Design a sequential circuit to control a phone answering

Chapter 14, Problem 14.44

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Design a sequential circuit to control a phone answering machine. The circuit should have three inputs (R, A, and S) and one output (Z).R = 1 for one clock cycle at the end of each phone ring. A = 1 when the phone is answered. S selects whether the machine should answer the phone after two rings (S = 0) or four rings (S = 1). To cause the tape recorder to answer the phone, the circuit should set the output Z = 1 after the end of the second (S = 0) or fourth (S = 1) ring, and hold Z = 1 until the recorder circuit answers the phone (i.e., when A goes to 1). If a person answers the phone at any point, A will become 1, and the circuit should reset. Assume that S is not changed while the phone is counting rings. Give a Moore state graph for this circuit, using the alphanumeric state graph notation given in Section 14.5.