Page 1079 - Basic Electrical Engineering

A truth table describes the behaviour of a logic gate. It shows the value of
the output for every possible combination of inputs. The simplest form of
electronic logic for AND and OR gates can be built using diodes. But a NOT

gate cannot be made using diodes. To build a complete logic system a
resistor–transistor logic, or RTL are used. RTL gates can be cascaded

indefinitely to produce complex logic function. RTL gates were used in early
integrated circuits. For achieving higher speed of functioning, diode–

transistor logic or DTL were used. To reduce the space required, transistor–
transistor logic (TTL) was created in the fabrication of ICs. Complementary

metal oxide semiconductor (CMOS) logic has been replacing TTL logic to
reduce size and power consumption. For small-scale logic, TTL 7400 series
and CMOS 4000 series of ICs are used. Gradually, the fixed function logic

gates mentioned earlier are being replaced by programmable logic devices,
which permit a large number of fixed function logic gates packed into an

integrated circuit.

Figure 16.5 Standard symbol and truth table of a NOR gate

16.6 BOOLEAN ALGEBRA

In 1854, George Boole developed a new form of algebra which has come to
be known as Boolean algebra. Boolean algebra is being applied for the

solution of logical problems.
The operation of logic gates in relation to one another may be represented
and analysed using a branch of mathematics called Boolean algebra.

Expressions used in Boolean algebra are called Boolean expressions. In
Boolean algebra only capital letters like A, B, C, D, etc. are used.