Inter-robot communication based on the conceptual mechanism of “sign-board” in Distributed Robotic Systems (DRS) is discussed. Equipped by each robot, a sign-board can be written only by the robot that carries it, and be read by robots in the neighborhood. Consistent with DRS principles, the sign-board model is not supported by any centralized mechanism, and is considered a natural way of interaction among autonomous robotic units. It is shown that along with message passing, the sign-board model is one of the two important mechanisms for inter-robot communication. Previous research on DRS algorithms employing the sign-board model assume zero signal propagation delay. These algorithms may fail if non-zero propagation delay is taken into account. A simple fix for these algorithms exists if the propagation delay is bounded. Implementation strategies for the conceptual sign-board are also discussed.