the computers power (current) negative and positive power logic and logic gates full tutorial|

Powers positive and Negative logic

the terms positive and negative logic specify the way logic 1 and 0 are defined, in a positive logic, the higher level is denoted as logic 1 and the lower level as logic 0. in negative logic, the higher level is denoted as logic 0 and the lower level as logic 1.negative logic is very rarely followed. in PC, it is used only in the RS232c communication interface.

Simple GATES

The simple gate are AND, OR, NOT, NAND, NOR, and EXCLUSIVE OR (XOR).

SPECIAL GATES

there are some gate which do not perform any logic function. they are used merely for one of the following special functions:

1. wave shaping
2. increasing signal driving capacity
3. Removing noise on a singnal
4. shifting signal levels
5. interfacing between different ICs families 
6. isolating an IC from its loads.
7. establishing bi-directional communication
8. forming bus.

These IC's are called by different names, such as BUFFERS, DRIVERS,LEVEL CONVERTER,LINE DRIVERS, SCHMITT TRIGGERS etc. depenting on the applications.

IC Families

Several types of IC's have been developed over the years. popular IC families are listed below:

1. Resister-transistor logic(RTL)
2. Diode-transistor logic (DTL)
3. Transistor-transistor logic(TTL)
4. Emitter-Coupled logicc(ECL)
5. High-threshold logic(H1L)
6. Metal oxcide semiconductor logic(MOS)
7. Complementary metal oxide semiconductor logic(CMOS)
8. Integrated injection logic(EL)

These IC families differ in their characteristics 

• Speed
• Power dissipation.
• Cost.
• Operating temperature ranges
• Noise margins (noise immunity)
• Packaging density
• Loading and driving limits
• Ease of interfacing 
• Operating supply voltage 
• Ease of handling

Digital logic

 Digital or binary logic has fascinated many people over the years.the very idea that a two-valued number system can possibly be the basic for the most powerful and sophisticated computers seems astounding, to say the least. Neverthless, it is so and the how and the why of this requires some explanation. Everything in the digital world is based on the binary number system. Numerically, this involves only two symbols; 0 and 1. Logically, we can use these symbols or we can equate them with others according to the needs of the moment.thus, when dealing with digital logic, we can specify that;

0 = False = no

1 = true = yes

Using this two-valued logic system, every statement or condition must be either "True" or "false", it cannot be partly true and partly false. while this approach may seem limited, it actually works quite nicely, and can be expanded to express very complex relationships and interactions among any number of individual conditions. one essential reason for basing logical operations on the binary number system is that it is easy to design simple, stable electronic circuits that can switch back and forth between two clearly-defined state unless and until they are deliberately switched to the other state. this makes it possible to construct a machine which can remember sequences of events and adjust its behavior accordingly.

Digital logic may be divided into two classes: combinational logic, in which the logical outputs are determined by the logical function being performed and states of those outputs. both classes of logic are used extensively in all digital computers. since both types of logic circuits begin with logic gates to combine logic input signals in various ways to produce the desired outputs, we will begin on the the next page by seeing how the basic logic gates work.

Basic logical functions and gates

While each logical element or condition must always have a logic value of either "0" or "1" we also need to have ways to combine different logical signals or conditions to provide a logical result. 

For example, consider the logical statement: "if move the switch on the wall up, the light will turn on " At first glance, this seems to be a correct statement. However, if we look at a few other factors, we realize that there's more to it than this. in this example, a more compete statement would be: "if move the switch on the wall up and the light bulb is good and the power is on, the light will turn on".

If we look at these two statements as logical expressions and use logical terminology, we can reduce the first statement to: light = switch

 This means nothing more than that the light will follow the action of the switch, so that when the switch up/on/true/0.

Light = Switch and bulb and power

Normally , we use symbols rather than words to designate the and function that we re using to combine the separate variables of switch,bulb, and power in this expression. the symbol normally used is a dot, which is the same symbol used for multiplication in some mathematical expressions. using this symbol, our three-variable expression becomes:

Light = switch bulb power

When we deal with logical circuits (as in computers), we not only need to deal with logical functions; we also need some special symbols to denote these functions in a logical diagram. there are three fundamental logical operations, from which all other functions, no matter how complex, can be derived. these functions are named and, or, and not. Each of these has a specific symbol and a clearly-defined behavior, as follows;

The AND gate

The AND gate implements the AND function. With the gate shown to the left, both inputs must have logic 1 signals applied to them in order for the output to be a logic 1 with either input at logic 0, the output will be held to logic 0. there is no limit to the number of inputs that may be practical reasons, commercial AND gates are most commonly manufactured with 2,3, or 4 inputs. A standard integrated circuit package contains 14 or 16 pins, for practical size and handling A standard 14-pin package can contain four 2-input gate, three 3-input gate, or two 4-input gates, and still have room for two pins for power supply connections.

And operations

The expression  X= A *B  read as "X equals A AND B"

The  multiplication sign stands for the AND operation, same for ordinary multiplication of 1s and 0s . the AND Operation produces a result of 1 occurs only for the single case when all of the input variables are 1. the output is for any case where one or more inputs are 0.

The or gate

the OR  gate is sort of the reverse of the AND gate. the OR function, like its verbal counterpart, allows the output to be true if any one or more of its inputs are true. verbally , we might say , if it is raining OR if turn on the sprinkler, the lawn will be wet. note that the lawn will still be wet if the sprinkler is on and it is also raining. this is correctly reflected  by the basic OR  function.

 in symbols, the OR  function is designated with a plus sign + . in logical diagrams, the symbol to the left designates the  OR  gate. As with tje AND  function , the OR function can have any number of inputs. however, practical commercial  OR  gates are mostly limited to 2,3, and 4 inputs, as with AND  gates.

OR operation 

The expression X=A+B read as "X equals A OR B".

The + sign stand for the OR operation, not for ordinary addition.

The OR operation produces a result of 1 when any of the input variable is 1.

The OR operation produces a result of 0 only when all the input variables are 0.

The  NOT Gate or Inverter

The inverter is a little different from AND  and OR  gates in that it always has exactly one input as well as one output. whatever logical state is applied to the input, the opposite state will appear at output. the NOT  function, as it is . called, is necessary in many applications and highly useful in others. A practical verbal application might be.the door is NOT locked = you may enter the NOT function is denoted by a horizontal bar over the value to be inverted , as shown in the figure to the in some case a single quote mark(') may also be use for this purpose : 0'= 1 and 1' =0. for greater clarity in some logical expressions, we will use the overbar most of the time 

in the inverter symbol, the triangle actually denotes only an amplifier, which in digital terms means that it "cleans up" pie signal but does not change its logical sense . it is the circle at the output which denotes the logical inversion. the circle could have been placed at  the input instead , and the logical meaning would still be the same  for example, if the variable A is subjected to the  NOT operation, the result x can be expressed as x= A where the prime (')  represent the NOT  operation. this expression is read as:

x equals NOT A 

x equals the inverse of A  Each of these is in common usage and all indicate that the logic value of x= A  is opposite to the logic value of A .
1= 0 because NOT 1is 0
0=1 because NOT 0 is 1
The  NOT operation is also referred to as inversion or complementation, and these terms are used interchangeably. the logic gates shown above are used in various combinations to perform tasks of any level of complexity. some functions are so commonly used that been given symbols of their own and are often packaged so as to provide that specific function directly. On the next page, we'll begin our coverage of these functions.

Derived logical Functions and gates

While the three basic functions AND, OR,  and NOT  are sufficient to accomplish all possible logical functions and operations, some combinations are used so commonly that they have been given names and  AND function symbols of their own, we will discuss three of these on this topic. the first is called NAND,  and consists of an followed by NOT  the third of these derived functions has a specific logic symbol and behavior, which we can summarize as follows:

The NAND gate 

The  NAND  gate implements the NAND  function, which is exactly inverted from the AND  function you already examined.with the gate shown to the left, both inputs must have logic 1 signals applied to them in order for the output to be a logic 0. with either input at logic 0, the output will be held to logic 1.

the circle of the output of the NAND gate denotes the logical inversion, just as it did at the output of the inverter . also in the figure to the left , note that the overbar is a solid bar over both input values at once. this shows that it is the NAND  function itself that is inverted, rather than each separate input. NAND  is the same as the AND  gate symbol except that it has a small circle on the output. this small circle represents the inversion operation . therefore, the output expression of the two input  NAND gate is :
As with AND, there is no limit to the number of input that may be applied to a NAND  function, so there is no functional limit to the number of inputs a NAND  gate may have. however, for practical reasons, commercial  NAND  gates are most  commonly manufactured with 2,3 or 4 inputs to fit in a 14-pin or 16-pin package.

The NOR gate

The NOR gate is an OR gate with the output inverted . where the OR  gate allows the output to be true if any one or more of its inputs are true, the NOR  gate inverts this and forces the output to logic 0 when any input is true. 

in symbols, the NOR  function is designated with a plus sign+ with an overbar  over the entire expression to indicate the inversion. in logical diagrams, the symbol to the left designates the  inversion. NOR is the same as the OR  gate symbol expect that it has a small circle on the output. this small circle represents the inversion operation. therefore the output expression of the two input  NOR  gate  

 The NOR function  have any number of inputs, but practical commercial  NOR 

 gates are mostly limited to 2,3 and  4 inputs, as with other gates in this class, to fit in standard IC  packages.

 The Exclusive-OR , or XOR Gate

The Exclusive-OR, or XOR  function is an interesting and useful variation on the basic OR  function. Verbally, it can be  stated as, either A  or B,   but not both. The XOR  gate produces a logic 1 output only if its two inputs are different. if the inputs are the same, the output is a logic 0.  

The XOR symbol is a variation of the standard OR  symbol. it consists of a plus (+)  sign with a circle around it . the logic  symbol, as shown here , is a variation on the standard OR  symbol. Unlike standard OR/NOR  and  AND/NAND  functions, the XOR  function always has exactly two inputs, and commercially manufactured XOR  gates are the same. four  XOR  gates fit in a standard 14-pm IC  package the three derived functions shown above are by no means the only ones, but these form the basic of all the others. next we will begin our look at practical application for logic gates in various combinations, to see just how these simple gates can be combined to perform every possible operation in a computer. choose the correct answers in the following questions.

• Boolean algebra is different from ordinary algebra in which way?
• Boolean algebra can represent more than 1 discrete level between 9 and 1
• Boolean algebra has only 2 discrete levels:o and 1
• Boolean algebra can describe up to 3 logic levels
• Boolean algebra can describe up to 3 logic levels
• They are actually the same
• NA

Some Advane Tips

If you like this article so please share this post , and if you know to interest about RAM, ROM, and PC so visit my site : https://www.digitaltech.net.in