A wired logic connection is a

logic gate A logic gate is an idealized or physical device implementing a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic ga ...

that implements

boolean algebra (logic) In mathematics and mathematical logic, Boolean algebra is a branch of algebra. It differs from elementary algebra in two ways. First, the values of the variables are the truth values ''true'' and ''false'', usually denoted 1 and 0, whereas in ...

using only passive components such as diodes and resistors. A wired logic connection can create an AND or an

OR gate The OR gate is a digital logic gate that implements logical disjunction. The OR gate returns true if either or both of its inputs are true; otherwise it returns false. The input and output states are normally represented by different voltage lev ...

. The limitations are the inability to create a NOT gate and the lack of level restoration.

The wired AND connection

The wired AND connection is a form of

AND gate The AND gate is a basic digital logic gate that implements logical conjunction (∧) from mathematical logic AND gate behaves according to the truth table. A HIGH output (1) results only if all the inputs to the AND gate are HIGH (1). If not all ...

. It uses a pull up resistor and one diode per input to create this function. In this example diagram 5V is considered logic HIGH (TRUE), and 0V is LOW (FALSE). The voltage on ''output C'' will be HIGH only when ''input sources A'' AND ''B'' are HIGH. If either input is LOW, output C will measure LOW. Usefully, an AND gate can have an arbitrary number of inputs. The behaviour of logic gates is often described by a

truth table A truth table is a mathematical table used in logic—specifically in connection with Boolean algebra (logic), Boolean algebra, boolean functions, and propositional calculus—which sets out the functional values of logical expression (mathematics) ...

:M. Morris Mano, ''Digital Logic and Computer Design'', Prentice-Hall, 1979 , page 571 The LOW state output voltage C will be approx 0.5V, due to the diodes' forward voltage. Output C would be approx 0.1V if transistors replace the diodes in the inputs A,B, perhaps in the form of open collector IC gate outputs. The wired AND function then consists of connecting both inputs A,B to the C output pull-up resistor.

The wired OR connection

The wired OR connection electrically performs the

Boolean logic In mathematics and mathematical logic, Boolean algebra is a branch of algebra. It differs from elementary algebra in two ways. First, the values of the variables are the truth values ''true'' and ''false'', usually denoted 1 and 0, whereas in ...

operation of an OR gate, using a pull down resistor and one diode per input. Voltage from any input is directed through its corresponding diode directly to the output ''C''. If no voltage/logic exists on any input there will be no output. The positive voltage source that is present in the AND connection is replaced by a ground in the OR connection. Also, notice the positioning of the diodes as compared to the AND gate. Similarly, the OR connection is capable of an arbitrary number of inputs and only one output. The output ''C'' is determined by the inputs at ''A'' and ''B'' according to the

and

square wave A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions b ...

(right).

Reversed levels

A wired AND can be converted into a wired OR, and vice versa, by using signals with reversed levels. For example,
#

C = A and B

\overline = \overline

\overline = \overline or \overline{B}

Compatibility of wired AND OR using diodes

With the wired AND or OR using diodes the output voltage will be shifted positive or negative by a diode drop so it may not be compatible with the primary logic family.

References

* Digital Techniques, Heathkit Educational Systems,1990 * Fundamental Physics, K.L Gomber and K.L Gogia,Pradeep Publications, 2005

External links

Introduction to Wired-OR Outputs and Open-Collector Circuits
Logic gates