Moore State Machine

Elevator floor indicators and digital lock displays use Moore machines because the output depends only on the current state, not the live input. This makes the output glitch-free and easy to decode in real hardware — every state has a fixed, stable output.

Core Concept

A Moore machine computes its output from the present state only. The output logic block receives only the state register bits — the input X does not feed into it. This separates input-driven state transitions from output generation.

Because the output changes only when the state changes (on a clock edge), Moore outputs are synchronised to the clock. This eliminates combinational glitches. The penalty is one extra state compared to an equivalent Mealy machine — the Moore model needs a dedicated output state, while Mealy produces output on the arc leading to that state.

Moore machines are implemented with D flip-flops (74HC74, 74HC175) plus combinational output gates. A 4-state Moore machine needs 2 flip-flops (74HC74 dual D-FF), while up to 16 states need 4 flip-flops (74HC175). The output gates are typically basic AND/OR/NOT combinations of the state bits.

Boolean Expression

Next state: NS = f(PS, X) (same as Mealy — input affects state transitions). Output: Z = g(PS) (state only — no X). In the state diagram, the output label Z appears inside the state circle alongside the state name. Each state has one fixed output value regardless of which arc brought the machine into that state.

Moore machine: detect input sequence 1011
States: S0(Z=0), S1(Z=0), S2(Z=0), S3(Z=0), S4(Z=1)
S4 is the dedicated output state (one more than Mealy''s 4 states)

State Transition Table:
PS   X=0    X=1    Z
S0   S0     S1     0
S1   S2     S1     0
S2   S0     S3     0
S3   S0     S4     0
S4   S2     S1     1  ← output 1 occurs while IN this state

Trace input: 1 0 1 1
  Start at S0, Z=0
  X=1: go to S1, Z=0
  X=0: go to S2, Z=0
  X=1: go to S3, Z=0
  X=1: go to S4, Z=1  ← sequence detected, Z=1 this clock cycle
  (next input decides whether machine goes to S2 or S1 from S4)

Comparison with Mealy:
  Mealy: 4 states, output on arc S3→S0 when X=1
  Moore: 5 states, output inside state S4
  Moore output is clock-synchronized; Mealy output appears with the input.

Final Answer:
  Moore machine needs 5 states for 1011 detection vs Mealy''s 4 states.
  Output Z=1 in the clock cycle when the machine is in state S4.

Exam Tip: Moore output is inside the state bubble; Mealy output is on the arc. GATE regularly tests this. Another trap: Moore machines need one more state than Mealy for the same sequence — be ready to justify this with the state table. Moore outputs are glitch-free because they change only on clock edges. If an exam question says ''output must be synchronous'' or ''glitch-free'', choose Moore over Mealy.

Key Properties

• Output Z = g(Present State) only — input X does not appear in output logic
• Output is synchronous (clock-edge aligned) — no combinational glitches on output
• One more state typically required compared to equivalent Mealy machine
• State diagram: output label inside the circle, transition label on arcs (input only)
• Implemented with 74HC175 (4-bit state) plus AND/OR output decoding gates
• n flip-flops support up to 2^n states; propagation delay 74HC175 = 14 ns at 5 V
• Preferred for output-to-display or actuator control where glitches cause problems

Quick Revision

• Moore: output depends on present state only; Z = g(PS)
• Output is registered — changes only on clock edges, so it is glitch-free
• One more state than equivalent Mealy machine
• State diagram: output inside the state circle, arc labels are input only
• Both Moore and Mealy share same next-state equation NS = f(PS, X)
• Choose Moore when glitch-free synchronous output is needed
• Choose Mealy when minimum state count or fastest output response is needed
• Exam trap: putting output on the arc in a Moore diagram — that is the Mealy convention, not Moore.