The fault code "8501" on Siemens G120 drives indicates that the cyclic communication between the drive and the connected PLC (Programmable Logic Controller) has been lost. This fault typically occurs when there is a disruption or interruption in the communication link between the drive and the PLC. Here are steps to address this issue:

1. Check Communication Cables: Inspect the communication cables between the drive and the PLC to ensure they are securely connected and properly terminated. Look for any signs of damage or wear on the cables that could affect communication.

2. Verify Communication Settings: Confirm that the communication settings in both the drive and the PLC are configured correctly and match each other. This includes parameters such as baud rate, parity, stop bits, and communication protocol.

3. Test Communication: Use diagnostic tools or software provided by Siemens to test the communication link between the drive and the PLC. This can help identify any issues with the communication hardware or configuration settings.

4. Check PLC Configuration: Verify that the PLC is configured to communicate with the drive and that the correct addressing and data exchange settings are configured in the PLC program.

5. Inspect Network Infrastructure: If the communication link between the drive and the PLC is established over a network (e.g., Ethernet), check the network infrastructure for any issues such as network congestion, packet loss, or configuration errors.

6. Review Fault History: Some Siemens drives store fault history, which can be accessed through the drive's control interface or diagnostic tools. Review the fault history to identify any recurring issues or patterns related to communication faults.

7. Update Firmware: Ensure that both the drive and the PLC are running the latest firmware versions available from Siemens. Firmware updates may include bug fixes or improvements related to communication stability.

8. Consult Technical Support: If you're unable to resolve the communication fault on your own, contact Siemens technical support for assistance. They can provide further guidance and troubleshooting steps based on the specific fault code you're encountering.

By following these steps and addressing the potential causes, you can troubleshoot and resolve the "8501" fault related to cyclic communication loss between the Siemens G120 drive and the connected PLC.