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Component Interconnect (PCI) Local BusOlder Bus Types
While most of the attention today is given to the current local bus standard, PCI, and the new AGP port that is likely to become the next standard interface for video, these have evolved from a series of older buses that you will still find in service today on older PCs. The oldest one, ISA, is in fact still used even on the newest PCs! This section takes a look at these older bus types in some detail.
Industry Standard Architecture (ISA) Bus
The most common bus in the PC world, ISA stands for Industry Standard Architecture, and unlike many uses of the word "standard", in this case it actually fits. The ISA bus is still a mainstay in even the newest computers, despite the fact that it is largely unchanged since it was expanded to 16 bits in 1984! The ISA bus eventually became a bottleneck to performance and was augmented with additional high-speed buses, but ISA persists because of the truly enormous base of existing peripherals using the standard. Also, there are still many devices for which the ISA's speed is more than sufficient, and will be for some time to come (standard modems being an example).
(As a side note, after 17 years it appears that ISA may finally be going the way of the dodo. Market leaders Intel and Microsoft want to move the industry away from the use of the ISA bus in new machines. My personal prediction is that they will succeed in this effort, but that it will take at least five years to do it fully. There are few standards in the PC world as pervasive as ISA, and the hundreds of millions of existing ISA cards will ensure that ISA sticks around for some time.)
The choices made in defining the main characteristics of the ISA bus--its width and speed--can be seen by looking at the processors with which it was paired on early machines. The original ISA bus on the IBM PC was 8 bits wide, reflecting the 8 bit data width of the Intel 8088 processor's system bus, and ran at 4.77 MHz, again, the speed of the first 8088s. In 1984 the IBM AT was introduced using the Intel 80286; at this time the bus was doubled to 16 bits (the 80286's data bus width) and increased to 8 MHz (the maximum speed of the original AT, which came in 6 MHz and 8 MHz versions).
Later, the AT processors of course got faster, and eventually data buses got wider, but by this time the desire for compatibility with existing devices led manufacturers to resist change to the standard, and it has remained pretty much identical since that time. The ISA bus provides reasonable throughput for low-bandwidth devices and virtually assures compatibility with almost every PC on the market.
Many expansion cards, even modern ones, are still only 8-bit cards (you can tell by looking at the edge connector on the card; 8-bit cards use only the first part of the ISA slot, while 16-bit cards use both parts). Generally, these are cards for which the lower performance of the ISA bus is not a concern. However, access to IRQs 9 through 15 is provided through wires in the 16-bit portion of the bus slots. This is why most modems, for example, cannot be set to the higher-number IRQs. IRQs cannot be shared among ISA devices.
Micro Channel Architecture (MCA) Bus
The MCA bus (also called the Micro Channel bus; MCA stands for "Micro Channel Architecture") was IBM's attempt to replace the ISA bus with something "bigger and better". When the 80386DX was introduced in the mid-80s with its 32-bit data bus, IBM decided (much like it did with the AT) to create a bus to match this width. MCA is 32 bits wide, and offers several significant improvements over ISA. (One of MCA's disadvantages was rather poor DMA controller circuitry.)
The MCA bus has some pretty impressive features considering that it was introduced in 1987, a full seven years before the PCI bus made similar features common on the PC. In some ways it was ahead of its time, because back then the ISA bus really wasn't a major performance limiting factor:
MCA had a great deal of potential. Unfortunately, IBM made two decisions that would doom MCA to utter failure in the marketplace. First, they made MCA incompatible with ISA; this means ISA cards will not work at all in an MCA system, one of the few categories of PCs for which this is true. The PC market is very sensitive to backwards-compatibility issues, as evidenced by the number of older standards that persist to this day (such as ISA!) Second, IBM decided to make the MCA bus proprietary. It in fact did this with ISA as well; however in 1981 IBM could afford to flex its muscles in this manner, while by this time the clone makers were starting to come into their own and weren't interested in bending to IBM's wishes.
These two factors, combined with the increased cost of MCA systems, led to the demise of the MCA bus. With the PS/2 now discontinued, MCA is dead. It is one of the classical examples in the field of computing of how non-technical issues often dominate over technical ones.
Extended Industry Standard Architecture (EISA) Bus
EISA stands for Extended Industry Standard Architecture. Unlike ISA, here the name is not indicative of reality, for the EISA bus never became widely used and cannot by any stretch of the imagination be considered an industry standard. EISA began as Compaq's answer to IBM's MCA bus, and followed a similar path of development--with very similar results.
Compaq avoided the two key mistakes that IBM made when they developed EISA. First, they made it compatible with the ISA bus. Second, they opened the design to all manufacturers instead of keeping it proprietary, by forming the non-profit EISA committee to manage the design of the standard. EISA was similar to MCA both in terms of technology and market acceptance: it had significant technical advantages over ISA, and it never caught on with the PC-buying public.
Some of the key features of the EISA bus:
EISA-based systems have today been mostly relegated to a specialty role; they are sometimes found in network fileservers. The EISA bus is virtually non-existent on desktop systems for several reasons. First, EISA-based systems tend to be much more expensive than other types of systems. Second, there are few EISA-based cards available. Finally, the performance of this bus is quite low compared to the popular local buses like the VESA Local Bus and PCI. EISA is not totally dead as a platform the way MCA is, but it is pretty close.
The first local bus to gain popularity, the VESA local bus (also called VL-Bus or VLB for short) was introduced in 1992. VESA stands for the Video Electronics Standards Association, a standards group that was formed in the late eighties to address video-related issues in personal computers. Indeed, the major reason for the development of VLB was to improve video performance in PCs.
The VLB is a 32-bit bus which is in a way a direct extension of the 486 processor/memory bus. A VLB slot is a 16-bit ISA slot with third and fourth slot connectors added on the end. The VLB normally runs at 33 MHz, although higher speeds are possible on some systems. Since it is an extension of the ISA bus, an ISA card can be used in a VLB slot, although it makes sense to use the regular ISA slots first and leave the (small number of) VLB slots open for VLB cards, which won't work in an ISA slot of course. Use of a VLB video card and I/O controller greatly increases system performance over an ISA-only system.
While VLB was extremely popular during the reign of the 486, with the introduction of the Pentium and its PCI local bus in 1994, wholesale abandonment of the VLB began in earnest. While Intel pushing PCI was one reason why this happened, there were also several key problems with the VLB implementation. First, the design was strongly based on the 486 processor, and adapting it to the Pentium caused a host of compatibility and other problems. Second, the bus itself was tricky electrically; for example, the number of cards that could be used on the bus was low (often only two or even one), and occasionally there could be timing problems on the bus when more than one card was used. Finally, the bus did not support bus mastering properly since there was no good arbitration scheme, and did not support Plug and Play.
Today VLB is obsolete for new systems; even the latest 486 motherboards use PCI, and all Pentiums and higher use PCI. However, these systems do still offer reasonable performance, and are now plentiful and very inexpensive--if you can still find them.
Next: Peripheral
Component Interconnect (PCI) Local Bus
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