Abstract: A loop-back device is particularly designed to fit line terminals in half-duplex optical transmission systems thereby facilitating fault location in these transmission systems. An optical fiber coupler having a high crosstalk generates an echo in a reception channel of the terminal, from a test signal transmitted in a transmission channel of the terminal, to an optical transmission medium. Means are provided for cancelling transmission of the echo in the reception channel when the loop-back is de-activated, i.e. when a data signal is carried in the transmission medium, and for amplifying and regenerating said echo when the loop-back is activated, i.e. when said test signal is transmitted. A memory is used for delaying the echo and for transmitting it to a receiving circuit in the terminal during a signal reception period of the half-duplex transmission.

Abstract: Supply circuit for an eddy current probe with two windings, comprising two parallel supply channels connected to the probe by a cable, each channel incorporating a reference winding and an amplification circuit having an input and an output, one of the ends of the reference winding being connected to the output of the amplification circuit and the other to the input of said circuit via a negative feedback resistor, wherein the amplification circuit of each channel comprises a first amplifier having an input constituting an input of the amplification circuit and an output connected to an absolute measurement access, a first resistor, whereof one end is connected to the output of the first amplifier, a second amplifier having an input connected to the other end of the first resistor and an output constituting the output of the amplification circuit, and a second resistor connected between the output and the input of the second amplifier.