Data Representation and Communications

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4 DATA REPRESENTATION AND COMMUNICATIONS

Learning Objectives

After you finish this chapter, you should be able to do the following: 1. Explain data and how it is represented. 2. Explain computer coding systems. 3. Define a parity bit and what it is used for. 4. Explain data storage concepts. 5. Describe three storage access methods. 6. Describe networks and data communications. 4.1 INTRODUCTION One of the major problems we face in using a digital computer is communicating with it. We must have one or more ways of getting data into the computer to be processed. You learned in chapter 2 that there are several types of input devices that read data into a computer. But how does one prepare the data to be used as input? How do we convert human-readable documents into a computer-readable form, and what type of input media do we use? If the data is to be used by another computer some distance away, how do we transmit it? Well, as you probably suspect, there are several ways to perform this conversion and transmission process, and that is the chapter of our discussion.

4.2 DATA Data is a general term used to describe raw facts. To put it simply, data is nothing more than a collection of related elements or items, that when properly coded into some type of input medium, can be processed by a computer. Data items might include your service number, your name, your paygrade, or any other fact. Until some meaning has been given to the data, nothing can really be determined about it; therefore, it remains data. When this data has been processed together with other facts, it then has meaning and it becomes information we can understand and properly use.

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4.2.1 Data Representation Data is represented by symbols. Symbols convey meaning only when understood. The symbol itself is not the information, but merely a representation of it. Symbol meaning is one of convention (fig. 4-1). Symbols may convey one meaning to you and me, another meaning to others, and no meaning at all to those that do not know their significance. Data must be reduced to a set of symbols that the computer can read and interpret before there can be any communication with the computer. The first computers were designed to manipulate numbers to solve arithmetic problems. But as you can see in figure 4-1, we create, use, and manipulate many other symbols to represent facts in the world in which we live. We are fortunate that early computer experts soon realized the need to manipulate nonnumerical symbols as well. Manipulating these symbols is possible if an identifying code or coded number is assigned to the symbol to be stored and processed.

Thus, the letters in a name such as ALBERT or CAROL can be represented by different codes, as can all special characters, such as #, (,), &, $, @, and yes, even the comma. The data to be represented is called source data. Figure 4-1 Communications symbols NEETS MODULE 22-Digital Computing

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4.2.2 Source Data Source data or raw data is typically written on some type of paper document, which we refer to as a source document. The data contained on the source document must be converted into a machine-readable form for processing either by direct or indirect means. The data may be entered directly into the computer in its original form; namely right from the source document on which it is recorded by way of magnetic ink characters, optically recognizable characters, or bar code recognition. Or the data on the documents may be entered indirectly on input media, such as punched cards, paper tape, magnetic tape, or magnetic disk. It may also be keyed directly into a computer from a keyboard.

If you look at figure 4-2, you see a list of SERVMART items that have been typed on a preprinted form. To most people this is just another piece of paper; however, to the Storekeeper (SK) it is a source document to be used to provide input data to the computer. In this example, the SERVMART form deals with requisitioning supplies. The form could be sent to the data-entry department to be used as a source document. There the data-entry operator can key the data into or on whatever computer medium is to be used, according to a prescribed format. The data elements are numbered in the order they are to be keyed: (1) document identification, (2) stock number, (3) unit of issue, (4) quantity, and so on. You'll notice we need more than numbers, and that is where coding systems come into play.

Figure 4-2 SERVMART shopping list (source document) NEETS MODULE 22-Digital Computing

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Q-1. What is a general term used to describe raw facts? Q-2. How is data represented? Q-3. What were the first computers designed to manipulate in order to solve arithmetic problems? Q-4. By what two means can the data contained on a source document be converted into a machine readable form for processing? Q-5. What are some of the types of input media on which data may be indirectly entered? 4.3 COMPUTER CODING SYSTEMS To represent numeric, alphabetic, and special characters in a computer's internal storage and on magnetic media, we must use some sort of coding system. In computers, the code is made up of fixed size groups of binary positions. Each binary position in a group is assigned a specific value; for example 8, 4, 2, or 1. In this way, every character can be represented by a combination of bits that is different from any other combination.

In this section you will learn how the selected coding systems are used to represent data. The coding systems included are Extended Binary Coded Decimal Interchange Code (EBCDIC), and American Standard Code for Information Interchange (ASCII). 4.3.1 Extended Binary Coded Decimal Interchange Code (EBCDIC) Using an 8-bit code, it is possible to represent 256 different characters or bit combinations. This provides a unique code for each decimal value 0 through 9 (for a total of 10), each uppercase and lowercase letter (for a total of 52), and for a variety of special characters. In addition to four numeric bits, four zone bit positions are used in 8-bit code as illustrated in figure 4-3. Each group of the eight bits makes up one alphabetic, numeric, or special character and is called a byte.

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When you look at figure 4-3, you will notice that the four rightmost bits in EBCDIC are assigned values of 8, 4, 2, and 1. The next four bits to the left are called the zone bits. The EBCDIC coding chart for uppercase and lowercase alphabetic characters and for the numeric digits 0 through 9 is shown in figure 4-4, with their hexadecimal equivalents. Hexadecimal is a number system used with some computer systems. It has a base of 16 (0-9 and A-F). A represents 10; B represents 11; C represents 12; D represents 13; E represents 14; and F represents 15. In EBCDIC, the bit pattern 1100 is the zone combination used for the alphabetic characters A through I, 1101 is used for the characters J through R, and 1110 is the zone combination used for characters S through Z. The bit pattern 1111 is the zone combination used when representing decimal digits.

For example, the code 11000001 is equivalent to the letter A; the code 11110001 is equivalent to the decimal digit 1. Other zone combinations are used when forming special characters. Not all of the 256 combinations of 8-bit code have been assigned characters. Figure 4-5 illustrates how the characters DP-3 are represented using EBCDIC. Figure 4-3 Format for EBCDIC and ASCII codes NEETS MODULE 22-Digital Computing

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Figure 4-4 Eight-bit EBCDIC coding chart (including hexadecimal equivalents) Figure 4-5 DP-3 represented using 8-bit EBCDIC code NEETS MODULE 22-Digital Computing

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Since one numeric character can be represented and stored using only four bits (8-4-2-1), using an 8-bit code allows the representation of two numeric characters (decimal digits) as illustrated in figure 4-6. Representing two numeric characters in one byte (eight bits) is referred to as packing or packed data. By packing data (numeric characters only) in this way, it allows us to conserve the amount of storage space required, and at the same time, increases processing speed. 4.3.2 American Standard Code for Information Interchange (ASCII) Another 8-bit code, known as the American Standard Code for Information Interchange (ASCII) (pronounced ASS-KEY), was originally designed as a 7-bit code. Several computer manufacturers cooperated to develop this code for transmitting and processing data. The purpose was to standardize a binary code to give the computer user the capability of using several machines to process data regardless of the manufacturer: IBM, HONEYWELL, UNIVAC, BURROUGHS, and so on. However, since most computers are designed to handle (store and manipulate) 8-bit code, an 8-bit version of ASCII was developed. ASCII is commonly used in the transmission of data through data communications and is used almost exclusively to represent data internally in microcomputers.

The concepts and advantages of ASCII are identical to those of EBCDIC. The important difference between the two coding systems lies in the 8-bit combinations assigned to represent the various alphabetic, numeric, and special characters. When using ASCII 8- bit code, you will notice the selection of bit patterns used in the positions differs from those used in EBCDIC. For example, let's look at the characters DP3 in both EBCDIC and ASCII to see how they compare. Character D P 3

Ebcdic 1100 0100 1101 0111 1111 0011

Ascii 0100 0100 0101 0000 0011 0011

In ASCII, rather than breaking letters into three groups, uppercase letters are assigned codes beginning with hexadecimal value 41 and continuing sequentially through hexadecimal value 5A. Similarly, lowercase letters are assigned hexadecimal values of 61 through 7A. The decimal values 1 through 9 are assigned the zone code 0011 in ASCII rather that 1111 as in EBCDIC. Figure 4-7 is the ASCII coding chart showing uppercase and lowercase alphabetic characters and numeric digits 0 through 9. Figure 4-6 Packed data NEETS MODULE 22-Digital Computing

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At this point you should understand how coding systems are used to represent data in both EBCDIC and ASCII. Regardless of what coding system is used, each character will have an additional bit called a check bit or parity bit. Figure 4-7 Eight-bit ASCII coding chart (including hexadecimal equivalents) NEETS MODULE 22-Digital Computing

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4.3.3 Parity Bit This additional check or parity bit in each storage location is used to detect errors in the circuitry. Therefore, a computer that uses an 8-bit code, such as EBCDIC or ASCII, will have a ninth bit for parity checking. The parity bit (also called a check bit, the C position in a code) provides an internal means for checking the validity, the correctness, of code construction. That is, the total number of bits in a character, including the parity bit, must always be odd or always be even, depending upon whether the particular computer system or device you are using is odd or even parity. Therefore, the coding is said to be in either odd or even parity code, and the test for bit count is called a parity check.

Now, let's talk about bits and bytes, primary storage, and storage capacities; or, to put it another way, the capacity of a storage location. Sit back, keep your memory cycling, and we will explain the ways data may be stored and retrieved inside the computer. Q-6. What does the acronym EBCDIC stand for? Q-7. By using an 8-bit code, how many characters or bit combinations can be represented? Q-8. What is the base of a hexadecimal number system? Q-9. What term is used for the representation of two numeric characters stored in eight bits?

Q-10. What does the acronym ASCII mean? Q-11. What was the purpose of several computer manufacturers cooperating to develop ASCII code for processing and transmitting data? NEETS MODULE 22-Digital Computing

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Q-12. Are there any differences in the concepts and advantages of ASCII and EBCDIC? Q-13. How is the parity bit in each storage location used? Q-14. A computer or device that uses 8-bit ASCII or EBCDIC will use how many bits to store each character? 4.4 DATA STORAGE CONCEPTS You learned in chapter 2 that a computer's primary storage area is divided into four areas, each serving a specific purpose. The input storage area accepts and holds input data to be processed. The working storage area holds intermediate processing results. The output storage area holds the final processing results. The program storage area holds the processing instructions (the program). You also learned that these separate areas do not have built-in physical boundaries, rather the boundaries are determined by the individual programs being used.

You also may recall in chapter 2, we talked about the different types of primary storage used in computers and how they differ from one another. Some were magnetic in nature, such as magnetic core storage; others were electronic, such as semiconductor and bubble storage. For purposes of simplicity, we have selected magnetic core storage to show you how data is represented and stored in the computer's primary memory. 4.4.1 Bits and Bytes A bit is a single binary digit. It represents the smallest unit of data just like the good old American penny). However, computers usually do not operate on single bits, rather they store and manipulate a fixed number of bits. Most often, the smallest unit or number of bits a computer works with is eight bits. These eight bits make up a byte. You just learned that both EBCDIC and ASCII codes use eight bits (excluding the parity bit), and that eight bits represent a single character, such as the letter A or the number 7. Thus, the computer can store and manipulate an individual byte (a single character) or a group of bytes (several characters, a word) at a time. These individual bytes, or groups of bytes, form the basic unit of memory.

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Primary storage capacities are usually specified in number of bytes. The symbol "K" is used whenever we refer to the size of memory, especially when the memory is quite large. The symbol K is equal to 1,024 units or positions of storage. Therefore, if a computer has 512K bytes (not bits) of primary storage, then it can hold 512 X 1,024 or 524,288 characters (bytes) of data in its memory. 4.4.2 Magnetic Core Storage In primary storage, many magnetic cores are strung together on a screen of wire to form what is called a core plane (fig. 4-8, view A). As you may know, each core can store one binary bit (0 or 1) of data. A core is magnetized by current flowing through the wires on which the core is strung. Hence, a core magnetized in one direction represents a binary 0, and when magnetized in the opposite direction, a binary 1. It is the direction that the core is magnetized that determines whether it contains a binary 0 or a binary 1 (refer to fig. 4- 8, view B). These core planes look very much like small window screens and are arranged vertically to represent data as shown in figure 4-8, view C. In looking at this figure, you will notice that nine planes are needed to code in 8-bit EBCDIC. The ninth plane provides for a parity (check) bit. Figure 4-8, view C, shows DP-3 in EBCDIC code, even parity.

Figure 4-8A Core storage with DP-3 represented using 8-bit EBCD1C code NEETS MODULE 22-Digital Computing

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Figure 4-8B.— Core storage with DP-3 represented using 8-bit EBCD1C code Figure 4-8C.— Core storage with DP-3 represented using 8-bit EBCD1C code NEETS MODULE 22-Digital Computing

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4.4.3 Storage Capacity and Addresses The storage capacity of an address is designed and built into the computer by the manufacturer. Over the years several different design approaches to partition primary storage have been used. With this in mind, let's take a look at some of the ways primary storage is partitioned into addresses. One way to design or organize the primary storage section is to store a fixed number of characters (bytes) at each address location. We can then reference these characters as a single entity called a word, as illustrated in figure 4-9, view A. The name CHARLIE (address location 400) or the amount he is paid, in this case $69.00 (address location 401), are each treated as a single word. Computers that are built to retrieve, manipulate, and store a fixed number of characters in each address are said to be word-oriented, word-addressable machines, or fixed-word-length computers.

Another way to design the primary storage section is to store a single character, such as the letter L or the number 8, in each address location. An address is assigned to each location in storage. Computers designed in this way are said to be character-oriented or character addressable. We also call them variable-word-length computers. Therefore, the name CHARLIE (fig. 4-9, view B) now requires seven address locations (300 through 306), while amount paid ($69.00) occupies six address locations (307 through 312). Figure 4-9A Fixed-word-length vs variable-word-length storage. FIXED-LENGTH WORDS,

Containing Eight Characters Each, Occupying Two Address Locations

(WORD ADDRESSABLE NEETS MODULE 22-Digital Computing

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Whether a computer addresses a group of bytes as a word or addresses each byte individually is a function of the circuitry. Both designs have advantages and disadvantages. Variable-word-length computers make the most efficient use of available storage space, since a character can be placed in every storage location. In a fixed-word- length computer, storage space may be wasted. For example, if the storage capacity in each address of a fixed-word-length computer is eight bytes, and some of the data elements to be stored contain only three or four characters, then many of the storage positions in each word are not being used.

Fixed-word-length computers have faster calculating speeds. They can add two data words in a single operation. This is not so with character-addressable computers. Here, only one digit (byte) in each number can be added during a single machine operation. Thus, eight steps are required to complete the calculation. The larger mainframe computers (super-computers like the CRAY-1 and CYBER 205) use only fixed-word-length storage. Most microcomputers use the variable-word-length approach allowing them to operate on one character at a time. Somewhere in between these two extremes are the dozens of existing minicomputer and mainframe models that have what is called built-in flexibility.

These flexible computers are byte-oriented but can operate in either a fixed- or variable- word-length mode through the use of proper program instructions. Let's take a look at how these flexible computers operate in a variable- and fixed-word-length environment. Figure 4-9B.— Fixed-word-length vs variable-word-length storage. VARIABLE-LENGTH

Words (Character Addressable)

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Working in a variable-word-length environment, each address holds one alphanumeric character as shown in figure 4-9, view B. Since a byte usually represents a single alphanumeric character, unless you are using packed decimal, a flexible computer is often said to be byte-addressable. Don't become confused; the terms character-addressable, character-oriented, and byte-addressable all have the same meaning. By using the appropriate program instructions, a programmer can retrieve a stored data element by identifying the address of the first character (say position 300 as in fig. 4-9, view B) and specifying the number of address locations to be included in the word. In this case there are seven, positions 300 through 306.

When a flexible computer is working in a fixed-word-length environment, each address identifies a group of bytes that can be operated on as a unit. This processing method helps to achieve faster calculating speeds. A programmer can use program instructions to cause the computer to automatically retrieve, manipulate, and store, as a unit, a fixed word of say, two, four, or eight bytes of data in one machine operation by identifying the address of the first character of data. At the same time all remaining bytes are acted upon as a unit moving from left to right. Figure 4-10 illustrates the different word lengths possible with many byte-addressable computers. They are half-word (2 bytes), full-word (4 bytes), and double-word (8 bytes).

Figure 4-10 Word lengths used on flexible byte- addressable computers NEETS MODULE 22-Digital Computing

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By now, you should have a good idea of how primary storage locations are identified by their storage addresses, how these addressable storage locations are used, and how the storage capacity at an address can vary depending on the design of the computer. Now, let's go one step further, to see how these bits and bytes are represented (coded) on some of the more common secondary storage media. 4.4.4 Secondary Storage Data Organization Remember, secondary storage devices (also called auxiliary or mass storage devices) are those devices which are not part of the central processing unit (cpu). They include: external core; semiconductor, thin film, and bubble memories; punched cards; paper tape; and several different types of mass storage, such as magnetic tape, disk, and drum.

You already know it takes a certain number of bits to make one byte (normally eight), and when bytes are grouped together at a single address they make up a word in the computer's memory. When data is recorded on some type of magnetic storage medium, such as disk or tape, it is normally organized by bits, characters (bytes), fields, records, and files (fig. 4-11). The following definitions should help you understand the relationship between bits, characters, bytes, words, fields, records, and files. Figure 4-11 Data organization NEETS MODULE 22-Digital Computing

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BIT—The smallest unit of data; it represents one binary digit (0 or 1). CHARACTER (BYTE)—A group of related bits (usually eight) that make up a single character letter, number, or special character. WORD—A group of related bytes that are treated as a single addressable unit or entity in memory. FIELD—One or more related characters that are treated as a unit of information. A field (also referred to as a data item) may be alphabetic, numeric, or alphanumeric, and may be either fixed or variable in length. For example, your social security number (SSN) is of a fixed length; that is, it's always 9 positions in length. Whereas, names are variable length because they may be from 2 to 25 positions in length.

RECORD—A group of rel ated fields, all pertaining to the same subject; a person, a thing, or an event. For example, your payroll record (LES statement) might include fields for your name, amount paid, taxes withheld, earned leave, and any allotments you might have. On the other hand, a supply inventory record might consist of fields containing stock number, the name of the item, its unit price, the quantity on hand, and its bin location. FILE—A collection of related records, such as the payroll or supply inventory records.

Normally, all records within the file are in the same format. When processing data, we think in terms of data files. For example, to process a parts inventory, you would need the master parts inventory file and the file that contains up-to- date information on each part that has been issued. The master parts inventory file would have a record for every part in the inventory. The update file, parts issued file, would have a record for each part issued. You would use a program to read the records on the parts issued file and update the matching records on the master parts inventory file.

Depending on whether the data is stored on magnetic tape or disk or in internal storage, the program would use different methods to access storage to obtain the data. In the next section you'll learn about storage access methods. Q-15. What area in the computer's primary storage area holds the processing instructions (the program)? Q-16. How are the boundaries determined for the separate areas of the computer's primary storage area? NEETS MODULE 22-Digital Computing

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Q-17. What is a bit? Q-18. How many bits make up a byte? Q-19. Primary storage capacities are usually specified in what unit of measure? Q-20. How are core planes formed? Q-21. Where are core planes used? Q-22. Who designs and builds the storage capacity of an address into a computer? Q-23. What is another name for computers designed to be character-oriented or character-addressable? Q-24. Which computer has the faster calculating speeds, the variable-word-length or the fixed-word-length? Q-25. What is the normal organization of data recorded on magnetic storage media?

Q-26. What is a file? NEETS MODULE 22-Digital Computing

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4.5 STORAGE ACCESS METHODS How data files are stored in secondary storage varies with the types of media and devices you are using. Data files may be stored on or in sequential-access storage, direct-access storage, or random-access storage. 4.5.1 Sequential-Access Storage Punched cards, paper tape, and magnetic tape are examples of sequential-access storage media. When operating in a sequential environment, a particular record can be read only by first reading all the records that come before it in the file. When you store a file on tape, the 125th record cannot be read until the 124 records in front of it are read. The records are read in sequence. You cannot read just any record at random. This is also true when reading punched cards or paper tape.

4.5.2 Direct-Access Storage Direct-access storage allows you to access the 125th record without first having to read the 124 records in front of it. Magnetic disks and drums are examples of direct-access storage media. Data can be obtained quickly from anywhere on the media. However, the amount of time it takes to access a record is dependent to some extent on the mechanical process involved. It is usually necessary to scan some (but not all) of the preceding data. 4.5.3 Random-Access Storage Random-access storage media refers to magnetic core, semiconductor, thin film, and bubble storage. Here, a given item of data can be selected from anywhere in storage without having to scan any preceding items. And, the access time is independent of the storage location.

Q-27. Punched cards, paper tape, and magnetic tape use what storage access method? Q-28. What kind of storage allows you to access the 125th record without having to read the 124 records in front of it? Q-29. Random-access storage media refers to what types of storage? NEETS MODULE 22-Digital Computing

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4.6 NETWORKS A network can be defined as any system composed of one or more computers and terminals; however, most are composed of multiple terminals and computers. In this section you will learn how this allows dissimilar computers to work together as a team. 4.6.1 Local Area Networks (LANs) In local area networks (LANs), various machines are linked together within a building or adjacent buildings. Figure 4-12 shows an example of a LAN. A LAN allows dissimilar machines to exchange information within one universal system. With the ability to communicate, the dissimilar machines act as a team. The information that exists in one system can be reused without being reentered via keyboard or disk into another separate system. A universal system for the integration and exchange of information is connected to all input devices. The entire system is usually housed within the same building or the same geographic area. A local area network is made up of a communications facility (for example, a coaxial cable, such as that used for cable television) and interface units creating a link for the computers and terminals to the communications facility. Two designs can be used: broadband or baseband.

A baseband communications channel uses the basic frequency band of radio waves and a coaxial cable. This coaxial cable has one channel, which is like a party line. Only two machines can use this cable at one time, even though many have the channel available, but there is no central switching unit to route traffic over the network. Figure 4-12 Local area network system NEETS MODULE 22-Digital Computing

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A more expensive channel, called a broadband communications channel, can handle more advanced applications. This includes transmission of voice as well as data and text. Because of the use of a controller to route traffic for a large number of simultaneous users, the users are able to share one of the many individual channels of the system. 4.6.2 Wide Area Networks Wide area networks provide for global connections and are sometimes referred to as global networks. Organizations are able to send information from city to city, across the nation, and to other countries throughout the world, through the expansion of local area networks into larger network configurations. Combinations of telephone lines, microwave radio links, and satellites are used by these larger telecommunications networks to send information. In 1965, the first successful communications satellite for business applications was launched. It was not the only try, it was preceded by many more primitive satellites. With the launching of larger and more complex satellites, the size and complexity of earth stations have been shrinking. Since satellite services' costs have been steadily decreasing, it is becoming more cost effective to employ them for business-type uses.

4.6.3 Modems Since both signals and data can be transmitted and received through cables (communications lines), we refer to them as input/output channels. And when we transmit data directly to a computer over long distances, it becomes necessary to add two other devices, one at each end of the communications line. These devices are called modems (fig. 4-13). The word modem is an acronym for modulator/demodulator (combines first syllable of each word). A modem converts the digital signal produced by your terminal or the computer to an audio signal suitable for transmission over the communications line. The modem at the other end of the line converts the audio signal back to a digital signal before it is supplied to the computers or your terminal. If this conversion were not carried out, the digital signal would degenerate during transmission and become garbled.

Figure 4-13 Modem NEETS MODULE 22-Digital Computing

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The physical link or medium that is used to carry (or transmit) data from one location to another is a communications channel. It allows remotely located input/output devices to communicate directly with the computer's central processing unit (cpu). Telephone lines (often referred to as land lines) are a frequently used type of communications channel. In a simple data communications system, terminals and other remote I/O devices are linked directly to one or more cpu's to allow users to enter data and programs and receive output information. Interface elements (those devices that serve to interconnect), such as modems are used to bridge and control the different data communications environments.

Modems are used to permit the system to switch back and forth from computer digital data to analog signals that can be transmitted over communications lines. A modem never knows exactly when to expect data; therefore, it must be given some type of signal warning that data is about to be transmitted. This gives the modem time to get itself aligned and in synchronization with the incoming signal. Special characters, known as message characters, provide this warning and are placed in front of and behind the data to mark the beginning and ending of the message. Two methods are used: asynchronous and synchronous.

With asynchronous transmission, each character of data must be surrounded by message characters. As a result, more total bits must be transmitted (transferred) than would be necessary if the synchronous method were used. With synchronous transmission, only a single set of start and stop message characters is needed per block of data, thus allowing more characters to be transmitted per second. As you can see, synchronous transmission is more efficient and faster. However, it has the disadvantage of requiring a more complex and expensive modem than does asynchronous transmission.

You should be aware that whenever data is transferred between devices, it also involves an exchange of prearranged signals. This is known as handshaking. These signals, in combination with a prearranged pattern of message characters, define the rules for exchanging data over a communications line. The exact rules depend upon each individual computer manufacturer, the telephone company, and the related devices (the modems) that make up the computer system. Protocol is the term used for the specific set of rules that govern handshaking and message characters.

In the system illustrated in figure 4-14, data to be sent to the main computer's cpu is entered through a remote online user terminal (far left). As the data is keyed, it is keyed in digital form and sent to a nearby modem to be converted into an analog signal suitable for transmission. This converted data is then transmitted over the telephone (or land) lines to another modem, that is located near the main computer system's cpu. The data, now in digital form, can be sent directly to the cpu for processing. The same route is followed when information is sent from the cpu back to the remote terminal.

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Data communications and networks expand our use of computer technology by providing a means for computers and other machines to talk to each other. Q-30. Any system composed of one or more computers and terminals can be defined as what? Q-31. A network system allows dissimilar machines to do what within one universal system? Q-32. What does the make-up of a local area network consist of? Q-33. How many designs of local area networks are there that can be used? Q-34. What are the different designs of local area networks called?

Q-35. What is a baseband communication channel like? Q-36. What do wide area networks provide for? Figure 4-14 Modems used in network system NEETS MODULE 22-Digital Computing

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Q-37. Where does the word modem come from? Q-38. What are interface elements? Q-39. How does a modem know when to expect data? Q-40. Whenever data is transferred between devices, it involves the exchange of prearranged signals; what is this process called? 4.7 SUMMARY Congratulations! You have just finished the last chapter in Introduction to Digital Computers. In this chapter you learned about many things that were mentioned in other chapters, without a detailed explanation. This was done intentionally, as some of the subjects would have been too difficult and hard to understand without background knowledge. Through your study of chapters 1, 2, and 3, you gained enough knowledge to understand chapter 4. This chapter should have answered a lot of questions for you and made certain subjects more clear.

DATA is a general term used to describe raw facts like your service number, name, and paygrade. SOURCE DATA is raw data typically written on some type of paper document. DATA REPRESENTATION is accomplished by the use of symbols. The symbol itself is not the information, but merely a representation of it. Symbols convey meaning only when understood. In computers, symbols are represented by CODES. COMPUTER CODING SYSTEMS are used to represent numeric, alphabetic, and special characters in computer storage and on magnetic media.

EXTENDED BINARY CODED DECIMAL INTERCHANGE CODE (EBCDIC) is an 8-bit code used in computers to represent numbers, letters, and special characters. NEETS MODULE 22-Digital Computing

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American Standard Code For Information Interchange (Ascii)

is another 8-bit code developed to standardize a binary code to give the computer user the capability of using several machines to process data regardless of the manufacturer. A PARITY (CHECK) BIT is used to detect errors in computer circuitry. MAGNETIC CORE STORAGE is used as primary storage in some computers. PRIMARY STORAGE CAPACITY AND ADDRESSES are designed and built into the computer by the manufacturer. Computers may be WORD-ADDRESSABLE, CHARACTER-ADDRESSABLE, or

Flexible.

Data in SECONDARY STORAGE like disk or tape is normally organized by bits, characters (bytes), fields, records, and files. STORAGE ACCESS METHODS vary with the types of media and devices you are using. SEQUENTIAL-ACCESS STORAGE is associated with punched cards, paper tape, and magnetic tape. DIRECT-ACCESS STORAGE is obtained by using magnetic disks and drums. RANDOM-ACCESS STORAGE refers to magnetic core, semiconductor, thin film, and bubble storage. A NETWORK is any system composed of one or more computers and terminals; however, most are composed of multiple terminals and computers.

LOCAL AREA NETWORKS (LANs) allow dissimilar machines to exchange information within one universal system within a building or small geographic area. WIDE AREA NETWORKS provide for global connections and are sometimes referred to as global networks. A MODEM converts the digital signal produced by your terminal or the computer to an audio signal suitable for transmission over a communications line. It also converts the audio signal back to a digital signal before it is supplied to your terminal or computer. NEETS MODULE 22-Digital Computing

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4-26

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Answers To Questions Q1. Through Q40.

A-1. Data. A-2. By symbols. A-3. Numbers. A-4. By either direct or indirect means. A-5. Punched cards, paper tape, magnetic tape, or magnetic disk. A-6. Extended Binary Coded Decimal Interchange Code.

A-7. 256.

A-8. 16. A-9. Packing or packed data. A-10. American Standard Code for Information Interchange. A-11. To standardize a binary code to give the computer user the capability of using several machines to process data regardless of the manufacturer. A-12. No, they are identical. A-13. To detect errors in the circuitry. A-14. Nine. A-15. Program storage area. A-16. By the individual programs being used. A-17. A single binary digit. A-18. Eight. A-19. Number of bytes. A-20. Magnetic cores are strung together on a screen of wire.

NEETS MODULE 22-Digital Computing

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A-21. In primary storage. A-22. The manufacturer. A-23. Variable-word-length or byte-addressable. A-24. Fixed-word-length. A-25. By bits, characters (bytes), fields, records, and files. A-26. A collection of related records. A-27. Sequential-access. A-28. Direct-access storage. A-29. Magnetic core, semiconductor, thin film, and bubble. A-30. A network. A-31. Exchange information. A-32. A communications facility and interface units. A-33. Two. A-34. Broadband and baseband. A-35. A party line. A-36. Global connections.

A-37. It is an acronym for modulator/demodulator. A-38. Those devices that serve to interconnect. A-39. It is given a signal warning that data is about to be transmitted. A-40. Handshaking.