Abstract: A method and apparatus are disclosed for closed loop testing of first and second modulators and demodulators from the modulated signal side of the first modulator and first demodulator. During ordinary communication, the first modulator and second demodulator are connected in series and the first demodulator and the second modulator are connected in series. In response to each test initiate signal, a loop is established between the second modulator and the second demodulator on their modulated signal side. Upon reception of some test initiate signals, a loop is established between the first modulator and first demodulator on their demodulated side and they are disconnected from the second demodulator and second modulator.

Abstract: A data communication system is described in which dial access control signals such as RING, DATA SET READY, DATA TERMINAL READY, CARRIER DETECT, and OUT OF SERVICE are transmitted between a central processing unit (CPU) and remote terminals in the form of frequency-modulated signals. A special frequency signal is transmitted from a remote terminal whenever a RING or DATA SET READY control signal is present and a CARRIER DETECT signal is not. Preferably, this special frequency is midway between the center frequency used for data communication and the frequency of either the MARK or the SPACE signal. Upon reception, this special frequency signal is processed in a particular fashion to generate signals comparable to the RING or DATA SET READY signals of the prior art. DATA TERMINAL READY and CARRIER DETECT control signals are transmitted as carrier signals and the OUT OF SERVICE signal is transmitted as a center frequency signal.