Expansion Slots
Expansion slots, also known as sockets, are receptacles on the
computer motherboard that accept printed circuit boards. All computers
have expansion slots that allow additional devices to be added. Video
cards, I/O cards, and sound cards are examples of components that are
located in expansion slots.
There are several types of expansion
slots on a motherboard. The number and type of expansion slots in the
computer determines future expansion possibilities. The common
expansion slots that are likely to be encountered include the
following:
- The Industry Standard Architecture
(ISA ) is a 16-bit expansion slot developed by IBM. It transfers
data with the motherboard at 8 MHz. ISA slots are
becoming obsolete and are being replaced by PCI slots in new
systems. However, many motherboard manufacturers may still include
one or two for backward compatibility with older expansion cards.
In 1987, IBM introduced the 32-bit, Extended ISA (EISA) bus, which
accommodates the Pentium chip. EISA became fairly popular in the
PC market.
- The Peripheral Component
Interconnect (PCI ) is a 32-bit local bus slot developed by Intel. Since they
"talk" to the motherboard at 33 MHz, the PCI bus slots
offer a significant improvement over ISA or EISA expansion slots.
With the PCI bus, each add-on card contains information that the
processor uses to automatically configure the card. The PCI bus is
one of the three components necessary for plug-n-play. The main
purpose of the PCI bus is to allow direct access to the CPU for
devices such as memory and video. PCI expansion slots are the most
commonly used type in motherboards today.
- The Accelerated Graphics Port (AGP)
was developed by Intel and is a dedicated high-speed bus that is
used to support the high demands of graphical software.
This slot is reserved for video adapters. This is the standard
graphics port in all new systems. On AGP-equipped motherboards, a
single AGP slot holds the display adapter, and the PCI slot can be
used for another device. Slightly shorter than the white PCI slot,
the AGP slot is usually a different color and is located about an
inch beyond the PCI slot. The latest version of the accelerated
graphics port, AGP-4, came out in 1992; it offers a 1GB transfer
rate.
Figure
shows the different slot
types, while Table
summarizes some useful information on the more
common slot types that are discussed above and others that have never
gained widespread use in the industry.
Bus Types
All the basic components of the computer are connected together by
communication paths that are referred to as buses. The system bus
is a parallel collection of conductors that carry data and control
signals from one component to the other. Recall that the conductors in
modern computers are actually metallic traces on the circuit board.
There are three major system bus types that can be identified based
on the type of information they carry. These include the address bus,
data bus, and control bus.
- The address bus is a uni-directional pathway, which means that
information can only flow one way. Its function is to
carry addresses generated by the CPU to the memory and I/O
elements of the computer. The number of conductors in the bus
determines the size of the address bus; this, in turn,
determines the number of memory locations and I/O elements that
the microprocessor can address.
- The data bus, unlike the address bus, is a bi-directional
pathway for data flow, which means that information can flow in
two directions. Data can flow along the
data bus from the CPU to memory during a write operation, and data
can move from the computer memory to the CPU during a read
operation. However, should two
devices attempt to use the bus at the same time, data errors
will occur. Any device connected to the data bus must have the
capability to temporarily put its output on hold (floating
state) when it is not involved in an operation with the
processor. The data bus size, measured in bits, represents the
word size of the computer. Generally, the larger the bus size, the
faster the system. Common data bus sizes are 8-bits or 16-bits
(older systems) and 32 bits (new systems). 64-bit data bus
systems are currently being developed.
- The control bus carries the control and timing signals needed
to coordinate the activities of the entire computer.
Control bus signals, unlike information carried by the data and
address buses, are not necessarily related to each other. Some
are output signals from the CPU, and others are input signals to
the CPU from I/O elements of the system. Every microprocessor
type generates or responds to its own set of control signals.
The most common control signals in use today are listed in
Figure
 .
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Lab
Activity (PDF, 8 KB) |
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In this lab,
students will be able to to identify safety issues,
specifications, and components relating to
expansion slots. Students will also be able to list
the advantages and disadvantages of each
expansion slot. |
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