Abstract: The invention relates to a high speed central office scanner for use with a plurality of transponding units connected by telephone lines. The scanner is comprised of N, I/O units, each comprised of a multiplexer for connecting the I/O units to M telephone lines; a microprocessor and multiplex logic which sequentially selects one of the M telephone lines; a busy detect circuit for checking the selected telephone line and providing a signal when the line is busy; a TX/RX unit connected to the selected line for transmitting an instruction code to a transponder connected thereto when the signal is absent and for receiving data from the transponder in response to the instruction code; and a storage unit. The scanner further includes a master microprocessing unit, which includes a main microprocessor for sequentially selecting one of the N, I/O units and for generating an interrogating code for transmission to each selected I/O unit.