the speed of an internal computer clock or to vary
the intensity of indicators used on a controlling
device.
DISPLAYS
Some computers use a display of alphanumeric
characters to provide computer status of the functional
areas and for operator interfacing. You can find the
interpretation of displayed information in the
technical manual or owner’s manual. The displays can
be used when the computer is in an operational
mode or a maintenance mode. Some computers use a
dot matrix display; others use light-emitting diodes
(LEDs) to display the alphanumeric characters.
Some computers use a small three- or four-digit
display to display an address and its contents. Other
computers use larger displays. For example, one
computer has a large display consisting of up to 44
alphanumeric characters per line and up to 6 lines. One
portion of the display, when used for status, does
not vary. The other portion, the operator interfacing
part, varies in accordance with the types of
operations being run at the time. The technology
used with the operator interfacing portion of the
display is ac plasma. This enables you to monitor
operations.
HOURS (TIME TOTALIZING
METER)
Time totalizing meters show the total number of
hours power has been applied to a unit or module. They
usually use a four-digit display to indicate the number
of hours. The display is similar to the odometer of an
automobile.
INDICATORS (LIGHTS)
The simplest way to show the status of an
operation or a selected item is to turn on a light.
Indicators usually come in several varieties—
backlit indicators, color indicators, and clear in-
dicators.
Backlit indicators —The light bulbs are covered
by a flat lens cover with clear alphanumeric cutouts that
appear lit when the lamp is on.
Color indicators —The light bulbs are inserted in
a small solid colored casings with or without letters or
characters. The whole casing glows when the lamp is
lit. The casing can be square or round.
3-2
Clear indicators —Clear indicator are the same
as the colored variety, but the casing is clear and usually
round.
PROTECTIVE DEVICES
Protective devices can serve as controls. They are
used in computers to prevent damage to the computer
or to warn you of conditions that could be potentially
dangerous. Circuit breakers and guards are two ways
we can protect the computer. Audible alarms are
sometimes used to alert us to potential problems.
Circuit breakers —Circuit breakers remove ac
input power when current becomes too high (i.e.,
internal short circuit).
Guards —Guards are used to prevent accidental
activation of selected keys and switches. A guard can
be a clear lens that covers the key/switch or it can be a
red cover. In either instance, the cover must be flipped
up to gain unrestricted access to the key or switch.
Audible alarms —Computers often use an
audible device, installed internally. These devices can
provide a warning that an abnormal condition is about
to take place, or they can sound an alarm when an
abnormal condition is in progress.
SWITCHES
Switches are used to turn a unit on or off, to activate
a function, or to set a parameter. You can activate
switches by depressing them or flipping them up or
down. Some switches are combined with an indicator
to tell when they are activated.
Switches can activate an immediate response in
computer operations.
They can also be used to set
parameters when the computer is being operated. Let’s
take a look at the different types of switches associated
with computers. They include key switches, position
switches, pushbutton switches, toggle switches, and
two-position switches. See figure 3-1 for examples.
Key Switches
To activate key switches, you lightly depress the
keys. Some keys are combined with an indicator. Two
types are generally found on computer controlling
units—alternate-action keys and momentary-action/
contact keys.
