VL BUS

VL-Bus
Short for VESA Local-Bus, a local bus architecture created by the Video Electronics Standards Association ( VESA ). Although it was quite popular in PCs made in 1993 and 1994, it has been overshadowed by a competing local bus architecture called PCI.

A VLB slot itself was an extension of an existing ISA slot. Indeed, both VLB and ISA cards could be plugged into a VLB slot (although not at the same time.) The extended portion was usually coloured a distinctive brown. This made VLB cards quite long, reminiscent of the expansion cards from the old XT days. The addition resembled a PCI slot, and indeed VLB and PCI use the same physical connector. The length of a VLB slot led to an alternate expansion of the acronym: Very Long Bus.

The VESA Local Bus was designed as a stopgap solution to the problem of the ISA bus's limited bandwidth. VLB had several flaws that served to limit its useful life substantially:

• 80486 dependence. The VESA Local Bus relied heavily on the 80486's memory bus design. When the Pentium processor started to gain mass acceptance, circa 1995, there were major differences in its bus design, and the VESA Local Bus was not easily adaptable. This also made moving the bus to non-x86 architectures nearly impossible. Few Pentium motherboards with VLB slots were ever made. IBM offered an OPAL motherboard based on the IBM 486SLC2 CPU with two VLB slots.

• Limited number of slots available. Most PCs that used VESA Local Bus had only one or two slots available, as opposed to 5 or 6 ISA slots. This was because, as a direct branch of the 80486 memory bus, the VESA Local Bus did not have the electrical ability to drive more than 1 or 2 (or 3 at the most) cards at a time.

• Reliability problems. The same electrical problems that limited the VESA Local Bus to 2 or 3 slots also limited its reliability. Glitches between cards were common, especially on low-end motherboards, and when important devices such as hard disk controllers were attached to the bus, there was the all-too-common possibility of massive data corruption.

• 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.

Despite these problems, the VESA Local Bus was very commonplace on 486 motherboards. Probably a majority of 486-based systems had a VESA Local Bus video card, although early 486 systems never had VLB slots, as VLB debuted years after the introduction of the 486 processor.

By 1996, the Pentium (driven by Intel's Triton chipset and PCI architecture) had eliminated the 80486 market, and the VESA Local Bus with it. Many of the last 80486 motherboards made have PCI slots in addition to (or completely replacing) the VLB slots.

Ron McCabe who founded MiraLink Corporation helped invent the VESA local bus.

VL-BUS 486 Motherboard Settings and Configuration

VL-BUS 486

Processor	80486SX/80486DX/80486DX2
Processor Speed	20/25/33/50(internal)/50/66(internal)MHz
Chip Set	Toshiba
Max. Onboard DRAM	32MB
Cache	32/64/128/256KB
BIOS	AMI
Dimensions	254mm x 220mm
I/O Options	32-bit VESA local bus slots (2)
NPU Options	None

CONNECTIONS
Purpose	Location	Purpose	Location
Turbo LED	JP3	Turbo switch	JP10
Speaker	JP7	External battery	JP25
Power LED & keylock	JP8	32-bit VESA local bus slots	SL1 & SL2
Reset switch	JP9

USER CONFIGURABLE SETTINGS
Function		Jumper	Position
»	Turbo switch select turbo	JP10	Closed
	Turbo switch select normal	JP10	Open
»	CMOS memory normal operation	JP24	pins 2 & 3 closed
	CMOS memory clear	JP24	pins 1 & 2 closed

DRAM CONFIGURATION
Size	Bank 0	Bank 1
1MB	(4) 256K x 9	NONE
2MB	(4) 256K x 9	(4) 256K x 9
4MB	(4) 1M x 9	NONE
5MB	(4) 256K x 9	(4) 1M x 9
8MB	(4) 1M x 9	(4) 1M x 9
16MB	(4) 4M x 9	NONE
20MB	(4) 1M x 9	(4) 4M x 9
20MB	(4) 4M x 9	(4) 1M x 9
32MB	(4) 4M x 9	(4) 4M x 9

CACHE CONFIGURATION
Size	Bank 0	Bank 1	TAG
32KB	(4) 8K x 8	NONE	(1) 8K x 8
64KB	(4) 8K x 8	(4) 8K x 8	(1) 8K x 8
128KB	(4) 32K x 8	NONE	(1) 8K x 8
256KB	(4) 32K x 8	(4) 32K x 8	(1) 32K x 8

CACHE JUMPER CONFIGURATION
Size	JP14	JP15	JP16	JP17	JP20	JP21	JP22
32KB	1 & 2	Open	Open	Open	2 & 3	2 & 3	1 & 2
64KB	2 & 3	Open	Open	Closed	1 & 2	2 & 3	1 & 2
128KB	1 & 2	Closed	Open	Closed	1 & 2	1 & 2	2 & 3
256KB	2 & 3	Closed	Closed	Closed	1 & 2	1 & 2	1 & 2
Note: Pins designated should be in the closed position.

CPU TYPE CONFIGURATION
Type	JP12	JP13	JP18
80486SX	pins 2 & 3 closed	Open	pins 1 & 2 closed
80486DX/DX2	pins 1 & 2 closed	Closed	pins 2 & 3 closed