In an apparatus for coating substrates, having sputtering cathodes (4, 5) disposed in a vacuum chamber (1), sputtering targets (6, 7), a medium-frequency generator (9) connected to the cathodes (4, 5), and a system (16) for detecting and suppressing undesired arcing, a cycle of the medium-frequency signal of the medium-frequency generator (9) is divided into a plurality of time segments, the electrical values of current and voltage for a predetermined time segment being determined so as to form a measured value signal and being entered into a ground-free meter island (16). The meter island (16) is tied as a remote station into a circular network (9, 16, 17, 18, 19, 11) whose master station is situated in the control unit (11) present in the generator (9). The blocking of the generator (9) when an arc occurs takes place through a line (19) connecting the meter island (16) to the generator (9). The parameters of the arc surveillance and the detection of measured values are preset through the network (17, 18, 19) by means of software.