Abstract: A device for protecting a modem against harmful telephone line signals or reversed polarity telephone line signals includes a telephone line interface couplable to a telephone line, a modem interface couplable to a modem communication port, a measurement circuit that measures the value of an electrical parameter of a signal on the telephone line, an indicator circuit that provides an indication if the value of the parameter is not within predetermined limits, an isolation circuit that electrically isolates the modem communication port from the telephone line if the value of the parameter is not within predetermined limits, and a polarity detection circuit that determines the polarity of the telephone line with respect to the polarity of the modem communication port and reverses the polarities with respect to one another if they are opposite. Such operation of the apparatus may be initiated automatically in response to detection of an off-hook condition of the modem.

Abstract: In a switchable impedance tester for measuring very small impedances, the effect of switch impedances is eliminated by connecting the test load between two switching transistors in their emitter-collector circuit, and driving them with operational amplifiers arranged to urge their inputs into equality. A test voltage source is connected across first inputs of the amplifiers, and the other inputs are connected between the respective transistors and the load. This produces an operational condition in which the voltage across the load is independent of the parameters of the switching transistors, i.e. the switching transistors appear to have zero impedance.