Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 409)

Program Troubleshooting
Chapter 26
26-10
An illegal opcode is an instruction code that theprocessor does not
recognize. It causes a run time error.
Important: The illegal opcode ERR message should not be confused with
ERR messages caused when a 1770-T1 or 1770-T2 industrial terminal is
connected to a processor that was using a 1770-T3 industrial terminal.
These industrial terminal ERR messages do not contain the 4-digit
hexadecimal value and will not cause the processor to fault.
If an illegal opcode occurs, compare the rung containing it to the
equivalent rung in a hard copy printout of the program. You must either
replace the error with its correct instruction or remove it. The ERR
message caused by an illegal opcode cannot be removed directly. Instead,
remove and replace the entire rung. You should identify and correct the
cause of the problem and correct the ERR message.
ERR Message for an
Illegal Opcode