VESA Local Bus - Historical Overview

Historical Overview

In the early 1990s the I/O bandwidth of the ISA bus was becoming a critical bottleneck to PC graphics performance. The need for faster graphics was being driven by increasing adoption of Graphical User Interfaces in PC operating systems. While IBM's attempt at producing a successor to ISA with the Micro Channel Architecture was a technically viable option, it failed in the market due to its proprietary nature and imposed licensing fees. The competing EISA open standard was still unable to offer enough performance improvement over ISA to provide a solution. Thus for a short time, hardware producers created proprietary implementations of local busses on their motherboards to give graphics cards direct access to the processor and system memory - and avoid the limitations of the ISA bus. However as these manufacturer specific solutions were not standardized, there were no provisions for providing interoperability between them. This led to the VESA consortium proposing and defining a Local Bus standard in 1992. Additionally while greater graphics card performance was a primary goal of VLB, other devices could also benefit from the VLB standard; notably many mass storage controllers were offered for VLB with increased hard disk performance.

A "VLB slot" itself was simply an additional edge connector placed in-line with the traditional ISA or EISA connector, with this extended portion often colored a distinctive brown. The result was a normal ISA or EISA slot being additionally capable of accepting VLB compatible cards. Traditional ISA cards remained compatible as they would not have pins past the normal ISA or EISA portion of the slot. The reverse was also true - VLB cards were by necessity quite long in order to reach the VLB connector, and were reminiscent of older full-length expansion cards from the earlier IBM XT era. Ironically the VLB portion of a slot looked similar to a IBM MCA slot, as indeed it was the same physical 116 pin connector used by MCA cards rotated by 180 degrees. The IBM MCA standard had not been as popular as IBM expected and there was an ample surplus of the connector, making it inexpensive and readily available.