following discussion it will be assumed to be a telephone the telephone subscriber line consisting of a tip wire 2 39 and a ring wire 2 34 and the resistor 1 39 form a remote circuit connected to a central office between tip and ring terminals shown in fig2 as nodes tip and ring respectively the final amplifier stages 4 and 5 of the interface device are shown separately as tip buffer 4 39 and ring buffer 5 39 the reference circuits are implemented as current sources 11 39 12 39 and 13 39 these current sources are coupled to the tip and ring buffers 4 39 and 5 39 through electronic switches 20 21 22 and 23 and legs 24 and 25 schematically shown in fig1 as connection and switching circuits 10 nodes c and d allow connection to the buffers 4 39 and 5 39 specifically the switches 20 and 21 connect the tip and ring buffers 4 39 and 5 39 either to the two current sources 11 39 or alternatively to the legs 24 and 25 coupled to the current sources 12 39 and 13 39 the switches 22 and 23 connect the legs 24 and 25 respectively to.