The drive’s analog inputs can be set to work either in the 0-5 V or in the +/-10 V range. In order to apply a voltage in the 0-20 V range to the drive, you need to set the drive in the 0-5 V voltage range, and use an external circuit to limit the input voltage in the drive. This can be done using a resistor in series with the drive’s analog input, which has a known impedance.