A man-machine interface for use with industrial processes is disclosed having the capability of design and configuration of the interrelationship of components forming an overall industrial process. The man-machine interface further provides operator use, including process monitoring and control, as well as alarm annunciation. Most user interaction with the man-machine interface is performed through a color CRT monitor having a touch panel on the surface of the CRT screen. Operator use may be limited to touch panel interaction while configurer and designer use normally further includes use of a keyboard.The man-machine interface utilizes distributed processing and incorporates a high level graphic language. The graphic language facilitates real time graphic display implementation through use of dynamic and static variables. Variable types are dynamically associated with variable values so that variables can undergo type changes without detrimental effect. Video bit bangers and shifters further enhance the versatility and ease of implementing rapid modifications of graphic displays. The man-machine interface further incorporates a new bus structure including a new bus arbitration technique, a unidirectional memory boundary protection mechanism, an improved interrupt system, and an improved watchdog timer circuit.