Abstract: A downhole system (10) comprises a tool (16) suspended on a reelable support (18). The support (18) has an optical fibre and the tool communicates with a surface controller via the fibre. The tool (16) has a number of units (24,16a,16b,16c), the unit (24) functioning as a router to transmit and receive signals to and from the other units (16a,16b,16c) and the surface controller via the optical fibre. Each of the units (24,16a,16b,16c) is assigned a unique address which permits secure communication between each unit (24,16a,16b,16c) and the controller over the fibre.