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Connecting too many devices risked interfering with the processor itself, particularly if the signals went through a slot. VESA recommended that only two slots be used at clock frequencies up to 33MHz, or three if they are electrically buffered from the bus.At higher frequencies no more than two devices should be connected, and at 50MHz or above they should both be built into themotherboard.
Tathagata Bhattacharjee18The fact that the VL-Bus ran at the same clock frequency as the host CPU became a problem as processor speeds increased. The faster the peripherals are required to run, the more expensive they are, due to the difficulties associated with manufacturing high-speed components. Consequently, the difficulties in implementing the VL-Bus on chips such as the 40MHz and 50MHz 486s and higher clock speed Pentiums created the perfect conditions for Intel's PCI (Peripheral Component Interconnect).
Tathagata Bhattacharjee19PCIThe Peripheral Component Interconnect(PCI) bus was developed by Intel and introduced in 1993. A typical PCI card
Tathagata Bhattacharjee20The PCI Local Bus is a high performance bus for interconnecting chips, expansion boards, and processor/memory subsystems.It originated at Intel in the early 1990s as a standard method of interconnecting chips on a board. It was later adopted as an industry standard administered by the PCI Special Interest Group, or "PCI SIG". Under the PCI SIG the definition of PCI was extended to define a standard expansion bus interface connector for add-in boards.
Tathagata Bhattacharjee21PCI BUSIntel's original work on the PCI standard was published as revision 1.0 and handed over to a separate organization, the PCI SIG (Special Interest Group). The SIG produced the PCI Local Bus Revision 2.0 specification in May 1993: –it took in the engineering requests from members, and gave a complete component and expansion connector definition, something which could be used to produce production- ready systems based on 5 volt technology.
Tathagata Bhattacharjee22Beyond the need for performance PCI sought to make expansion easier to implement by offering plug and play (PnP) hardware ––a system that enables the PC to adjust automatically to new cards as they are plugged in, obviating the need to check jumper settings and interrupt levels.
Tathagata Bhattacharjee23Windows-95, launched in the summer of that year, provided operating system software support for plug and play and all current motherboards incorporate BIOSeswhich are designed to specifically work with the PnP capabilities it provides. By 1994 PCI was established as the dominant Local Bus standard.