Patent ID: 7561683

Claim:
A method for ascertaining a resistance value between a first contact and a second contact in a subscriber line interface circuit, where a protective circuit for protecting the subscriber line interface circuit against overvoltages is provided between the two contacts and comprises a parallel circuit including a protective capacitor connected in parallel with two resistors connected in series via a node, the node being connected to a third contact in the subscriber line interface circuit, where the method has the following steps: a) a predetermined charging voltage is applied to the protective capacitor; b) a threshold voltage is calculated on the basis of the resistance values of the two resistors and the applied charging voltage; c) a measured voltage tapped off across one of the two resistors is measured while the protective capacitor is discharging; d) the measured voltage is compared with the calculated threshold voltage; e) a period between the start of the discharging of the protective capacitor and the time at which the measured voltage is the same as the threshold voltage is ascertained; and f) the resistance value is calculated using the ascertained period and the resistance values of the two resistors.