At first glance, there are many similarities between PCI and the older VLB specifications. Both are local bus systems with 32-bit data paths and burst modes. Also, the original PCI design operates at 33 MHz-roughly the same speed as the VLB. But the important differences between them gained PCI its dominant role in expansion-bus technology. These differences stem from the following features:
The PCI design's special bus and chip set are designed for advanced bus mastering techniques and full arbitration of the PCI local bus. This allows support of more than three slots.
The PCI bus has its own set of four interrupts, which are mapped to regular IRQs on the system. If a PC has more than four PCI slots, some will be sharing interrupts and IRQs.
In Windows 95 or with poorly designed PCI cards (both are becoming rarities), the shared addresses can lead to system conflicts and resource problems. Install PCI cards one at a time to minimize problems. Also, be aware that on many systems not all PCI slots offer full bus mastering. Check the owner's manual for details, especially on machines with more than four slots. In general, the PCI slots closest to the keyboard connector are the best choices for full bus mastering.
The PCI bus allows multiple bus-mastering devices. Advanced controllers such as SCSI (Small Computer System Interface) cards can incorporate their own internal bus mastering and directly control attached devices, then arbitrate with the PCI bus for data transfers across the system bus.
Autoconfiguration lets the PC's BIOS assign the IRQ linking the card to the system bus. Most PCI cards have no switches or jumpers to set, speeding installation and preventing many hardware conflicts.
Most PCs on the market today have one or more ISA slot for backward compatibility; however, most expansion cards are now built using the PCI interface. Although Intel was the original driving force behind PCI development, a PCI standards committee maintains the specification, and it is an open design-anyone can design hardware using PCI without being required to pay royalties.
Differences in PCI Versions
The earlier discussion makes PCI sound like a technician's dream interface: fast, reliable, and doing most of the work itself. In most cases, that's true; still, there is always a "but." PCI has gone through many changes, and there are some features to be aware of when you work with one:
The early PCI motherboards often have jumpers and BIOS settings that must be set to enable proper PCI operation. These are most often found on Pentium 60-MHz and 66-MHz machines.
The PCI bus speed is not fixed. Newer chip sets can drive it-and the cards on it-at 66 MHz. At full performance, the PCI bus can deliver data transfers at up to 132 MB per second.
PCI is not used only by PCs. Macintosh and some other non-PC-style computers incorporate PCI. Manufacturers appreciate this feature because it allows them to design core technology and port it to different models, with little effort and using the same production line. Although that's good, you need to be sure that a card is actually designed for the machine you are working on, even if it fits.
Keep in mind that PCI is evolving. That fact will help to keep it a viable interface for the foreseeable future, but it might also lead to incompatibilities between new cards and older machines.