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Upon completion of this chapter, you will be able to do the following:

  • Explain data and how it is represented.
  • Explain computer coding systems.
  • Define a parity bit and what it is used for.
  • Explain data storage concepts.
  • Describe three storage access methods.
  • Describe networks and data communications.


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.


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.


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.


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).

Q.1 What is a general term used to describe raw facts? answer.gif (214 bytes)
Q.2 How is data represented? answer.gif (214 bytes)
Q.3 What were the first computers designed to manipulate in order to solve arithmetic problems? answer.gif (214 bytes)
Q.4 By what two means can the data contained on a source document be converted into a machine-readable form for processing? answer.gif (214 bytes)
Q.5 What are some of the types of input media on which data may be indirectly entered? answer.gif (214 bytes)

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