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Photovoltaic (PV) systems can generate less energy than expected owing to inefficiency of the inverter, deterioration of the PV modules or conditions of partial shading and soiling. The monitoring and diagnosis techniques of PV systems are therefore quite important to recognize the causes of the performances reduction and optimize the PV energy production. For a more in-depth diagnostic capability, it would be necessary to know the short-circuit current (Isc) and the open-circuit voltage (Voc) of the PV system. These quantities are indeed useful to recognize non-uniform operating conditions of the PV modules (partial shadings, malfunctions, generic electro-optical mismatch conditions), estimate the PV module junction temperature and the voltage temperature coefficient. However, so far, these quantities cannot be measured by the inverter in operative conditions. With this contribution we propose and test a new electronic interface for a more advanced diagnostics of the PV system to be easily installed between the PV string and the inverter, able to measure the values of Isc and Voc as well as the inverter voltage, current and power working points, avoiding any downtime of the PV system and minimizing the power losses during the diagnosis activity.