Abstract: In a return loop control system in a data transmission system having data transmission units such as modems connected at opposite ends of a transmission line, a return loop being formed at one of the data transmission units by a command signal from the other data transmission unit when the transmission line, modulator, demodulator, etc. are to be tested. Where a data to be transmitted is sent out a predetermined time period later than the send-out of a signal for establishing a communication status to the transmission line, a command signal for forming a return loop is sent out within the predetermined time period.

Abstract: A variable equalizer is provided which, using a single variable resistor, can make compensations in both the directions of the gain side, and the loss side and can set a reference gain as desired.An input signal to be equalized is received as an antiphase input signal of a differential amplifier, an output signal which has been equalized is fed back to the antiphase input signal, and a difference signal between an in-phase input signal and the antiphase input signal is delivered as an output. A resistor is connected between an input terminal and ground, first and second impedance circuits and a variable resistor are connected between an intermediate point of the first-mentioned resistor and ground in the order mentioned, and a voltage at the junction point between the first and second impedance circuits is used as the in-phase input signal.

Abstract: To form a simple and economical variable equalizer, the equalizer is constructed of a differential amplifier having two input terminals to which an input signal to be equalized and an equalized output signal derived from the output terminal are applied, a first impedance circuit connected between the differential amplifier and the output terminal, and a series circuit having a second impedance circuit and a variable resistor connected in series between the output terminal and a ground terminal.