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

摘要

A plurality of serially addressable devices connected to transmit data to or from a central controller via a two-wire polling loop has a global poll capability. The transmissions of data are via a tri- level line signal to provide both power and data transmission on a two- wire line. Global polling is initiated by a preamble recognized by each device in the loop. The preamble is followed by a single bit sequence to allow the devices to signal the central controller that one or more devices will respond to the global poll. If one or more devices will respond to the global poll, a series of bit sequences is generated by the central controller equal to the number of bits that will be transmitted by the serially addressable device to transmit its address and status. In order that only a single device respond to a global poll, if two or more devices are in contention for control of the loop to transmit their message to the central controller, each device measures the voltage on the loop and drops out of contention if it measures a digital zero while it is trying to transmit a digital one in the address field.