Communication bus system with protocol for detecting presence of slave device

An apparatus can be used as a hub station in a communication bus system that contains a host station connected to a host connector of the apparatus and a slave station connected to a slave connector of the appara tus. In the communication bus system operates according to a communication protocol wherein the hub station generates a presence signal on the host connector upon incorporation of the hub station into the system via the host connector. The presence signal is dependent on whether the hub station detects a slave device connected to the slave connector. The apparatus comprises an alternative host port and a hub circuit operatively connected to the host connector, the slave connector, and the alternative host port. The hub circuit is arranged to pass communication between an alternative host processor and the slave device via the alternative host port and the slave connector, generating the presence signal to simulate absence of a connection to the slave connector when the hub station is incorporated into the system at a time when the alternative host processor uses the slave device connected to the slave connector.