Yes, a digital signal can be transmitted on a balanced two-wire network without an analog carrier signal. The technique used by Foundation Fieldbus H1 and PROFIBUS-PA uses Manchester encoding. A trapezoidal wave signal is generated similar to an AC sine wave at a synchronous clock frequency. This signal is modulated by the data stream to shift the clock pulses differently for "one" bits and "zero" bits. For each bit, the signal crosses zero voltage once, but the voltage between the two wires will be + followed by zero followed by - for a value of binary one, and then -, zero, an + for a value of binary zero.