DC microgrids has been proposed to adopt more renewable energy sources in future smart electric power distribution networks. DC bus signalling is one of the secondary layer control methods that has been widely used in DC microgrids when the communication infrastructure is absent or under failure. It has advantages in regulating specific DC microgrids with countable working states, such as household applications. This method has achieved fast development over the past decades and there is still no existing literature review that classifies and compares those methods. Driven by this idea, this paper provides a review based on reported DC bus signalling methods in the IEEE Xplore library.