it isn't try the other locations if there are more.
If there isn't more, then it is likely that a device is writing to that address over a network. Like the HMI muusic_man suggested.