VESA Local Bus
The VESA Local Bus ( VLB ) is a standardized by the Video Electronics Standards Association Local Bus. He came in 1992 to market, saw its golden age from 1993 to 1994 and disappeared in 1995 again. It was used almost only for systems with i486 microprocessors. Since he is a direct lead-out of the i486 signals essentially, he was on non- i486 systems can only be achieved with great effort and therefore disappeared soon after the introduction of the Pentium processor.
Properties
The bus complements the ISA bus and allows for quick transfers from and to the memory via DMA. The VESA Local Bus was introduced because of the limited throughput of the ISA bus, the time for fast graphics cards increasingly became the bottleneck. Apart from graphics cards VLB like for hard disk controller was (IDE and SCSI, some with built-in floppy disk controller and / or built-in parallel and / or serial interfaces ) are used. Other types of cards are not needed as much data throughput and therefore were still running for VLB computer as ISA cards.
The term local bus means in that it is directly connected to the address and data lines of the processor. He therefore ensures a 32-bit data transfer. According to VESA specification V2.0 Data Transfer by address and data multiplexing has been extended to 64 bits. You can access a VLB slot card on IRQ and I / O lines on the ISA bus, hence the VLB is inextricably connected to the ISA bus. Therefore VLB slots consist of a normal two-piece 16 -bit ISA slot (usually black ) and an adjoining towards the computer front third slot section with closer spaced pins (often brown). By this arrangement VLB cards are necessarily quite long. In VLB slots and conventional ISA cards were used when needed, the third portion remains blank.
According to VESA specification only a maximum of three VLB slots may be present and the clock frequency must not exceed 40 MHz on the motherboard. The clock frequency is derived here from the external clock frequency of the processor used. By this fact, the use of an i486 processor with 50 MHz external clock frequency is extremely critical, as this could cause a malfunction in the VLB cards. Often on the motherboard near the VLB slots a jumper in place which for VLB cards a wait state (WS ) can be set, but making them work slower, but more stable. Up to a clock frequency of 33 MHz is not normally a wait state is required ( Jumper = 0 WS), above this frequency, a wait state (jumper = 1 WS ) stabilize the system.
VLB motherboard for the Intel Pentium were produced only in small quantities due to the large circuit-related expenses. This is also a problem that has to be operated at the non-specified frequency of 30 MHz, when using a 60 MHz Pentium or Pentium -S with 90 MHz VL bus. Furthermore VLB cards were partially incompatible with each other through ongoing changes to the VESA definitions. The industry could not rely on the fact that there would be some point a uniform standardization.
Intel chose not to support the VESA bus and instead developed the processor independent PCI bus and an adapter circuit, the PCI - ISA bridge. The latter allows the use of ISA cards, but none VLB cards in any PCI system, provided that the manufacturer installs corresponding so-called "legacy" slots. The VESA Local Bus was therefore granted only a short period of success.
VIP boards
From the late period of the VL- bus, there are so-called VIP boards. VIP stands for VL / ISA / PCI. These have, in addition VL-Bus and ISA bus slots and PCI bus slots. Thus, the use of existing ISA and VLB cards was just as possible as the use of the then new and therefore less widely used PCI cards.