Insertion and removal of a circuit device into a bus network

Abstract

A circuit device (e.g. mounted on a printed circuit board) is connected to a data network, to which network are also connected a plurality of other circuit devices under power. On removal of such a circuit device, the output driver connecting the circuit components of the output device to the communications bus of the bus network is first disconnected from power, and only when its disconnection is stable is the device withdrawn. A control device of the circuit device is connected to the control bus of the data network, and before power is disconnected from the output driver, the control device signals to the network so as to prevent other circuit devices connected to the network from signalling on the network. Effectively, the circuit device to be removed 'occupies' the bus. This occupation is maintained until the disconnection of the output driver is stable, and then the circuit device can be removed from the network. Similarly, on insertion, the network is first occupied before power is connected to the output driver. In this way, it becomes possible to prevent faults due to missignalling between the other circuit devices connected to the network when one circuit device is inserted or removed.