State problems in programming human-controlled devices

Many consumer goods are complicated enough to benefit from programmed control. Today's home electronics devices support a wide range of options and controls. At the same time, personal digital assistants and programmable remote controls are now capable of learning and generating control sequences to control a wide range of devices. Unfortunately, most device interfaces are designed for interactive human control rather than programmed control. This paper analyzes state-based obstacles to programming devices designed for interactive human control. It develops a theory of statelock, a condition in which a control program is unable to synchronize with the state machine underlying the controlled device. The paper also presents design strategies to avoid statelock and applies these strategies to the home audio/video and telephone autodialer domains.