PCI BUSES

The Peripheral Component Interface (PCI). This bus was introduced by Intel Corporation in July 1992, a little more than a month before the VESA Local Bus specifications were announced. It was designed primarily for high speed operation of the expansion bus. It was released again as PCI Release 2.0 in May 1993. PCI is the most popular 'bus' in the mid-1990s and is usually combined on a motherboard with an ISA or EISA expansion bus. For example, many motherboards have a number of pure ISA or EISA slots, some PCI slots, and one or more PCI/ISA combination slots.

The PCI bus was originally supposed to be a "local bus", but it is said it is a high speed interconnection system. However, it runs at superior speed, for instance some of the SCSI interfaces can run up to 40 MB/S transfer rates, although some books indicate the original PCI bus was a 32 bit, 33MHz bus, which could move data at up to 132MBytes/sec as a theoretical maximum - this includes overhead. The May 1993 release broadened the data path to 64 bits to conform to the Pentium processors release in 1993. Please note the second release of the VESA Local Bus standards did the same.

The PCI standard provides an interface to the ISA, EISA, and MCA buses, but PCI can replace these older buses in a motherboard design. A pure PCI bus machine is possible, but most motherboards for years will still have an interface to ISA and EISA expansion slots. Remember: Most motherboards have both PCI and ISA, or PCI and EISA, or even PCI and MCA slots. The PCI I/O controller will route traffic from the CPU to the proper bus - either the PCI bus or the ISA/EISA bus.

Some points about the PCI Bus:

  1. Intel intended the PCI to be the single industry standard for buses.
  2. It was designed for CPUs from 33 MHz clock speeds and up.
  3. PCI is basically process or independent.
  4. PCI is not a true "local" bus.
  5. PCI has 3 flow control signals which enable it to accomodate devices that cannot operate at the full speed of the PCI bus.
  6. The version 2.0 release is designed to work in PCs based on the ISA, MCA, and EISA buses.
  7. The revised standard defined compatibility so that the same board can be adapted to fit the form factor of an ISA, EISA, or MCA system.
  8. The PCI bus is ideal for the Pentium and P6 type machines because it's speed is theoretically 528MB/s at the 66MHz bus speed and 64 bit bus. It appears to work also with the 75 and 83MHz buses being introduced in motherboards.

The PCI Bus has an I/O Controller connected to the CPU. The system is supposed to operate as follows:

  1. Signals from the CPU go to the I/O controller for the PCI local bus operations. The controller is between any ISA or EISA controller.
  2. The PCI controller examines all signals from the CPU to determine their destination.
  3. The PCI controller routes all signals meant for the ISA/EISA/MCA bus to the controller for that bus. The speeds to the ISA/EISA/MCA bus will be at 33MHz, and then slowed to the 8-10MHz range used by the destination bus.
  4. The PCI controller routes all signals for the PCI local bus to the local bus adapter slots. The data along this path travels at 33MHZ and is 32 bits.

In effect, the traffic is routed by the I/O controller to the appropriate type of expansion slot for which it is destined. If it is to go to a card in a PCI slot, that is where it is sent. All traffic going to the CPU is collected by the PCI I/O controller and forwarded to the CPU.

PCI buses will probably become the "de facto" bus for the industry. PCI is designed to replace older buses such as ISA, EISA, and MCA. However, you will see PCI/ISA or PCI/EISA mixes for some time - there is a large base of these cards in the world; however, these mixes are already falling by the wayside. PCI and EISA combinations are becoming more common, and even this combinations will gradually disappear.

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