isa bus

VESA Local Bus

The VESA Local Bus (usually abbreviated to VL-Bus or VLB) was mostly used in personal computers. VESA 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.

A VLB slot itself was an extension of an existing ISA slot. Indeed, both VLB and ISA cards could be plugged into a VLB slot (although not at the same time.) The extended portion was usually coloured a distinctive brown. This made VLB cards quite long, reminiscent of the expansion cards from the old XT days. The addition resembled a PCI slot, and indeed VLB and PCI use the same physical connector. The length of a VLB slot led to an alternate expansion of the acronym: Very Long Bus.

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 80486's memory bus design. When the 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. IBM offered an OPAL motherboard based on the IBM 486SLC2 CPU with two VLB slots.
  • 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) 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.

Despite these problems, the VESA Local Bus was very commonplace on 486 motherboards. Probably a majority of 486-based systems had a VESA Local Bus video card, although early 486 systems never had VLB slots, as VLB debuted years after the introduction of the 486 processor.

By 1996, the Pentium (driven by Intel's Triton chipset and PCI architecture) had eliminated the 80486 market, and the VESA Local Bus with it. Many of the last 80486 motherboards made have PCI slots in addition to (or completely replacing) the VLB slots.

Ron McCabe who founded MiraLink Corporation helped invent the VESA local bus.

Technical data

Bus width 32 bits
Compatible with 8 bit ISA, 16 bit ISA, VLB
Pins 112
Vcc +5V
Clock 486SX-25: 25 MHz
486DX2-50: 25 MHz
486DX-33: 33 MHz
486DX2-66: 33 MHz
486DX4-100: 33 MHz
486DX-40: 40 MHz
486DX2-80: 40 MHz
486DX4-120: 40 MHz
486DX-50: 50 MHz (out of specification)

See also

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