To reset the "8501" fault code related to cyclic communication loss on a Siemens G120 drive, you can follow these general steps:

1. Isolate Power: Before performing any maintenance or reset procedures, ensure the drive and associated equipment are safely isolated from power sources. Follow appropriate lockout/tagout procedures to prevent accidental re-energization.

2. Check Communication Cables: Inspect the communication cables between the drive and the PLC. Ensure they are securely connected and not damaged. Re-seat any loose connections and replace any damaged cables if necessary.

3. Verify Communication Settings: Confirm that the communication settings (baud rate, parity, stop bits, etc.) in both the drive and the PLC match each other. Misconfigured settings can lead to communication errors.

4. Power Cycle the Drive: Turn off the power to the drive and wait for a few minutes. Then, turn the power back on. This power cycle can sometimes reset communication faults and restore normal operation.

5. Perform a Drive Reset: Follow the procedure outlined in the drive's user manual or technical documentation to perform a drive reset. This procedure may involve pressing specific buttons or accessing menus on the drive's control panel or interface.

6. Check PLC Configuration: Ensure that the PLC is properly configured to communicate with the drive. Verify the addressing and data exchange settings in the PLC program.

7. Test Communication: Use diagnostic tools or software provided by Siemens to test the communication link between the drive and the PLC. Monitor for any errors or anomalies that may indicate ongoing communication issues.

8. Update Firmware: If applicable, consider updating the firmware of both the drive and the PLC to the latest versions available from Siemens. Firmware updates may include bug fixes or improvements related to communication stability.

Remember to document any steps taken and record any observations made during the troubleshooting process.