Editing VESA Local Bus (section)

==Limitations==
[[File:Vlb.jpg|thumb|right|upright=1.5|Computer [[motherboard]] with 7 [[Industry Standard Architecture|ISA]] slots of various feature levels. The top three are 16-bit ISA. The middle three are VLB; 16-bit ISA with the added slot (leftmost brown sections). The bottom (shorter) slot is 8-bit ISA. A card installed in this motherboard would have its mounting bracket on the right, which normally would be the "back" of the computer case.]]

The VESA Local Bus was designed as a [[wiktionary:stopgap|stopgap]] solution to the problem of the ISA bus's limited [[Bandwidth (computing)|bandwidth]]. As such, one requirement for VLB to gain industry adoption was that it had to be a minimal burden for manufacturers 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 ties 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.<ref name="Infinite expansion" />

This simplicity of VLB unfortunately created several factors that served to limit its useful life substantially:

; 80486 dependence
: The VESA Local Bus relies heavily on the [[Intel 80486]] [[CPU]]'s memory bus design.<ref>{{cite web |url=https://www.karlstechnology.com/blog/vesa-local-bus/ |title=VESA Local Bus |publisher=The PC Guide |last=Kozierok |first=Charles  |date=24 October 2018 |access-date=May 27, 2019}}</ref>{{Failed verification|date=June 2019}} When the [[Pentium (brand)|Pentium]] processor arrived, there were major differences [[P5 (microarchitecture)|in its bus design]], not easily adaptable to a VESA Local Bus implementation.  Few Pentium motherboards with VLB slots were ever made and use VLB-to-PCI bridges such as the [[OPTi]] 82C822.<ref>{{Cite web |url=http://bitsavers.informatik.uni-stuttgart.de/pdf/opti/dataSheets/82C822_VESA_to_PCI_Apr94.pdf |title=Archived copy |access-date=2016-06-28 |archive-date=2016-08-06 |archive-url=https://web.archive.org/web/20160806064640/http://bitsavers.informatik.uni-stuttgart.de/pdf/opti/dataSheets/82C822_VESA_to_PCI_Apr94.pdf |url-status=dead }}</ref>  This also meant that moving the bus to a computer with a non-[[x86 architecture|x86]] architecture was nearly impossible, within practical economic constraints.<ref name="vlb"/>
; Limited number of slots available
: Most PCs that use VESA Local Bus have only one or two VLB-capable ISA slots out of the total five or six available; thus, four ISA slots generally are just that, ISA-only.  This is a result of VESA Local Bus being a direct branch of the 80486 memory bus.  The processor does not have sufficient electrical capacity to correctly drive (signal and power) more than two or three devices at a time directly from this bus.<ref name=vlb>{{cite web |url=https://www.karlstechnology.com/blog/vlb-for-x86/ |title=VESA Local Bus |publisher=The PC Guide |last=Kozierok |first=Charles |date=4 October 2018 |access-date=May 27, 2019}}</ref>
; Reliability problems
: The strict electrical limitations on the bus also reduce any safety margin available, negatively influencing reliability. Glitches between cards are common, as the interaction between individual cards, combinations of cards, motherboard implementation, and even the processor itself are difficult to predict. This is especially prevalent on lower-end [[motherboard]]s, as the addition of more VLB cards could overwhelm an already marginal implementation. Results can be rather spectacular when often important devices such as [[hard disk]] controllers are involved with a bus conflict with a memory-intensive device such as the ubiquitous video card.<p>As VLB devices have direct high-speed access to system memory at the same level as the main processor, there is no way for the system to intervene if devices were mis-configured or became unstable. If two devices overwrite the same memory location in a conflict, and the hard-disk controller relies on this location (the HDD controller often ''being'' the second conflicting device), there is the all-too-common{{fact|date=April 2022}} possibility of massive [[data corruption]].</p>
; Limited scalability
: As bus speeds of 486 systems increased, VLB stability became increasingly difficult to manage. The tightly coupled local bus design that gives VLB its speed became increasingly intolerant of timing variations, notably past 40&nbsp;MHz. Intel's original [[Intel 80486#Models|50&nbsp;MHz 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 50&nbsp;MHz. If one could achieve reliable operation of VLB at 50&nbsp;MHz, it was faster{{snd}} but again, this was notoriously difficult to achieve, and often it was discovered not to be possible with a given hardware configuration.<ref>BrainBell.com [http://www.brainbell.com/tutors/A+/Hardware/VESA_Local_Bus_VLB.htm "A+ Tutorials > Expansion Buses > VESA Local Bus (VLB)"], accessed January 8, 2012.</ref><p>The 486DX-50's successor, the 486DX2-66, circumvents this problem by using a slower but more compatible bus speed (33&nbsp;MHz) and a [[CPU multiplier|multiplier]] (×2) to derive the processor clock speed.</p>
; Difficulty of installation
: The length of the slot and number of pins makes VLB cards notoriously difficult to install and remove.<ref>{{cite book |last1=Slone |first1=John P. |title=Local Area Network Handbook, Sixth Edition |date=28 September 1999 |publisher=CRC Press |isbn=9780849398384 |page=43 |url=https://books.google.com/books?id=AAnzJXtXQTUC&pg=PA43 |language=en}}</ref> The sheer mechanical effort required is stressful to both the card and the motherboard, and breakages are not uncommon. This is compounded by the extended length of the card logic board; often there is 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 is not always the case, and the person inserting the board had to distribute the downward force evenly across its top edge.<p>Due to the length of a VLB slot and the difficult installation that results from its length, a slang alternative use of the acronym VLB is ''Very Long Bus''.<ref>{{cite web |url=http://www.vintagecomputing.com/index.php/archives/1069/retro-scan-of-the-week-the-micron-millennia |title= The Micron Millennia |publisher=Vintage Computing and Gaming Adventures in Classic Technology |last=Edwards |first=Benj |access-date=May 27, 2019}}</ref></p>