Run control in MODEL: the state manager

The MODEL software is a set of modules for online applications, running principally on VAX-family computers. It provides data flow, human interface, process control, and error-reporting facilities. Recently, facilities have been developed to tackle the complex problem of controlling the various activities that constitute the data-acquisition system of a large physics experiment. The approach adopted is based on a state manager. The physicist describes the experiment in terms of objects, i.e. logical subsystems, for each of which a number of states are defined. Commands can be sent to these objects, causing them to perform actions and to change state. The complete description of all objects, states, and actions, in a simple language, is used to generate a state manager for the experiment, which runs as a VMS process. The concepts embodied in the state manager, its generation, its interactions with other processes, and the system configuration it supports are examined