Method and apparatus for the detection of the presence of a device on a high speed digital data bus

Abstract

An interface comprises a management entity including a first state machine, a bidirectional serial data bus and a clock line, intended for connection to physical layer devices each of which includes at least one register into which data can be written by way of the serial data bus and at least one register from which data can be read by way of the serial data bus, the management entity providing a clock signal having transitions defining bit periods for said data and further comprising a line driver for the serial data bus and an amplifier which is connected to sense the voltage level of the data bus. The management entity is organised to detect during a specified period while the line driver is quiescent whether the physical layer device is present or not according as the voltage of the bus approaches that of a voltage supply rail in the physical layer device or that of a datum for the management entity. The management entity is arranged to cause the line driver to drive the bus towards the voltage of the supply rail in an interval between the penultimate clock transition before the beginning of said time period and the clock transition denoting the beginning of said time period. The said interval may occur during a reading cycle of the management entity and, in accordance with IEEE Standard 802.3 Clause 22, the reading cycle may be defined to include an address word identifying a register in a physical layer device and a turn around time before the physical layer device provides data and wherein said interval occurs after the provision of a final bit in said address word when the final bit is such as to drive the bus toward the datum.