# Switchgear (technical informatics)

A switching mechanism ( English sequential circuit ) processes various input values ( Boolean variables ) according to the specified switching matrix into an output value. In contrast to switching networks , in which by definition there is no feedback , at least one of the outputs in a switching mechanism is fed back to at least one of the inputs, so that the circuit has a storing character (a memory ).

A switching mechanism is called "synchronous" if the inputs and feedback are synchronized by clock signals , otherwise it is called "asynchronous".

## Basics

Asynchronous rear derailleur

### Asynchronous rear derailleur

There are many methods of linking input values ​​according to a specification to an output value . Linking through a switching network is a forgetful process, since at any given point in time the output value only ever depends on the input value applied at that point in time. However, in order to solve tasks that are not only dependent on a snapshot, one needs circuits with memory , i.e. a circuit that links input values ​​at a certain point in time with values ​​created before this point in time.

The asynchronous switching mechanism is the basic form of these circuits. It represents the circuitry implementation of a Boolean machine and, as shown in the picture, can be viewed as an interconnection of two switching networks that implement the Boolean functions f and g . The function f takes the input values x and the state values u that exist at a point in time t and links them to a new state vector u d . The vector u represents the time-delayed thereby, that in the memory stored previous result of the function f . The time delay ( English delay ) is separate from the switching power f represented by the gray box. At the same time, at time t, the values x and u are linked by function g to form the output vector y . The gray box is replaced by flip-flops in synchronous switchgear .

Asynchronous and synchronous rear derailleur

### Synchronous rear derailleur

In reality, every switching network only generates its result after a certain time, which depends on its structure. In order to link input values ​​present at a certain point in time with the correct, temporarily stored values, the input signals and the fed-back outputs are often synchronized with the aid of flip-flops and a clock signal. This signal pulsates at a fixed frequency . A unit of time has always passed after the same sequence occurs regularly in the signal. The intervals between these sequences are chosen so large that in the meantime all switching networks in the switchgear can complete their calculations, ie all the gate runtimes involved have elapsed, possibly one after the other (namely if intermediate results are relevant). Then, for example, result y is present at the output at time 3 , which was obtained by combining the values x and u that were applied to g at time 2 . From the same values x and u at time 2, the new value u d at f is also set at time 3, but retained by flip-flops . The value u at time 2 was obtained by clocking the flip-flops from the value u d at time 1, which in turn resulted from the values x and u that were applied to f at time 0 .

## realization

Switching mechanisms are implemented by logic circuits that are built with individual transistors or manufactured on wafers . In the early days of computer technology , electron tubes were used instead of transistors . In digital technology , synchronous switching mechanisms are often implemented with sliding registers . Such a shift register is in itself a synchronous switching mechanism. In circuits with processors, switching mechanisms are often implemented as programs and the memory of the preceding inputs is designed as a ring structure .

## Applications

Almost every device for electronic data processing ( computer ) uses switching mechanisms in its chips . Switching mechanisms are also important in data transmission , for example every serial interface is based on a switching mechanism. In particular, the receiving end must collect all the individual bits one after the other so that a complete byte is available after eight cycles .

## literature

• Hans Liebig, Stefan Thome: Logical design of digital systems. 3rd edition, Springer, Heidelberg 1996, ISBN 3-540-61062-6 .
• Wolfram Schiffmann, Robert Schmitz: Technical computer science 1. Basics of digital electronics. 5th edition, Springer, Berlin 2003, ISBN 3-540-40418-X .
• Heinz-Dietrich Wuttke, Karsten Henke: Switching systems - a machine-oriented introduction. Pearson Studium, Munich 2003, ISBN 3-8273-7035-3 .
• Clemens Hackl: Switching mechanism and automaton theory , de Gruyter, Berlin, Vol. I: 1972, ISBN 3-11-003948-6 ; Vol. II: 1973, ISBN 3-11-004213-4 .