Abstract: A control system has a central unit for applying DC operating current and control signals to a two line bus. The central unit applies a DC operating current to the bus, as well as signals that have first and second parts. A terminal connected to the bus detects first signal parts on the bus that are unique to that terminal, and, in response thereto, applies the second signal parts to its output terminals. A manual control unit is connected to receive operating current and signals from the output terminals. The manual control unit has a plurality of keys connected to load the bus, and is responsive to determined second signal parts for inhibiting such loading of the bus, thereby enabling the central unit to determine and address of the manual control unit.