Local Bus Interfaces

Intel 80286 motherboards were capable of running expansion slots and the processor at different speeds over the same bus. However, dating from the introduction of the 386 chip in 1987, motherboards provided two bus systems. In addition to the official bus – whether ISA, EISA or MCA – there was also a 32-bit system bus connecting the processor itself to the main memory. It was the rise in popularity of the Graphical User Interface (GUI) – such as Microsoft Windows – and the consequent need for faster graphics that originally drove the concept of local bus peripherals. The bus by which they were connected was commonly referred to as the local bus because its high speed and the delicate nature of the processor means that it can only function over short distances.

Initial efforts to boost speed were proprietary: manufacturers integrated the graphics and hard disk controller into the system bus. This achieved significant performance improvements but limited the upgrade potential of the system. As a result, in the early 1990s, a group of graphics chipset and adapter manufacturers, the Video Electronics Standards Association (VESA), established a non-proprietary high-performance bus standard. Essentially, this extended the electronics of the 486 system bus to include two or three expansion slots: the VESA Local Bus (VL-Bus). The VL-Bus worked well and many cards became available, predominately graphics and IDE controllers.

The main problem with VL-Bus was its close coupling with the main processor. 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 the motherboard.

The 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 newer chips such as the 40MHz and 50MHz 486s and the new 60/66MHz Pentium created the perfect conditions for Intel’s PCI (Peripheral Component Interconnect).