Also contained in the frame or cabinet of the
computer is the peripheral equipment-floppy and/or
hard disk units. Computers that use motherboards or
backplanes use a keyboard external to the frame or
cabinet as their method to control the computer. With
some micros, however, the keyboard is part of the
cabinet assembly.
The motherboard or backplane
usually rests on the bottom of the frame or cabinet of
the computer. The motherboard or backplane contains
all the pcb’s for the whole computer, a keyboard
connector, a battery backup circuit, and power supply
status LEDs. A motherboard has IC chips included on
the motherboard; a backplane does not. Each pcb
contains one or more functional areas. Figure 2-13 is
an illustration of a motherboard or backplane design
used in a computer.
It is easier to perform maintenance on computers
with motherboards or backplanes than on modular- or
chassis-designed computers because of their size and
the easy accessibility to the interior of the computer.
Extending pcb’s for maintenance is usually not
necessary because everything can be readily accessed
once the cover is removed; this includes maintenance
with the power still applied. Remember, you must still
exercise safety precautions when removing and
installing any parts inside the frame or cabinet by
securing power to the computer.
Safety and Security Design Features of
Computer Frames/Cabinets
The frame or cabinet can provide limited protection
for a computer by use of gaskets and filters. Gaskets
and filters are not used on all types of computers, but
they serve important safety and security functions on
those where they are used.
GASKETS.— Gaskets are used for two main
purposes on computers.
Gaskets provide moisture
sealing protection and protection against interference
(radio frequency interference [RFI] and
electromagnetic interference [EMI]). The gaskets are
usually located around the edges of an item to protect
its contents or internal parts. For example, gaskets are
used in heat exchangers for a module to protect the pcb’s
inside the module from moisture and electronic
interference.
Gaskets are also used in electrical
connectors inside a frame or cabinet to protect the
connection from electronic interference.
FILTERS.— There are two types of filters you will
encounter. They are electronic (EMI and RFI) and
environmental (foreign particles such as dust and dirt)
filters. Both filters provide protection for the computer.
The computer’s technical manual and/or the Planned
Maintenance System (PMS) will provide you with the
requirements for the maintenance of these two filter
types.
Figure 2-13.—Examp1e of a motherboard- or backplane-designed computer.
2-12
