Cascade control system for network units

Cascade control logic for use in a switch or other network unit that can be used in a cascaded stack can maintain normally a point-to-point half-duplex connection for control data with each of the next preceding and next succeeding units in the cascade. Each cascade logic device is organised so that for one direction, conveniently called the up direction, a device is a master and in the other direction the device is a slave in respect of the control path. A control device will generate master control frames in the up direction and deliver slave control frames in the down direction. The control device is organised so that in the absence of reception of valid control frames on a control link control data which would otherwise be sent out on that link is looped back within the control device. In this manner the control device can maintain under normal circumstances two virtual control channels which can 'self-heal' notwithstanding the failure or powering-down of a unit in the cascade. Status information represented by the control frames can be used to control a switching engine to provide self healing of the data path in the cascade.