A method performed by a controlling unit in a master-slave system of interconnected units (101, 102), wherein each of said interconnected units (101, 102) comprises at least one port via which said interconnection is realized, and wherein at least a subset of said interconnected units (101, 102) comprises at least one cascading port, said method comprising a system start-up procedure that comprises: - transmitting at least one first request for information to at least one port on the controlling unit, - receiving, from a first unit of said interconnected units (101, 102), a response to said at least one first request, said response comprising information that identifies the first unit, the information further comprising information that identifies at least one cascading port on the first unit, wherein the first unit is connected to the controlling unit via said at least one port on the controlling unit, - transmitting a second request for information to said at least one cascading port on the first unit, - receiving, from a second unit of said interconnected units (101, 102), a response to said second request, said response comprising information that identifies the second unit, the information further comprising information that identifies any cascading port on the second unit, wherein the second unit is connected to the first unit via said at least one cascading port on the first unit, - assigning a respective unique address associated with the first unit and the second unit, - recording the assigned unique addresses together with the information that identifies the first and second units and the information that identifies at least one cascading port on the first and second units, and - transmitting the recorded unique addresses to the first unit and to the second unit. - transmitting at least one first request for information to at least one port on the controlling unit, - receiving, from a first unit of said interconnected units (101, 102), a response to said at least one first request, said response comprising information that identifies the first unit, the information further comprising information that identifies at least one cascading port on the first unit, wherein the first unit is connected to the controlling unit via said at least one port on the controlling unit, - transmitting a second request for information to said at least one cascading port on the first unit, - receiving, from a second unit of said interconnected units (101, 102), a response to said second request, said response comprising information that identifies the second unit, the information further comprising information that identifies any cascading port on the second unit, wherein the second unit is connected to the first unit via said at least one cascading port on the first unit, - assigning a respective unique address associated with the first unit and the second unit, - recording the assigned unique addresses together with the information that identifies the first and second units and the information that identifies at least one cascading port on the first and second units, and - transmitting the recorded unique addresses to the first unit and to the second unit.