Some high-end FANUC models (31i-A, 31i-B) feature Handle Interrupt – a mode allowing the operator to manually jog an axis during an automatic cycle using the MPG (handwheel). If the operator exits this mode without canceling the interrupt or if the program expects synchronized axes, WN57 appears as a safety warning.
For maintenance engineers and integrators, understanding why WN57 triggers requires a look at the PMC (Programmable Machine Controller) ladder logic.
WN57 is not generated by the CNC core itself; it is generated by the PMC based on specific diagnostic signals. The CNC sends a "mode change request" to the PMC. The PMC checks the following signals (typical addresses – confirm against your ladder diagram):
| Function | FANUC Signal Name | Typical Address |
| :--- | :--- | :--- |
| Machine Lock 1 | G104.0 | G116.0 (older) |
| Dry Run | G104.7 | G116.7 |
| Aux Funct Lock | G104.1 | N/A |
| Handle Interrupt Mode | Input from rotary switch | Varies |
If any of these signals are "1" (active) and you attempt to transition from STOP to START or EDIT to MEM, the PMC explicitly sets the WN57 message in the message table (address range: 3000-4000 in the message list).