Port Not Available Extra Quality: Modbus Poll
Cheap adapters (especially Prolific PL2303 clones) have buggy drivers. After disconnecting the adapter, Windows may keep the COM port "ghosted" — still listed but not physically available. When you reconnect, the driver might create a new COM number (e.g., COM5 instead of COM3), leaving the original stuck.
If Modbus Poll crashes or you force-close it via Task Manager, it may not release the COM port handle. Windows can hang onto the port for several minutes or until a reboot. modbus poll port not available
Modbus Poll also throws "Port not available" for (usually port 502). This is almost always a firewall issue. If Modbus Poll crashes or you force-close it
By following this guide, you can diagnose the error in under five minutes and implement a permanent fix that keeps your Modbus polling reliable for years to come. This is almost always a firewall issue