Difference Between Mealy and Moore Machine
The output of a Mealy machine depends on both the current state and the current input. A combinational logic function, which takes the current state and input as arguments, determines the output. In contrast, the output of a Moore machine depends solely on the current state. The current state determines the output, regardless of the input.
Table of Content (TOC)
- Tabular Difference Between Mealy and Moore Machine
- What is a Mealy Machine?
- What is a Moore Machine?
- Key Differences Between Mealy and Moore Machine
Tabular Difference Between Mealy and Moore Machine
| Difference | Mealy Machine | Moore Machine |
|---|---|---|
| Output Dependency | Output depends on both the current state and the current input. | Output depends solely on the current state, regardless of the input. |
| Output Association | Output is associated with the transition between states. | Output is associated with the states themselves. |
| Generality | More general and can represent a wider range of behaviors. | Less general than Mealy machines. |
| Implementation | Can be more complex to design and implement in some cases. | Generally simpler to design and implement. |
| Output Changes | Output can change when either the state changes or the input changes (while in the same state). | Output can only change when the state changes. |
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What is a Mealy Machine?
A Mealy machine is a finite state machine (FSM) that produces output based on its current state and input. In lay terms, the output of a Mealy machine depends on the transition between states rather than just the state itself.
The transition and output functions of a Mealy machine can be represented as:
- Transition Function: δ(current_state, input) → next_state
- Output Function: λ(current_state, input) → output
Mealy Machine Example
Consider a Mealy machine with two states (S0 and S1), the input alphabet (0, 1), and the output alphabet (A, B). The state transition table for this Mealy machine is:
| Current State | Input | Next State | Output |
|---|---|---|---|
| S0 | 0 | S0 | A |
| S0 | 1 | S1 | B |
| S1 | 0 | S1 | B |
| S1 | 1 | S0 | A |
In this example, the output depends on the current and input state. For example, when the machine is in state S0 and receives input 1, it transitions to state S1 and produces output B.
What is a Moore Machine?
A Moore machine is another type of finite state machine (FSM) that produces output based solely on its current state. In lay terms, the output of a Moore machine is associated with the states themselves, not the transitions between states.
The transition and output functions of a Moore machine can be represented as:
- Transition Function: δ(current_state, input) → next_state
- Output Function: λ(current_state) → output
Moore Machine Example
Consider a Moore machine with two states (S0 and S1), the input alphabet (0, 1), and the output alphabet (A, B). The state transition table for this Moore machine is:
| Current State | Input | Next State | Output |
|---|---|---|---|
| S0 | 0 | S0 | A |
| S0 | 1 | S1 | A |
| S1 | 0 | S1 | B |
| S1 | 1 | S0 | B |
In this example, the output depends solely on the current state, regardless of the input. For example, when the machine is in state S0, it always produces output A, irrespective of whether the input is 0 or 1.
Key Differences Between Mealy and Moore Machine
Here are the key differences:
- In a Mealy machine, the output is determined by a combinational logic function that takes both the current state and the current input as arguments. In contrast, the output of a Moore machine depends solely on the current state, irrespective of the input.
- In a Mealy machine, the output is associated with the transition between states, meaning that the output is produced during the transition from one state to another. Whereas, in a Moore machine, the output is associated with the states themselves, and it is produced while the machine is in a particular state, not during a transition.
- Mealy machines are more general than Moore machines and can represent a wider range of behaviors. In contrast, Moore machines, being less general, cannot represent some behaviors that Mealy machines can.
- In a Mealy machine, the output can change not only when the state changes but also when the input changes, even if the state remains the same. On the other hand, in a Moore machine, the output can only change when the state changes, and it remains constant as long as the state remains the same, regardless of any changes in the input.
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FAQs
Can a Mealy machine be converted into an equivalent Moore machine, or vice versa?
A Mealy machine can be converted into an equivalent Moore machine, and vice versa. However, in general, the number of states in the Mealy machine explodes when converted into an equivalent Moore machine. The additional states are added to account for the output dependency upon the present state and input (in case of converting the Mealy machine to Moore) or for capturing the output dependency on the transitions (in case of converting Moore machine to Mealy machine).
Which type of finite state machine is more suitable for modeling real-world systems that involve complex input-output relationships?
Mealy machines are more appropriate to model real-world systems with complex input-output relationships. Mealy machines can describe more behavior because their output is based not only on the current state but on the input; they can give rise to complex output sequences. On the other hand, Moore machines have a further limitation of their output capabilities because the output depends on the current state and only that, without any dependence on the input.
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