1.  Field of the Invention
The invention relates to a circuitry arrangement for controlling a plurality of consumers, such as lamps for example.  In particular with the employment of gas discharge lamps, each of these lamps requires its own ballast in order to deliver the respectively necessary ignition or operating voltage to the gas discharge lamp.  With the employment of many such lamps there is the possibility of centrally controlling the respective ballasts from a central unit via a data line or bus. 
2.  Description of the Related Art
From GB-A-2 072 467, there is known a circuitry arrangement in which a central unit is connected with a plurality of peripheral units via a data bus, the functions of the peripheral units being controlled via the data bus.  Each peripheral unit has a coupling unit which is divided into a receiving branch and a transmission branch.  The receiving branch serves for the transfer of information from the central unit to the peripheral unit, the central unit applying information to the data bus in the form of a modulated a.c.  or pulse voltage.  The transmission branch serves for the transfer of information from the peripheral unit to the central unit.  Here, information is applied from the peripheral unit to the data bus by means of the variation of the output resistance of the transmission branch. 
Further, from EP-A-0 490 329, there is known a circuitry arrangement for controlling the brightness and the operational behaviour of gas discharge lamps.  Here, a central unit controls a plurality of ballasts for gas discharge lamps via a bus.  Each ballast is connected with the bus via a coupling unit.  Data is transferred from the bus to the respective ballast concerned via a capacitance circuit or by means of a transformer.  If return signals are to be passed from the ballast to the central unit it is possible, when no data is present on the bus, to couple these signals from the ballast into the bus via the same coupling unit. 
Both known circuitry arrangements offer the possibility of bringing a peripheral unit into a quiescent state, in which it has only a reduced current, by means of corresponding control signals, but the receiving branch of the coupling unit remains fully activated even in the quiescent state, so that the bus can be subject to load even when no signals are present.  In particular when a large number of peripheral units are employed, this leads to a significant current consumption in the central unit. 
Further, in both circuitry arrangements, it is disadvantageous that the coupling unit can easily be damaged by an overvoltage on the bus.  This can happen, for example, if the circuitry arrangement is laid in the context of normal building installations, for example lighting arrangements.  An unintended connection of the bus with a mains voltage line--220 volts or 380 volts--would immediately damage the coupling unit of the circuitry arrangement, whereby the entire arrangement would be made inoperable.