Method and apparatus for global polling having contention-based address identification

In a loop having a plurality of serially addressable devices each having a unique address and being coupled to each other by a pair of wires, in which data is transferred from or to each device by means of a multi-voltage level line signal generated by a central controller, apparatus for global polling said devices comprising:

• means in said central controller for transmitting a preamble signal for instituting global polling;
• means in said central controller for transmitting a plurality of bit intervals corresponding to a number of bits in an address for each device plus a number of bits in a message;
• self-identifying means in each device in contention for responding to said central controller for identifying each device by said unique address, wherein any of said devices in said loop can respond to the poll and wherein said central controller does not know the addresses of the devices that will respond to the poll prior to said identification of each device by its unique address, said self-identifying means comprising means for providing a low impedance across output terminals of said device when an address bit is a first logic state and means for providing a high impedance across said output terminals when an address bit is a second logic state;
• means for monitoring the voltage level on said loop by each device in contention;
• means in said self-identifying means for terminating output of said device address if, when said device provides said second logic state at said output terminals, said monitoring means indicates a voltage level corresponding to said first logic state on said loop.

In a serially addressable device having a unique address for being coupled to other such devices by a two-wire loop in which data is transferred to or from each device by means of a multi-voltage level line signal generated by a central controller for said loop, means for global polling comprising:

• starting means responsive to a preamble signal for instituting global polling address means responsive to said starting means and a series of bit intervals generated by said central controller for generating a plurality of bits representing the unique address of said device, where any of said devices in said loop can respond to the poll and where said central controller does not know the addresses of the devices that will respond to the poll prior to the generation of said unique address, said address means comprising driver means for providing a low impedance across output terminals of said device when one of said address bits is a first logic state and for providing a high impedance across said output terminals when said address bit is a second logic state;
• monitoring means coupled to said loop for monitoring the voltage level on said loop during each bit sequences;
• stopping means responsive to said monitoring means for ceasing generation of address bits by said driver means if said driver means provides said second logic state at said output terminals and said monitoring means indicates a voltage level on said loop corresponding to said first voltage level.