Page 80 - A Practical Guide from Design Planning to Manufacturing

56 Chapter Two

Intel 8088 processors used in the first IBM PC. Released in 1984, the
IBM AT used the Intel 286 processor. The ISA bus was expanded to
match the 16-bit data bus width of that processor and its higher clock
frequency. This 16-bit version was also backward compatible with 8-bit
cards and became enormously popular. IBM did not try to control the
ISA standard and dozens of companies built IBM PC clones and ISA
expansion cards for PCs. Both 8- and 16-bit ISA cards were still widely
used into the late 1990s.
With the release of the Intel 386, which transferred data 32 bits at a
time, it made sense that “The Bus” needed to change again. In 1987, IBM
proposed a 32-bit-wide standard called Micro Channel Architecture
(MCA), but made it clear that any company wishing to build MCA com-
ponents or computers would have to pay licensing fees to IBM. Also, the
MCAbus would not allow the use of ISAcards. This was a chance for IBM
to regain control of the PC standard it had created and time for compa-
nies that had grown rich making ISA components to pay IBM its due.
Instead, a group of seven companies led by Compaq, the largest PC
clone manufacturer at the time, created a separate 32-bit bus standard
called Extended ISA (EISA). EISA would be backward compatible with
older 8 and 16-bit ISA cards, and most importantly no licensing fees
would be charged. As a result, the MCA standard was doomed and never
®
appeared outside of IBM’s own PS/2 line. EISA never became popular
either, but the message was clear: the PC standard was now bigger than
any one company, even the original creator, IBM.
The Peripheral Component Interconnect (PCI) standard was proposed
in 1992 and has now replaced ISA. PCI offers high bandwidth but per-
haps more importantly supports Plug-n-Play (PnP) functionality. ISA
cards required the user to set switches on each card to determine which
interrupt line the card would use as well as other system resources. If
two cards tried to use the same resource, the card might not function,
and in some cases the computer wouldn’t be able to boot successfully.
The PCI standard includes protocols that allow the system to poll for new
devices on the expansion bus each time the system is started and dynam-
ically assign resources to avoid conflicts. Updates to the PCI standard
have allowed for steadily more bandwidth.
Starting in 2004, systems began appearing using PCI-Express, which
cuts the number of data lines but vastly increases frequencies. PCI-
Express is software compatible with PCI and expected to gradually
replace it. The standard allows for bus widths of 1, 4, 8, or 16 bits to allow
for varying levels of performance. Eventually PCI-Express may replace
other buses in the system. Already some systems are replacing the AGP
graphics bus with 16-bit-wide PCI-Express.
As users continue to put computers to new uses, there will always be
a need for a high-performance expansion bus.