• A sequence detector is a sequential state machine. In a Mealy machine, output depends on the present state and the external input (x). Hence in the diagram, the output is written outside the states, along with inputs. The state diagram of a Mealy machine for a 1010 detector is:

• The state table for the above diagram:

• State assignments: Let $S_0$ = 00

$S_1$ = 01

$S_2$ = 10

$S_3$ = 11

The above state table becomes:

• Four states will require two flip flops. Consider two D flip flops. Their excitation table is shown below.

• Excitation table:

• K-maps to determine inputs to D Flip flop:

• Circuit diagram for the sequence detector:

for present state 11 ('S3') input X=1 changes it to next state 01 ('S1'). So there should be modification in excitation table in the last row in next state column.