PURPOSE: To specify a transmission node of a data frame and to recognize the data frame collision from plural nodes by setting a voltage amplitude at mark signal output specifically to each terminal equipment and identifying an output terminal equipment occupying a signal lien at present externally independently of the content of communication on a common signal line.

CONSTITUTION: An npn transistor(TR) 6a is employed at mark signal transmission in a terminal equipment A to bring a potential of an R×ψ line toward a GND level from a potential on an R×1 line. Moreover, a pnp TR 6b is employed to increase a potential on an R×1 line toward a Vcc side from the potential of an R×ψ line. Reception differential comparators 4a, 4b of each terminal equipment can correctly identify a mark signal in any drive form and no disturbance is imposed on a physical protocol itself for the balanced 2-line transmission. Thus, frame collision is detected externally independently of a sequence of, a transmission node through the detection of the voltage amplitude.