If you see errors, uninstall the device, unplug it, reboot, and reinstall the latest driver from the chip manufacturer (FTDI, Silicon Labs, or genuine Prolific).
In Device Manager, right-click your COM port > Properties > Power Management. Uncheck "Allow the computer to turn off this device to save power" to prevent the port from dropping out. Summary Checklist Close all other Serial/Modbus software. Verify the COM port number in Device Manager Unplug and re-plug the USB adapter. Restart Modbus Poll as an Administrator Modbus TCP modbus poll port not available
The error message means:
"Port not available" in Modbus Poll typically means the software cannot gain exclusive access to the COM port or network socket you are trying to use If you see errors, uninstall the device, unplug
Within Modbus Poll, if you close the connection window but the poll cycle is still running in the background (rare bug), the software thinks it still owns the port. Summary Checklist Close all other Serial/Modbus software