VESA Local Bus - Limitations

Limitations

The VESA Local Bus was designed as a stopgap solution to the problem of the ISA bus's limited bandwidth. As such, one requirement for VLB to gain industry adoption was that it had to be a minimal burden for manufactures to implement, in terms of board re-design and component costs - otherwise manufacturers would not have been convinced to change from their own proprietary solutions. As VLB fundamentally tied a card directly to the 486 processor bus with minimal intermediary logic (reducing logic design and component costs), timing and arbitration duties were strongly dependent on the cards and CPU. This simplicity of VLB unfortunately created several factors 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 Pentium processor arrived there were major differences in its bus design, and was not easily adaptable to a VESA Local Bus implementation. Few Pentium motherboards with VLB slots were ever made. Also moving the bus to non-x86 architectures was nearly impossible.

• Limited number of slots available. Most PCs that used VESA Local Bus had only one or two VLB capable ISA slots from the 5 or 6 available (thus 4 ISA slots generally were just that, ISA only). This was a result of VESA Local Bus being a direct branch of the 80486 memory bus. The processor did not have the electrical ability to correctly drive (signal and power) more than 2 or 3 devices at a time directly from this bus.

• Reliability problems. The strict electrical limitations on the bus also reduced any "safety margin" available - negatively influencing reliability. Glitches between cards were common, as the interaction between individual cards, combinations of cards, motherboard implementation, and even the processor itself was difficult to predict. This was especially prevalent on lower-end motherboards, as the addition of more VLB cards could overwhelm an already marginal implementation. Results could be rather spectacular when often important devices such as hard disk controllers were involved with a bus conflict with a memory intensive device such as the ubiquitous video card. As VLB devices had direct high-speed access to system memory at the same level as the main processor, there was no way for the system to intervene if devices were mis-configured or became unstable. If two devices overwrote the same memory location in a conflict, and the hard disk controller relied on this location (the HDD controller often being the 2nd conflicting device) there was the all-too-common possibility of massive data corruption.

• Limited scalability. As bus speeds of 486 systems increased, VLB stability became increasingly difficult to manage. The tightly coupled local bus design that gave VLB its speed became increasingly intolerant of timing variations - notably past 40mhz. Intel's original 50mhz 486 processor faced difficulty in the market as many existing motherboards (even non-VLB designs) did not cope well with the increase in front side bus speed to 50mhz. If one could achieve reliable operation of VLB at 50mhz it was extremely fast - but again this was notoriously difficult to achieve, and often it was discovered not to be possible with a given hardware configuration. The 486DX-50's successor, the 486DX2-66 circumvented this problem by using a slower but more compatible bus speed (33mhz) and using a multiplier (x2) to derive the processor clock speed.

• 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.