Switching circuit theory

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Switching circuit theory is the mathematical study of the properties of networks of idealized switches. Such networks may be strictly combinational logic, in which their output state is only a function of the present state of their inputs; or may also contain sequential elements, where the present state depends on the present state and past states; in that sense, sequential circuits are said to include "memory" of past states. An important class of sequential circuits are state machines. Switching circuit theory is applicable to the design of telephone systems, computers, and similar systems. Switching circuit theory provided the mathematical foundations and tools for digital system design in almost all areas of modern technology. [1]

Combinational logic type of digital logic which is implemented by boolean circuits

In digital circuit theory, combinational logic is a type of digital logic which is implemented by Boolean circuits, where the output is a pure function of the present input only. This is in contrast to sequential logic, in which the output depends not only on the present input but also on the history of the input. In other words, sequential logic has memory while combinational logic does not.

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. 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.

Digital electronics Electronic circuits that utilize digital signals

Digital electronics or digital (electronic) circuits are electronics that operate on digital signals. In contrast, analog circuits manipulate analog signals whose performance is more subject to manufacturing tolerance, signal attenuation and noise. Digital techniques are helpful because it is a lot easier to get an electronic device to switch into one of a number of known states than to accurately reproduce a continuous range of values.


From 1934 to 1936, NEC engineer Akira Nakashima published a series of papers showing that the two-valued Boolean algebra, which he discovered independently, can describe the operation of switching circuits. [2] [3] [4] [1] His work was later cited and elaborated on in Claude Shannon's seminal 1938 paper "A Symbolic Analysis of Relay and Switching Circuits". [4] The principles of Boolean algebra are applied to switches, providing mathematical tools for analysis and synthesis of any switching system.

NEC Japanese technology corporation

NEC Corporation is a Japanese multinational provider of information technology (IT) services and products, headquartered in Minato, Tokyo, Japan. It provides IT and network solutions to business enterprises, communications services providers and to government agencies, and has also been the biggest PC vendor in Japan since the 1980s. The company was known as the Nippon Electric Company, Limited, before rebranding in 1983 as NEC.

In mathematics and abstract algebra, the two-element Boolean algebra is the Boolean algebra whose underlying setB is the Boolean domain. The elements of the Boolean domain are 1 and 0 by convention, so that B = {0, 1}. Paul Halmos's name for this algebra "2" has some following in the literature, and will be employed here.

Claude Shannon American mathematician and information theorist

Claude Elwood Shannon was an American mathematician, electrical engineer, and cryptographer known as "the father of information theory". Shannon is noted for having founded information theory with a landmark paper, A Mathematical Theory of Communication, that he published in 1948.

Ideal switches are considered as having only two exclusive states, for example, open or closed. In some analysis, the state of a switch can be considered to have no influence on the output of the system and is designated as a "don't care" state. In complex networks it is necessary to also account for the finite switching time of physical switches; where two or more different paths in a network may affect the output, these delays may result in a "logic hazard" or "race condition" where the output state changes due to the different propagation times through the network.

In digital logic, a hazard in a system is an undesirable effect caused by either a deficiency in the system or external influences. Logic hazards are manifestations of a problem in which changes in the input variables do not change the output correctly due to some form of delay caused by logic elements This results in the logic not performing its function properly. The three different most common kinds of hazards are usually referred to as static, dynamic and function hazards.

Race condition type of software defect

A race condition or race hazard is the behavior of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of other uncontrollable events. It becomes a bug when one or more of the possible behaviors is undesirable.

See also

Karnaugh map method to simplify boolean algebra expressions; refinement of Edward Veitchs 1952 Veitch diagram

The Karnaugh map is a method of simplifying Boolean algebra expressions. Maurice Karnaugh introduced it in 1953 as a refinement of Edward Veitch's 1952 Veitch chart, which actually was a rediscovery of Allan Marquand's 1881 logical diagram aka Marquand diagram but with a focus now set on its utility for switching circuits. Veitch charts are therefore also known as Marquand–Veitch diagrams, and Karnaugh maps as Karnaugh–Veitch maps.

In computational complexity theory and circuit complexity, a Boolean circuit is a mathematical model for digital logic circuits. A formal language can be decided by a family of Boolean circuits, one circuit for each possible input length. Boolean circuits are also used as a formal model for combinational logic in digital electronics.


The Muller C-element is a small digital block widely used in design of asynchronous circuits and systems. It has been specified formally in 1955 by David E. Muller and first used in ILLIAC II computer. In terms of the theory of lattices, the C-element is a semimodular distributive circuit, whose operation in time is described by a Hasse diagram. The C-element is closely related to the rendezvous and join elements, where an input is not allowed to change twice in succession. In some cases, when relations between delays are known, the C-element can be realized as a sum-of-product (SOP) circuit ,. Earlier techniques for implementing the C-element include Schmidt trigger, Eccles-Jordan flip-flop and last moving point flip-flop.


  1. 1 2 Radomir S. Stanković, Jaakko Astola (2008), Reprints from the Early Days of Information Sciences: TICSP Series On the Contributions of Akira Nakashima to Switching Theory, TICSP Series #40, Tampere International Center for Signal Processing, Tampere University of Technology
  2. History of Research on Switching Theory in Japan, IEEJ Transactions on Fundamentals and Materials, Vol. 124 (2004) No. 8, pp. 720-726, Institute of Electrical Engineers of Japan
  3. Switching Theory/Relay Circuit Network Theory/Theory of Logical Mathematics, IPSJ Computer Museum, Information Processing Society of Japan
  4. 1 2 Radomir S. Stanković (University of Niš), Jaakko T. Astola (Tampere University of Technology), Mark G. Karpovsky (Boston University), Some Historical Remarks on Switching Theory, 2007, DOI

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