We normally think of communication as an act that involves people and activities like talking, writing, and reading. In this book, though, we are interested also with communication between computers and between people and computers. Communicating is the act of giving, transmitting, or exchanging information. In this chapter, we discuss how a computer processes data and communicates (transmits information) with its user. Understanding this process is fundamental to understanding how computers work.
Computer Communication
In this lesson, we examine the fundamentals of electronic communication and explore how computer communication differs from human communication.- Understand how a computer transmits and receives information
- Explain the principles of computer language
Early Forms of Communication
Humans communicate primarily through words, both spoken and written. From ancient times until about 150 years ago, messages were either verbal or written in form. Getting a message to a distant recipient was often slow, and sometimes the message (or the messenger) got lost in the process.As time and technology progressed, people developed devices to help them communicate faster over greater distances. Items such as lanterns, mirrors, and flags were used to send messages quickly over an extended visual range.
All "out of earshot" communications have one thing in common: They require some type of "code" to convert human language to a form of information that can be packaged and sent to the remote location. It might be a set of letters in an alphabet, a series of analog pulses over a telephone line, or a sequence of binary numbers in a computer. On the receiving end, this code needs to be converted back to language that people can understand.
Obstacles to effective communications include differences in languages and in how the speaker and listener give meaning to words. Language between people is made up of more than words. Gestures, emphasis, body language, and social concepts have an impact on how we interpret interpersonal communications. Most of these elements have no bearing on human–machine interactions. There are other issues we must understand to be able deal and interact with computers.
Dots and Dashes, Bits and Bytes
Telegraphs and early radio communication used codes for transmissions. The most common, Morse code (named after its creator, Samuel F. B. Morse), is based on assigning a series of pulses to represent each letter of the alphabet. These pulses are sent over a wire in a series. The operator on the receiving end converts the code back into letters and words. Morse code remained in official use for messages at sea almost until the end of the twentieth century—it was officially retired in late 1999.Morse used a code in which any single transmitted value had two possible states: a dot or a dash. By combining the dots and dashes into groups, an operator was able to represent letters and, by stringing them together, words. That form of on–off notation can also be used to provide two numbers, 0 and 1. The value 0 represents no signal, or off, and the value 1 represents a signal, or on, state.
This type of number language is called binary notation because it uses only two digits, usually 0 and 1. It was first used by the ancient Chinese, who used the terms yin (empty) and yang (full) to build complex philosophical models of how the universe works.
Our computers are complex switch boxes that have two states and use a binary scheme. The value of a given switch's state—on or off—represents a value that can be used as a code. Modern computer technology uses terms other than yin and yang, but the same binary mathematics creates virtual worlds inside our modern machines.
The Binary Language of Computers
The binary math terms that follow are fundamental to understanding PC technology.Bits
A bit is the smallest unit of information that is recognized by a computer: a single on or off event.Bytes
A byte is a group of 8 bits. A byte is required to represent one character of information. Pressing one key on a keyboard is equivalent to sending 1 byte of information to the computer's central processing unit (CPU). A byte is the standard unit by which memory is measured in a computer—values are expressed in terms of kilobytes (KB) or megabytes (MB). The table that follows lists units of computer memory and their values.| Memory Unit | Value |
| Bit | Smallest unit of information; shorthand term for binary digit |
| Nibble | 4 bits (half of a byte) |
| Byte | 8 bits (equal to one character) |
| Word | 16 bits on most personal computers (longer words possible on larger computers) |
| Kilobyte (KB) | 1024 bytes |
| Megabyte (MB) | 1,048,576 bytes (approximately 1 million bytes or 1024 KB) |
| Gigabyte (GB) | 1,073,741,824 bytes (approximately 1 billion bytes or 1024 MB) |
The Binary System
The binary system of numbers uses the base of 2 (0 and 1). As described earlier, a bit can exist in only two states, on or off. When bits are represented visually:- 0 equals off.
- 1 equals on.
0 0 0 0 0 0 0 0The binary system is one of several numerical systems that can be used for counting. It is similar to the decimal system, which we use to calculate everyday numbers and values. The prefix dec in the term decimal system comes from the Latin word for 10 and denotes a base of 10, which means the decimal system is based on the 10 numbers 0 through 9. The binary system has a base of 2, the numbers 0 and 1.
Counting in Binary Notation
There are some similarities in counting with binary notation and the decimal system we all learned in grade school. In the decimal system, the rightmost whole number (the number to the left of the decimal point) is the "digits" column. Numbers written there have a value of 0 to 9. The number to the left of the digits column (if present) is valued from 10 to 90—the "10s" column. The factor of each additional row is 10 in the decimal system of notation. To get the total value of a number, we add together all columns in both systems: 111 is the sum of 100 + 10 + 1.NOTE
A factor is an item that is multiplied in a multiplication problem. For example, 2 and 3 are factors in the problem 2 × 3.
Binary notation uses the same system of right-to-left columns of ascending values, but each row has only two (0 or 1) instead of 10 (0–9) possible values.Thus, in the binary system, the first row to the right can be only 0 or 1; the next row to the left can be 2 or 3 (if a number exists in that position). The columns that follow have values of 4, then 8, then 16, and so on, each column doubling the possible value of the one to its right. The factor used in the binary system is 2, and—just as in the decimal system—0 is a number counted in that tally. Examples of bytes of information (eight rows) follow.Byte—Example A
The value of this byte is 0 because all bits are off (0 = off).0 0 0 0 0 0 0 0 8 bits
128 64 32 16 8 4 2 1 # values
Byte—Example B
In this example, two of the bits are turned on (1 = on). The total value of this byte is determined by adding the values associated with the bit positions that are on. This byte represents the number 5 (4 + 1).0 0 0 0 0 1 0 1 8 bits
128 64 32 16 8 4 2 1 # values
Byte—Example C
In this example, two different bits are turned on to represent the number 9 (8 + 1).0 0 0 0 1 0 0 1 8 bits
128 64 32 16 8 4 2 1 # valuesThe mathematically inclined will quickly realize that 255 is the largest value that can be represented by a single byte. (Keep in mind that we start with 0 and go to 255, which corresponds to a possible 256 places on a number line.)
Because computers use binary numbers and humans use decimal numbers, A+ technicians must be able to perform simple conversions. The following table shows decimal numbers and their binary equivalents (0_9). You will need to know this information. The best way to prepare is to learn how to add in binary numbers rather than merely memorizing the values.
| Decimal Number | Binary Equivalent |
| 0 | 0000 |
| 1 | 0001 |
| 2 | 0010 |
| 3 | 0011 |
| 4 | 0100 |
| 5 | 0101 |
| 6 | 0110 |
| 7 | 0111 |
| 8 | 1000 |
| 9 | 1001 |
Parallel and Serial Devices
The telegraph and the individual wires in our PCs are serial devices. This means that only one element of code can be sent at a time. Like a one-lane tunnel, there is only room for one person to pass through at one time. All electronic communications are—at some level—serial, because a single wire can have only two states: on or off.To speed things up, we can add more wires. This allows simultaneous transmission of signals. Or, to continue our analogy, it's like adding another set of tunnels next to the first one: We still have only one person per tunnel, but we can get more people through because they are traveling in parallel. That is the difference between parallel and serial data transmission. In PC technology, we often string eight wires in a parallel set, allowing 8 bits to be sent at once. Figure 2.1 illustrates serial and parallel communication.
Figure 2.1 Serial and parallel communication
ASCII Code
The standard code for handling text characters on most modern computers is called ASCII (American Standard Code for Information Interchange). The basic ASCII standard consists of 128 codes representing the English alphabet, punctuation, and certain control characters. Most systems today recognize 256 codes: the original 128 and an additional 128 codes called the extended character set.Remember that a byte represents one character of information; 4 bytes are needed to represent a string of four characters. The following 4 bytes represent the text string 12AB (using ASCII code):
00110001 00110010 01000001 01000010
1 2 A BThe following illustrates how the binary language spells the word binary:
B I N A R Y
01000010 01001001 01001110 01000001 01010010 01011001
NOTE
It is very important to understand that in computer processing, the "space" is a significant character. All items in a code must be set out for the machine to process. Like any other character, the space has a binary value that must be included in the data stream. In computing, the absence or presence of a space is critical and sometimes causes confusion or frustration among new users. Uppercase and lowercase letters also have different values. Some operating systems (for example, UNIX) distinguish between them for commands, whereas others (for example, MS-DOS) translate the uppercase and lowercase into the same word no matter how it is cased.
The following table is a partial representation of the ASCII character set. Even in present-day computing, laden with multimedia and sophisticated programming, ASCII retains an honored and important position.| Symbol | Binary 1 Byte | Decimal | Symbol | Binary 1 Byte | Decimal |
| 0 | 00110000 | 48 | V | 01010110 | 86 |
| 1 | 00110001 | 49 | W | 01010111 | 87 |
| 2 | 00110010 | 50 | X | 01011000 | 88 |
| 3 | 00110011 | 51 | Y | 01011001 | 89 |
| 4 | 00110100 | 52 | Z | 01011010 | 90 |
| 5 | 00110101 | 53 | a | 01100001 | 97 |
| 6 | 00110110 | 54 | b | 01100010 | 98 |
| 7 | 00110111 | 55 | c | 01100011 | 99 |
| 8 | 00111000 | 56 | d | 01100100 | 100 |
| 9 | 00111001 | 57 | e | 01100101 | 101 |
| A | 01000001 | 65 | f | 01100110 | 102 |
| B | 01000010 | 66 | g | 01100111 | 103 |
| C | 01000011 | 67 | h | 01101000 | 104 |
| D | 01000100 | 68 | i | 01101001 | 105 |
| E | 01000101 | 69 | j | 01101010 | 106 |
| F | 01000110 | 70 | k | 01101011 | 107 |
| G | 01000111 | 71 | l | 01101100 | 108 |
| H | 01001000 | 72 | m | 01101101 | 109 |
| I | 01001001 | 73 | n | 01101110 | 110 |
| J | 01001010 | 74 | o | 01101111 | 111 |
| K | 01001011 | 75 | p | 01110000 | 112 |
| L | 01001100 | 76 | q | 01110001 | 113 |
| M | 01001101 | 77 | r | 01110010 | 114 |
| N | 01001110 | 78 | s | 01110011 | 115 |
| O | 01001111 | 79 | t | 01110100 | 116 |
| P | 01010000 | 80 | u | 01110101 | 117 |
| Q | 01010001 | 81 | v | 01110110 | 118 |
| R | 01010010 | 82 | w | 01110111 | 119 |
| S | 01010011 | 83 | x | 01111000 | 120 |
| T | 01010100 | 84 | y | 01111001 | 121 |
| U | 01010101 | 85 | z | 01111010 | 122 |
NOTE
All letters have a separate ASCII value for uppercase and lowercase. The capital letter "A" is 65, and the lowercase "a" is 97.
Keep in mind that computers are machines, and they do not really "perceive" numbers as anything other than electrical charges setting a switch on or off. Like binary numbers, electrical charges can exist in only two states—positive or negative. Computers interpret the presence of a charge as 1 and the absence of a charge as 0. This technology allows a computer to process information.Lesson Summary
The following points summarize the main elements of this lesson:- Computers communicate using binary language.
- A bit is the smallest unit of information that is recognized by a computer.
- ASCII is the standard code that handles text characters for computers.
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