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"Sequential circuit" redirects here. For the synthesizer company, see Sequential Circuits. In digital circuit theory, sequential logic is a type of logic circuit whose output depends not only on the present value of its input signals but on the sequence of past inputs, the input history as well. [1] [2] [3] [4] This is in contrast to combinational logic , whose output is a function of only the present input. That is, sequential logic has state ( memory ) while combinational logic does not. Sequential logic is used to construct finite state machines, a basic building block in all digital circuitry. Virtually all circuits in practical digital devices are a mixture of combinational and sequential logic. A familiar example of a device with sequential logic is a television set with "channel up" and "channel down" buttons. [1] Pressing the "up" button gives the television an input telling it to switch to the next channel above the one it

For the concept in quantum mechanics, see observable. In control theory, observability is a measure of how well internal states of a system can be inferred from knowledge of its external outputs. The observability and controllability of a system are mathematical duals. The concept of observability was introduced by Hungarian-American engineer Rudolf E. Kálmán for linear dynamic systems. [1] [2] Contents 1 Definition 2 Continuous time-varying system 2.1 Observability 2.2 Example 3 Nonlinear case 4 Static systems and general topological spaces 5 See also 6 References 7 External links Definition Formally, a system is said to be observable if, for any possible sequence of state and control vectors (the latter being variables whose values one can choose), the current state (the values of the underlying dynamically evolving variables) can be determined in finite time using only the outputs. (This definition uses the state space represe