A protocol for fiber-optic communication systems, or other communication systems based on transmission of unipolar pulses having wide dynamic range provides for information to be transmitted in packets having a predictable time slot for each transmitter. The receiver for such protocol has a first relatively long RC time constant mode conditioned for reception of data packets whose time of arrival is well predictable and a second relatively short RC time constant mode conditioned for reception of asynchronous randomly received packets. In the relatively long RC time constant mode, each packet includes a preamble having a first clamp interval in which no pulse is transmitted, and a second clamp interval in which a continual pulse is transmitted. A transducer on the receiver translates the packets of pulses into differential electronic signals on first and second outputs. First and second coupling capacitors, receive respective outputs of the transducer and AC couples the signals to respective second terminals of the capacitors. First and second switches connect the second terminals of the respective coupling capacitors to ground during the first and second clamping intervals, respectively, of the preamble. By clamping the outputs of the transducer to ground during the first and second clamping intervals, a DC level for each incoming packet is instantly established independent of the magnitude of the incoming packet. In the asynchronous mode, relatively low impedance discharge paths are established to reduce the time constant of the receiver. An output circuit, connected to the second terminal of each of the first and second coupling capacitors, supplies sequences of digital output signals in response to the differential signals.