Hardware Reference
In-Depth Information
• Increasing software demands
• Greater multimedia requirements
Each of these areas requires the I/O bus to be as fast as possible.
One of the primary reasons new I/O bus structures have been slow in coming is compatibility—that
old catch-22 that anchors much of the PC industry to the past. One of the hallmarks of the PC's
success is its standardization. This standardization spawned thousands of third-party I/O cards, each
originally built for the early bus specifications of the PC. If a new high-performance bus system was
introduced, it often had to be compatible with the older bus systems so the older I/O cards would not
be obsolete. Therefore, bus technologies seem to evolve rather than make quantum leaps forward.
You can identify different types of I/O buses by their architectures. The main types of I/O buses are
detailed earlier in this chapter.
The main differences among buses consist primarily of the amounts of data they can transfer at one
time and the speeds at which they can do it. The following sections describe the various types of PC
buses.
The ISA Bus
Industry Standard Architecture (ISA) is the bus architecture that was introduced as an 8-bit bus with
the original IBM PC in 1981; it was later expanded to 16 bits with the IBM PC/AT in 1984. ISA is
the basis of the modern personal computer and was the primary architecture used in the majority of
PC systems until the late 1990s.
To learn more about the ISA Bus, including diagrams of 8-bit and 16-bit ISA slots and pinouts, see
“The ISA Bus” in Chapter 4 in
Upgrading and Repairing PCs,
19
th
Edition, available in its entirety
on the DVD packaged with this topic.
The Micro Channel Bus
The introduction of 32-bit chips meant that the ISA bus could not handle the power of another new
generation of CPUs. The chips could transfer 32 bits of data at a time, but the ISA bus can handle a
maximum of only 16 bits. Rather than extend the ISA bus again, IBM decided to build a new bus; the
result was the MCA bus. MCA (an abbreviation for
MicroChannel Architecture
) was completely
different from the ISA bus and was technically superior in every way. However, because IBM was
the only vendor to use it, MCA never became a common standard, and was phased out in 1996.
The EISA Bus
The EISA standard was announced in September 1988 as a response to IBM's introduction of the
MCA bus—more specifically, to the way IBM wanted to handle licensing of the MCA bus. Vendors
did not feel obligated to pay retroactive royalties on the ISA bus, so they turned their backs on IBM
and created their own buses.
Compaq was the primary developer of the EISA standard, which was intended to be the company's
way of taking over future development of the PC bus from IBM. Compaq knew that nobody would
clone its bus if it was the only company that had it, so it essentially gave the design to other leading
manufacturers. Compaq formed the EISA committee, a nonprofit organization designed specifically to
control development of the EISA bus. Few EISA adapters were developed. Those that were
developed centered mainly on disk array controllers and server-type network cards.
