four-wire line and connects a loop-back amplifier between the receive pair and the transmit pair. On the second count output, the switching circuitry disconnects the loop-back amplifier and provides quiet termination for each of the receive and transmit pairs. On the third count output, the loop-back amplifier is briefly reconnected between the receive pair and the transmit pair to provide a brief tone pulse, and thereafter the loop-back amplifier is disconnected and the equipment at the second location is reconnected to the four-wire line. At this time, the counter is reset to be ready for the next cycle of switching and testing.
In the second preferred embodiment of the invention disclosed in our prior patent, the counter counts four detection signals, and since this count results in both stagesof the counter being in their initial states, there is no need to provide a reset for the counter. On the first count output, the switching circuitry disconnects the equipment at the second location from the four-wire line and connects a loop-back amplifier between the receive pair and the transmit pair. On the second count output, the switching circuitry disconnects the loop-back amplifier and provides quiet terminations for each of the receive and transmit pairs. On the third count output, a milliwatt test signal source is connected to the transmit pair. On the fourth count output, the milliwatt test signal source is disconnected and the connection of the equipment at the second location to the four-wire line is re-established.
In the case where the second location includes a plurality