VL BUS

11/1/97

VESA Local Bus: VLB was designed by a consortium of PC manufacturers in order to provide a high speed replacement for the EISA bus suitable for graphics and hard disk I/O for commodity 486 computers. VLB added yet another connector in back of the XT and AT connectors in order to allow high speed communications to the card.

The VLB was released in 1991 as a 32bit, 33MHz bus although it was unofficially pushed to 40MHz in some designs. The specification was extended to 66 MHz operation in 1994 and to a 64 bit, 66MHz configuration in the same year. Few 66MHz VLB motherboards were ever sold.

The VLB is basically an extension of the 486 signal set, and VLB is widely used on 486 motherboards. The specification permitted cards to act as bus masters -- controlling the bus signals directly instead of having them provided by the CPU. Bursting was supported. VLB motherboards usually had three VLB slots. Often one or two slots were "slave" slots which could not accommodate bus mastering cards. The presence of second and third VLB cards may slow the maximum speed of the first card to some extent. The extreme length of the VLB cards caused some difficulty in that motherboard flexing sometimes loosened the cards if the PC was lifted or moved. XT or AT cards will run in the forward connectors of a VLB slot.

The maximum theoretical speed of VLB is 136 MBytes/sec for 32 bit, 33 MHz operation. For 66MHz operation, maximum rates are 272 MBytes/Sec for the 32 bit variant and 544 MBytes/Sec for the 64 bit variant. Practical maxima are, as for all other buses, much lower.

Return To Index Copyright 1994-2002 by Donald Kenney.