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|VESA Local Bus|
Multi-I/O-Controller with 1×IDE/SCSI-2/FDD/parallel/2×RS232/Game
|Superseded by||PCI (1993)|
|Width in bits||32|
|Number of devices||3|
|Capacity||33 MHz|
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.
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 buses 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.
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:
Despite these problems, the VESA Local Bus became very commonplace on later 486 motherboards, with a majority of later (post 1993) 486-based systems featuring a VESA Local Bus video card. VLB importantly offered an affordable high speed interface for consumer systems, as only by 1996 was PCI commonly available outside of the server market via the Pentium and Intel's Triton chipset. PCI finally did displace the VESA Local Bus (and also EISA) in the last years of the 486 market, with the last generation of 80486 motherboards featuring PCI slots in lieu of VLB capable ISA slots. However a few manufacturers did develop and offer "VIP" (VESA/ISA/PCI) motherboards with all three slot types.
|Bus width||32 bits|
|Compatible with||8 bit ISA, 16 bit ISA, VLB|
|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
5x86@133 MHz: 33 MHz
5x86@160 MHz: 40 MHz
486DX-50: 50 MHz (out of specification)
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