Communications network

1. A packet switched communications network comprising:

a controller and a plurality of controlled units arranged in a ring, wherein each of the controlled units comprises a switch at its input, the switch including control means responsive to control instructions contained in packets transmitted by the controller, the control means being arranged to configure the switch so that the switch passes control instructions and data into or out of the unit in a first configuration and the switch bypasses the unit and passes the control instructions and data to a further unit connected to the controller or back to the controller in a second configuration, wherein the controller is arranged to transmit periodically broadcast packets which any unit not previously registered with the controller is able to detect, each broadcast packet including a unique registration address to be adopted by an unregistered unit for registration with and for subsequent control by and communication with the controller.

2. A network as claimed in claim 1 in which each controlled unit includes a watchdog timer which is set by receipt of the broadcast packet.

3. A network as claimed in claim 2 in which the controller transmits at regular intervals a control instruction to cause the watchdog timer to reset.

4. A network as claimed in claim 3 in which if the watchdog timer times out, the unit is arranged to reset so that it reacts to a subsequent broadcast packet to adopt a new registration address.

5. A network as claimed in claim 4 in which each of the controlled units comprises a number of registers to or from which the controller may write or read instructions or data.

6. A network as claimed in claim 3 in which each of the controlled units comprises a number of registers to or from which the controller may write or read instructions or data.

7. A network as claimed in claim 2 in which if the watchdog timer times out, the unit is arranged to reset so that it reacts to a subsequent broadcast packet to adopt a new registration address.

8. A network as claimed in claim 7 in which each of the controlled units comprises a number of registers to or from which the controller may write or read instructions or data.

9. A network as claimed in claim 2 in which each of the controlled units comprises a number of registers to or from which the controller may write or read instructions or data.

10. A network as claimed in claim 1 in which each of the controlled units comprises a number of registers to or from which the controller may write or read instructions or data.

11. A network as claimed in claim 10 in which registers which may be read by the controller contain information entered during manufacture identifying the controlled unit and its properties.

12. A network as claimed in claim 1 in which each controlled unit includes a watchdog timer which is set by receipt of the broadcast packet.

13. A network as claimed in claim 12 in which if the watchdog timer times out, the unit is arranged to reset so that it reacts to a subsequent broadcast packet to adopt a new registration address.

14. A network as claimed in claim 12 in which each of the controlled units comprises a number of registers to or from which the controller may write or read instructions or data.

15. A network as claimed in claim 1 in which each of the controlled units comprises a number of registers to or from which the controller may write or read instructions or data.

16. A packet switched communications network comprising:

a controller and a plurality of controlled units arranged in a ring, wherein each of the controlled units comprises a switch at its input, the switch including control means responsive to control instruction contained in packets transmitted by the controller, the control means being arranged to configured the switch so that the switch passes control instructions and data into or out of the unit in a first configuration and the switch bypasses the unit and passes the controller or back to the controller in a second configuration, wherein the controller is arranged to transmit periodically broadcast packets which any unit not previously registered with the controller is able to detect, each broadcast packet including a unique registration address to be adopted by an unregistered unit for registration with and for subsequent control by and communication with the controller,

wherein an unregistered unit, on detection of the broadcast packet, adopts the unique address and modifies the broadcast packet before passing it on to further units to prevent any subsequent unit from adopting the unique address.

17. A network as claimed in claim 16 in which each controlled unit includes a watchdog timer which is set by receipt of the broadcast packet.

18. A network as claimed in claim 17 in which if the watchdog timer times out, the unit is arranged to reset so that it reacts to a subsequent broadcast packet to adopt a new registration address.

19. A network as claimed in claim 17 in which each of the controlled units comprises a number of registers to or from which the controller may write or read instructions or data.

20. A network as claimed in claim 16 in which each of the controlled units comprises a number of registers to or from which the controller may write or read instructions or data.