Abstract: A communication system capable of continuously exchanging digital information in a well logging installation between apparatus arranged on the earth's surface and apparatus arranged in an earth borehole, interconnected by a cable. Both the surface apparatus and the downhole apparatus include a controller and a modem for sending digital command messages downhole and digital data messages uphole. The downhole apparatus also includes one or more tools which are coupled in parallel to the downhole controller via a bi-directional bus. Each tool has a universal (identical) interface adapted to recognize a specific address unique to itself and a universal address common to all interfaces. A command message with a universal address may activate all of the tools in a "free-running" mode to transmit a predetermined quantity of data in a predetermined sequence.