VESA Local Bus
VESA Local Bus | |
VESA | |
Superseded by | PCI (1993) |
---|---|
Width in bits | 32 |
No. of devices | 3[1] |
Speed | 25–40 MHz |
Style | Parallel |
Hotplugging interface | no |
External interface | no |
The VESA Local Bus (usually abbreviated to VL-Bus or VLB) is a short-lived
Historical overview
In the early 1990s, the
Thus for a short time, a market opening occurred where video card manufacturers and motherboard chipset makers created their own proprietary implementations of local buses to provide graphics cards direct access to the processor and system memory. This avoided the limitations of the ISA bus while being less costly than a "licensed IBM MCA machine". It is important to note that at the time the cost to migrate to an MCA architecture machine from an ISA machine was substantial. MCA machines generally did not offer ISA slots, thus a migration to MCA architecture meant that any prior investment in ISA cards was made unusable. Additionally, makers of MCA-compatible cards were subject to IBM's licensing fees, which combined with MCA's greater technical requirements and expense to implement. It did have the effect of making an MCA version of a peripheral card significantly more expensive than its ISA counterpart.
So while these ad-hoc manufacturer-specific solutions were effective, they were not standardized, and there were no provisions for providing interoperability. This drew the attention of the
Implementation
A "VLB slot" itself is 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 is a normal ISA or EISA slot being additionally capable of accepting VLB-compatible cards. Traditional ISA cards remain compatible, as they do not have pins past the normal ISA or EISA portion of the slot. The reverse was also true – VLB cards are 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. The VLB portion of a slot looks similar to an IBM MCA slot, as indeed it is 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.[citation needed]
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 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.[1]
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 x86 architecture was nearly impossible, within practical economic constraints.[5]
- 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.[5]
- 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 hard disk controllers are involved with a bus conflict with a memory-intensive device such as the ubiquitous video card..
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[citation needed] 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 gives VLB its speed became increasingly intolerant of timing variations, notably past 40 MHz. Intel's original 50 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 MHz. If one could achieve reliable operation of VLB at 50 MHz, it was faster – but again, this was notoriously difficult to achieve, and often it was discovered not to be possible with a given hardware configuration.[6](×2) to derive the processor clock speed.
The 486DX-50's successor, the 486DX2-66, circumvents this problem by using a slower but more compatible bus speed (33 MHz) and a multiplier
- Difficulty of installation
- The length of the slot and number of pins makes VLB cards notoriously difficult to install and remove.[7] 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.
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.[8]
Legacy
Despite these problems, the VESA Local Bus became very commonplace on later 486 motherboards, with a majority of later (post-1992) 486-based systems featuring a VESA Local Bus video card. VLB importantly offers a less costly high-speed interface for mainstream systems, as only by 1994 was PCI commonly available outside of the server market through the Pentium and Intel's chipsets. PCI finally displaced 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 instead of VLB-capable ISA slots. However, some manufacturers did develop and offer "VIP" (VLB/ISA/PCI) motherboards with all three slot types.
Technical data
Bus width | 32 bits |
---|---|
Compatible with | 8 bit ISA, 16 bit ISA, VLB |
Pins | 112 |
Vcc | +5 V |
Clock |
|
Bandwidth |
|
See also
- Industry Standard Architecture (ISA)
- Extended Industry Standard Architecture (EISA)
- Micro Channel architecture (MCA)
- NuBus
- Peripheral Component Interconnect (PCI)
- Accelerated Graphics Port (AGP)
- PCI Express (PCIe)
- List of device bandwidths(A useful listing of device bandwidths that include VLB)
References
- ^ a b Schuytema, Paul. "Infinite expansion. (computer buses)". Atari Magazine, COMPUTE! ISSUE 158 / NOVEMBER 1993 / PAGE 68. Retrieved May 27, 2019.
- ^ Richter, Jake. "Local-bus architecture: A little-understood, much-cited graphics technology", InfoWorld, May 18, 1992, accessed March 9, 2011.
- ^ Kozierok, Charles (24 October 2018). "VESA Local Bus". The PC Guide. Retrieved May 27, 2019.
- ^ "Archived copy" (PDF). Archived from the original (PDF) on 2016-08-06. Retrieved 2016-06-28.
{{cite web}}
: CS1 maint: archived copy as title (link) - ^ a b Kozierok, Charles (4 October 2018). "VESA Local Bus". The PC Guide. Retrieved May 27, 2019.
- ^ BrainBell.com "A+ Tutorials > Expansion Buses > VESA Local Bus (VLB)", accessed January 8, 2012.
- ISBN 9780849398384.
- ^ Edwards, Benj. "The Micron Millennia". Vintage Computing and Gaming Adventures in Classic Technology. Retrieved May 27, 2019.