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On the system board are a number of expansion slots that are used to expand on
the computer’s capabilities. When it is first purchased, a computer only has so
many capabilities—the nice thing is that you can expand on those capabilities.
Expansion slots expand on what the computer can do. The problem is that there
are different types of expansion slots in the system, so when you go to purchase
that sound card or network card, you have to make sure that you purchase the
right type. In the following sections, we will look at the different type of expansion
slots and compare their characteristics.
It is important to mention that another term for the expansion slots is bus architectures.
There are a number of different bus architectures (or expansion slots) that
have been developed over time. It is important to identify the differences between
each of these architectures and also mention which ones are a little more popular
today.
ISA
The Industry Standard Architecture (ISA) was the first major expansion bus architecture
that was originally developed as an 8-bit architecture and then a 16-bit architecture.
The ISA bus architecture had a speed of 8 MHz, which is extremely slow
when looking at the speed of processors and other system components today.
One of the reasons why you still see 16-bit ISA slots in systems today is because ISA
was the popular bus architecture for many years. This means that companies who
have been buying ISA network cards for many years and are upgrading to a new
system can use their old network card if they want to, thereby avoiding the additional
cost of a new network card to go with the new system.
MCA
The expansion card concept was a big hit, and there was a lot of money to be made
in manufacturing components that could be added to a computer to give the system
more functionality, so everyone wanted a piece of this market. One of the major
downfalls of the ISA bus architecture is its performance. It runs at only 8 MHz, and
it is only a 16-bit architecture—that was fine years ago, but everything evolves,
and new and improved standards arise.
The Micro Channel Architecture (MCA), which was developed by IBM, was a 32-bit
architecture. The MCA architecture ran at 10 MHz and was not compatible with ISA.
You would usually find the MCA slots in high-end IBM machines, such as those that
might be used as a server.
With the MCA, IBM came up with a feature called bus mastering. Bus mastering
works like this: devices in the bus don’t have to send information through the CPU
if they want to talk to one another; they just send the information directly. This
takes some of the workload off the processor and allows it to perform other tasks.
Bus mastering became an important feature in future bus architectures.
EISA
In 1988 the industry standard for expansion cards was still ISA, but bus architectures
had already been created that performed better. So a number of companies
got together with the goal of extending ISA, while maintaining backward compatibility
so that companies could use their existing ISA cards.
As a result, the Extended Industry Standard Architecture (EISA) was developed as a
16- and 32-bit architecture. The big advantage to EISA is that it maintained support
for the ISA cards that companies already had in large quantities, and it also supported
32-bit EISA cards. EISA also included the major advancement in expansion
bus technology that MCA created, known as bus mastering. Because both types of
cards fit into the same slot, they had to keep the speed of 8 MHz.
The bus architecture holds both 16- and 32-bit cards because the EISA slots have
two levels. The EISA cards have very deep edge connectors that fill the two levels
(32-bit) of the slot, but ISA cards only filled the top level (16-bit).
VESA
In 1992 Video Electronics Standard Association (VESA) developed a bus architecture
that outperformed ISA. VESA was a 32-bit architecture, supported bus mastering,
and ran at the same speed as the processor, which back then was around 25 to 33
MHz. Because the bus ran at the speed of the processor, they called this the VESA
local bus. VESA slots were typically used for video cards because the bus ran at the
speed of the local system.
VESA slots are extremely easy to identify because they are tan in color and act as
an extension to the ISA slot. You will notice the black ISA slots and then right beside
them may be a tan slot. The VESA card fills the entire ISA slot and the additional
extension to make the full 32-bit path for VESA. This allows an ISA card to be
inserted into the slot for backward compatibility or, with the extension slot, it can
hold a VESA card.
PCI
Peripheral Component Interconnect (PCI) is one of the newer bus architectures that
has hit the market. PCI has two flavors: 32-bit cards and 64-bit cards. When Pentium
systems hit the market, they had motherboards shipped with them that featured
both ISA/EISA slots and PCI slots. If you want to buy a new card today, you would
most likely buy a PCI device for one of the PCI slots in your system.
The PCI bus has a speed of 33 MHz and also supports bus mastering. One of the
other major benefits of PCI is that it is a plug and play architecture. If you are running
a plug and play operating system like Windows 95/98/2000 (Windows NT is not
a plug and play operating system) and you have a plug and play BIOS, then the system
resources like IRQs and IO addresses can be dynamically assigned for PCI
components.
PCI slots are easily identified on the system board as the small white slots on the
motherboard, usually alongside the ISA or EISA slots.
PCMCIA
Personal Computer Memory Card Industry Association (PCMCIA) is a unique type of
expansion bus architecture because of its small size. PCMCIA is popular in laptop
computers.How are you going to get a big network card like the one that is used in a
desktop computer into a little laptop to add network support to the laptop? The
answer is that you can’t; you have to purchase a PCMCIA network card for the laptop
to add network support. PCMCIA cards, also known as PC Cards, are a little bit
larger than a credit card and can fit into your back pocket.
PCMCIA (say that five times fast!) is a 16-bit architecture that runs at 33 MHz and
has three different types of slots named type 1, type 2, and type 3.
Types Of PCMCIA Slots
Type 1 cards were originally used to add memory to laptop computers or personal
computers. This is where the “personal computer memory card” part of the PCMCIA
name comes from. Type 1 cards are 3.3 millimeters thick, and type 2 cards are 5
millimeters thick. Type 2 cards are used for everyday devices like modems and network
cards. Type 3 cards are 10.5 mm thick and are used for removable drives.
AGP
Advanced GraphicsPort(AGP) has been around since the Pentium II processor in 1997
and runs at 66 MHz, which is double the speed of the PCI bus. Today’s motherboards
have one AGP slot to insert an AGP video card. The performance gain from the AGP
port not only comes from the increase in speed, but also the fact that the AGP bus has
a direct path to the processor so that information travels quickly from the processor
to the AGP card.
USB
Universal Serial Bus (USB) is a high-speed serial technology that transfers data at 12
Mbps. One of the major benefits of USB is the fact that USB devices use a similar
type of connector so that with USB you will not have to guess what port on the
back of the computer the mouse, keyboard, or monitor is to connect to. If they are
all USB devices, they connect to the same type of port.
USB devices also support daisy chaining. For example, you can connect device A to
the back of the computer and then connect device B to device A, and so on. You
can connect up to 127 devices in a USB chain.
A USB device that connects to the computer and then has other devices connected
to it is considered a hub device. If you don’t have a USB device that can act as a hub device,
then you can purchase a specific USB hub device that will allow you to chain other
devices off of it.
As far as the “real world” is concerned, or from an exam perspective, you need to
be extremely strong in the area of bus architectures. A big part of servicing computers
is installing network cards, sound cards, or video cards—these components
come as ISA, PCI, or AGP cards. You will have to know how to look at a system and
say, “We are going to buy a PCI network card for this system.”
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