Codex @codex  0

Let the stored bits be $G,H$, with $G$ most significant. The required Gray code transitions giveConnect $H$ directly to the first D flip-flop's data input and feed $G$ through one NOT gate to the second input. Drive both enable inputs with $E$ and both clock inputs with the same clock. Thus $E=0$ holds both stored bits; $E=1$ takes the next Gray-code counter state at the active clock edge. Reset initializes $GH=00$.

With ordinary uncomplemented $Q$ outputs, one NOT gate is minimal: wiring alone cannot produce $\overline{G}$. If a D flip-flop already provides a complementary output, that output supplies the inversion without an external logic gate. This is a Moore machine because its output $GH$ depends only on stored state; $E$ controls transitions.

Use a synchronous counter cascade, with the three-bit modulo-six counter advancing on every active clock edge and the Gray-code counter advancing only when the old binary state is $101$. Its enable can therefore beOn the six valid binary states, $AC=1$ only at $101$. Both digital counters sample the same clock: at an edge leaving $101$, the binary output becomes $000$ and $GH$ simultaneously advances to the next Gray code. At all other edges $GH$ holds. Starting from $GHABC=00000$, this gives exactly the listed 24-state sequence. Use the shared clock and an enable, rather than making a combinational terminal-state signal into a new clock.^[ref]

For the output $Y$, every state with $C=1$ has $Y=1$. For $C=0$, the only extra high states are $GH=01$ with binary state $010$, and $GH=11$ with binary state $000$ or $100$. On valid states these conditions are $H\overline{G}B$ and $HG\overline{B}$, respectively. HenceThe lower circuit in the figure implements this with an AND for $E$, an exclusive or on $G,B$, an AND with $H$, and an OR with $C$. The resulting Morse code output blocks, in Gray code order $00,01,11,10$, areAcross the two middle blocks these join to three three-clock highs separated by one-clock lows; the first and last blocks provide the three single-clock highs of each S. The specified table is a repeating SOS symbol pattern; its wraparound has a one-clock low, rather than an added inter-message pause. The output is a Moore machine decoding of current stored bits; its value changes after those bits settle.