VESA Local Bus

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The VESA Local Bus (usually abbreviated to VL-Bus or VLB) was mostly used in personal computers. VESA (Video Electronics Standards Association) Local Bus worked alongside the ISA bus; it acted as a high-speed conduit for memory-mapped I/O and DMA, while the ISA bus handled interrupts and port-mapped I/O.


Historical overview

A VLB slot itself was an extension of an existing ISA slot. Indeed, either a VLB or an ISA card could be plugged into a VLB slot. The extended portion was usually colored a distinctive brown. This made VLB cards quite long, reminiscent of the ISA expansion cards from the old XT days (which were long because of low component density and high chip count requiring more printed circuit board space, rather than because of a long edge connector.) The addition resembled a PCI slot, and indeed VLB and PCI use the same physical connector.

The VESA Local Bus was designed as a stopgap solution to the problem of the ISA bus's limited bandwidth. VLB had several flaws that served to limit its useful life substantially:

  • 80486 dependence. The VESA Local Bus relied heavily on the Intel 80486 CPU's memory bus design. When the P5 Pentium processor started to gain mass acceptance, circa 1995, there were major differences in its bus design, and the VESA Local Bus was not easily adaptable. This also made moving the bus to non-x86 architectures nearly impossible. Few Pentium motherboards with VLB slots were ever made.
  • Limited number of slots available. Most PCs that used VESA Local Bus had only one or two slots available, as opposed to 5 or 6 ISA slots. This was because, as a direct branch of the 80486 memory bus, the VESA Local Bus did not have the electrical ability to drive more than 1 or 2 (or 3 at the most)[citation needed] cards at a time.
  • Reliability problems. The same electrical problems that limited the VESA Local Bus to 2 or 3 slots also limited its reliability. Glitches between cards were common, especially on low-end motherboards, and when important devices such as hard disk controllers were attached to the bus, there was the all-too-common possibility of massive data corruption.
  • Installation woes. The length of the slot and number of pins made VLB cards notoriously difficult to install and remove. The sheer mechanical effort required was stressful to both the card and the motherboard, and breakages were not uncommon. This was compounded by the extended length of the card logic board; often there was not enough room in the PC case to angle the card into the slot, requiring it to be pushed with great force straight down into the slot. To avoid excessive flexing of the motherboard during this action the chassis and motherboard had to be designed with good, relatively closely spaced supports for the motherboard, which was not always the case, and the person inserting the board had to distribute the downward force evenly across its top edge. The length of a VLB slot, and the difficult installation that resulted from it, led to an alternate expansion of the acronym: Very Long Bus.

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