Mealy machine

In the theory of computation, a Mealy machine is a finite-state machine whose output values are determined both by its current state and the current inputs. The outputs change asynchronously with respect to the clock, meaning that the outputs change at unpredictable times, making timing analysis harder. (This is in contrast to a Moore machine, whose output values are determined solely by its current state.) 

History

The Mealy machine is named after George H. Mealy, who presented the concept in a 1955 paper, “A Method for Synthesizing Sequential Circuits”.^[1]

Formal definition

A Mealy machine is a 6-tuple, (S, S₀, Σ, Λ, T, G), consisting of the following:

• a finite set of states (S)
• a start state (also called initial state) S₀ which is an element of (S)
• a finite set called the input alphabet (Σ)
• a finite set called the output alphabet (Λ)
• a transition function (T : S × Σ → S) mapping pairs of a state and an input symbol to the corresponding next state.
• an output function (G : S × Σ → Λ) mapping pairs of a state and an input symbol to the corresponding output symbol.

In some formulations, the transition and output functions are coalesced into a single function (T : S × Σ → S × Λ).

Diagram

The state diagram for a Mealy machine associates an output value with each transition edge (in contrast to the state diagram for a Moore machine, which associates an output value with each state).

Examples

Simple

State diagram for a simple Mealy machine with one input and one output.

Simple Mealy machine has one input and one output. Each transition edge is labeled with the value of the input (shown in red) and the value of the output (shown in blue).  The machine starts in state S_i.  (In this example, the output is the exclusive-or of the two most-recent input values; thus, the machine implements an edge detector, outputting a one every time the input flips and a zero otherwise.) 

Complex

More complex Mealy machines can have multiple inputs as well as multiple outputs.

Applications

Mealy machines provide a rudimentary mathematical model for cipher machines. Considering the input and output alphabet the Latin alphabet, for example, then a Mealy machine can be designed that given a string of letters (a sequence of inputs) can process it into a ciphered string (a sequence of outputs). However, although one could use a Mealy model to describe the Enigma, the state diagram would be too complex to provide feasible means of designing complex ciphering machines.

See also

• Synchronous circuit
• Moore machine
• Algorithmic State Machine

Footnotes

1. ^ Mealy, George H. (September 1955). "A Method for Synthesizing Sequential Circuits". Bell Systems Technical Journal 34: 1045–1079. 

References

• Mealy, George H. (1955). A Method to Synthesizing Sequential Circuits. Bell Systems Technical Journal. pp. 1045–1079. 

• Roth, Charles H., Jr. (2004). Fundamentals of Logic Design. Thomson-Engineering. pp. 364–367. ISBN 0534378048.