An improved state diagram

The conventional state diagram for a synchronous sequential circuit places the circuit's output labels on arrows representing state transitions. This notation leads students to think that the labels denote output values during, or associated with, changes of state; in fact, the output is not defined when the state is changing. The misconception fostered by this notation creates difficulties for students when they encounter real circuits. A state diagram is proposed which clarifies signal timing by associating outputs with states, and not with state transitions. The proposed diagram provides a number of additional benefits. It includes markedly fewer transition labels, enabling one to design larger systems before the diagram becomes too cluttered to handle. The construction of next state and output maps is simplified. The proposed diagram readily admits register-transfer notation, and thus serves as the initial step in the design of a wired controller. The authors have employed the proposed diagram successfully in elementary and advanced classes in logical design.