INPUT/OUTPUT DEVICES (EXTERNAL)

Input and output devices are similar in operation but perform opposite functions. It is through the use of these devices that the computer is able to communicate with the outside world. Input data may be in any one of three forms:

• Manual inputs from a keyboard or console
• Analog inputs from instruments or sensors
• Inputs from a source on or in which data has previously been stored in a form intelligible to the computer

Computers can process hundreds of thousands of computer words or characters per second. Thus, a study of the first method (manual input) reflects the inability of human-operated keyboards or keypunches to supply data at a speed that matches the speed of digital computers. A high average speed for keyboard operation is two or three characters per second, that, when coded to form computer words, would reduce the data input rate to the computer to less than a computer word per second. Since mainframe computers are capable of reading several thousand times this amount of information per second, it is clear that manual inputs should be minimized to make more efficient use of computer time. However, as a rule, the keyboard is the normal input media for microcomputers.

Input data that has previously been recorded on paper tapes, magnetic tapes, magnetic disks, or floppy disks in a form understood by the program may also be entered into the computer. These are much faster methods than entering data manually from a keyboard. The most commonly used input devices in this category are magnetic tape units, magnetic disk drive units, and floppy disk drive units.

Output information is also made available in three forms:

• Displayed information: codes, numbers, words, or symbols presented on a display device like a cathode-ray screen
• Control signals: information that operates a control device, such as a lever, aileron, or actuator
• Recordings: information that is stored in a machine language or human language on tapes, disks, or printed media

Devices that display, store, or read information include magnetic tape units, magnetic disk drive units, floppy disk drive units, printers, and display devices.

MAGNETIC TAPE UNITS (INPUT/OUTPUT)

The purpose of any magnetic tape unit (drive or device) is to write data on or read data from a magnetic tape (fig. 2-17). Tape stores data in a sequential manner. In sequential processing, the computer must begin searching at the beginning and check each record until the desired data is found. Like a tape cassette with recorded music, to play the fifth song recorded, you must play or fast forward the tape past the first four songs before you can play the fifth.

Figure 2-17. - Magnetic tape unit.

Two reels are used, tape moves from a supply reel to a take-up reel (both are mounted on hubs). Figure 2-18 shows the basic tape drive mechanism. The magnetic oxide coated side of the tape passes directly over the read/write head assembly, making contact with the heads. The magnetic tape unit reads and writes data in parallel channels or tracks along the length of the tape as shown in figure 2-19, view A. Each channel or track is used by a read/write head (one for each channel), as the tape moves across the magnetic gap of the head. Read/write heads may be either one gap or two gap as shown in figure 2-19, views B and C. The one-gap head has only one magnetic gap at which both reading and writing occur.

The two-gap head has one gap for reading and another for writing. Although the one gap is satisfactory, the two-gap head gives increased speed by checking while writing. For example, a tape being written on passes over the write gap where the data is recorded, and then the data is read as it passes over the read gap to make a comparison. With this method, errors are detected almost instantly. When you look closely at figure 2-19, view B (top view), you will notice that there is one read/write coil in the write head for each channel (or track). In this particular case, there are seven. It is the electrical current flowing through these coils that magnetizes the iron-oxide coating on the surface of the tape.

Figure 2-18. - A basic tape drive mechanism.

Figure 2-19. - Read/write head assemblies.

The major differences between magnetic tape units are the speed at which the tape is moved past the read/write head and the density of the recorded information. You know that density describes the number of binary digits, bytes, or frames we can record on an inch of tape. The most common tape densities are 800 and 1,600 BPI (or FPI). Tape speed (or tape movement) varies to a great extent, from less than 50 inches per second to more than 100 inches per second. How fast a tape unit reads and writes is specified as the character transfer rate which is calculated by multiplying the speed of the magnetic tape unit by the character density.

MAGNETIC DISK DRIVE UNITS (INPUT/OUTPUT)

Magnetic disk drive units are storage devices that read and write information on the magnetized surfaces of rotating disks (fig. 2-20). The disks are made of thin metal, coated on each side so that data can be recorded in the form of magnetized spots. As the disks spin around like music records, characters can be stored on them or retrieved in a direct manner. This direct accessing of data has a big advantage over the sequential accessing of data. It gives us fast, immediate access to specific data without having to examine each and every record from the beginning. You can direct the disk drive to begin reading at any point. This is like the phonograph record, you can place the needle at any point and begin playing at any point.

Figure 2-20. - Magnetic disk drive unit.

Located within each disk drive unit is a drive motor that rotates the disk at a constant speed, normally 3,600 revolutions per minute (rpm); or, if you prefer, 60 revolutions per second. The rotational speed for floppy disks is usually between 300 and 400 rpm because of their plastic base. Data is written on the tracks of a spinning disk surface and read from the surface by one or more (multiple) read/write heads. When reading from and writing to hard disks (rigid disks), the read/write heads float on a cushion of air and do not actually touch the surface of the disk. The distance between the head and the surface varies from a millionth of an inch to one-half millionth of an inch. This distance is called the flying height. When multiple disks (platters) are packaged together as a unit in a disk pack, a number of access arms and read/write heads are used to access both surfaces of each platter (fig. 2-21).The disk pack shown consists of six metal disks mounted on a central spindle. Data can be recorded on all surfaces except the top surface of the top disk, and the bottom surface of the bottom disk. These two surfaces are intentionally left blank for protection.

Figure 2-21. - Multiple access arms, read/write heads used with disk packs.