We have designed an interface board between the Medipix2 single photon counting chip and USB (Universal Serial Bus), which is presently the most widespread PC interface. All necessary detector support is integrated into one compact system (80 x 50 x 20 mm3) including the detector bias source (up to 100 V). Power supply is internally derived from the voltage provided by the USB connection, so no external device is required. This solution allows to achieve maximum portability of the measurement setup. One of the most significant advantages is the support of back-side pulse processing. The charge generated by an ionizing particle is measured in the bias circuit and can be used for spectroscopic purposes or for triggering. With the present design the energy resolution of such spectrometry is about 44 keV. The interface board has been designed for connection with present chip-boards carrying one or four Medipix2 chips.