Abstract: A device for electrically isolating an external device or internal device from an internal signal bus on a semiconductor substrate is disclosed. The device includes a set of drivers which couple an internal signal bus to an external signal bus and a set of drivers which couple the internal signal bus to an internal device signal bus. A driver enable signal generator generates a signal which enables one set of drivers to electrically couple the internal signal bus to either the external device signal bus or the internal device signal bus. The driver enable signal generator generates the driver enable signal in correspondence with the state of a signal indicating the presence or absence of an external device and a signal indicating whether a data transfer operation is occurring. If the external device is present and a data transfer operation is occurring, the internal signal bus is coupled to the external device signal bus.