LOGIC GATES

Digital Logic GatesStandard commercially available Digital Logic Gates are available in two basic forms, TTL which stands for Transistor-Transistor Logic such as the 7400 series, and CMOS which stands for Complementary Metal-Oxide-Silicon which is the 4000 series of chips. Generally speaking, this refers to the logic technology used to manufacture the Integrated Circuit, (IC) or "chip" as it is commonly called. Generally speaking, TTL IC's use NPN type Bipolar Junction Transistors while CMOS IC's use Field Effect Transistors or FET's for both their input and output circuitry.

How are they Made?Simple digital logic gates can also be made by connecting together diodes and resistors to produce RTL, Resistor-Transistor Logic circuits but these are now less common.

Integrated Circuits or IC's as they are more commonly called, can be grouped together into families according to the number of transistors or "gates" that they contain. For example, a simple AND gate my contain only a few individual transistors, where as a more complex microprocessor may contain many thousands of individual transistor gates

Digital Logic StatesAll digital electronic circuits and microprocessor based systems contain hardware elements called Digital Logic Gates that perform the logical operations of AND, OR and NOT on binary numbers. In digital logic only two voltage levels or states are allowed and these states are generally referred to as Logic "1" or Logic "0", High or Low, True or False and which are represented in Boolean Algebra and Truth Tables by the numbers "1" and "0" respectively. A good example of a digital logic level is a simple light as it is "ON" or "OFF".

The three basic types of logic gates are:AND gate.OR gate.NOT gate

NOT gatesNOT gates or inverters have a single bit input and a single bit of output.

The truth table defines the behavior of this gate.

Boolean algebraThis is a method of mathematics used to describe logic circuits. Although it looks ordinary algebra its symbols and operations are used differently.A = F

OR GateOR gates have two bits of input and a single bit of output. The subscript, 2, indicates how many inputs this OR gate has. For example, OR3 gates have 3 inputs.

Switch Representation

TRUTH TABLEINPUTSOUTPUTABC000011101111Boolean Expression A + B = C

AND GateThe output of AND2 gate is 1 only if both inputs are 1. Otherwise, the output is 0.

TRUTH TABLEINPUTSOUTPUTABM000010100111