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7.1 / EXTERNAL DEVICES 221

Control logic associated with the device controls the device’s operation in re-
sponse to direction from the I/O module.The transducer converts data from electri-
cal to other forms of energy during output and from other forms to electrical during
input. Typically, a buffer is associated with the transducer to temporarily hold data
being transferred between the I/O module and the external environment; a buffer
size of 8 to 16 bits is common.
The interface between the I/O module and the external device will be exam-
ined in Section 7.7. The interface between the external device and the environment
is beyond the scope of this book, but several brief examples are given here.

Keyboard/Monitor
The most common means of computer/user interaction is a keyboard/monitor
arrangement. The user provides input through the keyboard. This input is then
transmitted to the computer and may also be displayed on the monitor. In addition,
the monitor displays data provided by the computer.
The basic unit of exchange is the character.Associated with each character is a
code, typically 7 or 8 bits in length. The most commonly used text code is the Inter-
1
national Reference Alphabet (IRA). Each character in this code is represented by
a unique 7-bit binary code; thus, 128 different characters can be represented. Char-
acters are of two types: printable and control. Printable characters are the alpha-
betic, numeric, and special characters that can be printed on paper or displayed on a
screen. Some of the control characters have to do with controlling the printing or
displaying of characters; an example is carriage return. Other control characters are
concerned with communications procedures. See Appendix F for details.
For keyboard input, when the user depresses a key, this generates an electronic
signal that is interpreted by the transducer in the keyboard and translated into the
bit pattern of the corresponding IRA code. This bit pattern is then transmitted to
the I/O module in the computer.At the computer, the text can be stored in the same
IRA code. On output, IRA code characters are transmitted to an external device
from the I/O module.The transducer at the device interprets this code and sends the
required electronic signals to the output device either to display the indicated char-
acter or perform the requested control function.

Disk Drive
A disk drive contains electronics for exchanging data, control, and status signals
with an I/O module plus the electronics for controlling the disk read/write mecha-
nism. In a fixed-head disk, the transducer is capable of converting between the mag-
netic patterns on the moving disk surface and bits in the device’s buffer (Figure 7.2).
A moving-head disk must also be able to cause the disk arm to move radially in and
out across the disk’s surface.

1 IRA is defined in ITU-T Recommendation T.50 and was formerly known as International Alphabet
Number 5 (IA5). The U.S. national version of IRA is referred to as the American Standard Code for
Information Interchange (ASCII).