Abstract: Apparatus for detecting and recording status changes of two-state devices in a power network substation, a pipeline substation, or the like. At a substation a remote terminal unit records status changes in its digital memory. When a sequence of status changes occurs, each change is recorded, together with its time of occurrence as measured from the time of occurrence of the first change, thereby reducing the size of memory for recording sequence data. Upon request, data related to a sequence of changes is transmitted to a master station in chronological order of the changes, to simplify processing of such data at the master station. Provision is made to permit the master station to convert elapsed time to real time, and to reduce the time required of the master station to retrieve sequence data from a remote terminal.