The "Illegal word operand Access" error can present itself due to application logic error or due to EMI. (electromagnetic interference)

Please follow next steps to help you determine the issue.

1. Check if there are any devices near the PLC that can create EMI.
2. Check if the electrical cabinet correctly wired according to the wiring guidelines attached
3. Try to replicated the issue at your desk
4. Check if the message appears after a specific event or at random.
5. Check if the LDR code is the same each time you get this message.

For example: Error code `42C`

If the error is at random and it's different all the time then it is highly possible that is cause by and external factor such as EMI, in this case please refer to the link below about "Wiring Guidelines".

http://www.unitronicsplc.com/Download/SoftwareUtilities/General_Wiring_Guidelines.pdf

If the LDR error is the same all the time, you may determine the subroutine that is implicated in this error by following the next steps:

• Open the application that is running in the PLC using VisiLogic.
• Go to the Edit menu and select Find subroutine by ladder error code.

• Introduce the error you see on screen.
  

This will point you to the suspected subroutine.