Abstract: A stored program digital signal processor (e.g., Digital Equipment Corporation PDP 11/40) is used to collect billing data (e.g., Initial Entry data for Automatic Message Accounting) from a common control circuit (e.g., marker) of a common control automatic telephone switching system (e.g., the Bell System No. 5 Crossbar system). Ten marker control status indicating leads are scanned by the signal processor fast enough to detect any change in the signals on the 10 status leads which, in signal content as a group, represent the control status of the marker at any given time. When two successive prescribed control status changes occur, a large number of marker data leads are scanned by the signal processor so as to allow collection of that data and the storage thereof in a marker core buffer area of the signal processor.