VESA Local Bus

VLB is a motherboard slot type created by VESA (Video Electronics Standards Association) before PCI slots were created.

At the time it was released the most popular motherboard slot type was ISA, which had a maximum theoretical transfer rate of only 8 MB/s. VESA wanted PCs with a faster connection to the video card. VLB was a high-speed extension to ISA.

VLB slot is connected directly to the CPU external bus thus its performance is the same from the CPU local bus. For example, a 486DX-33, a 486DX2-66 and a 486DX4-100 have a 33 MHz x 32 bits (132 MB/s) local bus, so VLB slots run at this rate when one of these CPUs are installed on the motherboard. On computers based on 486DX-25, 486DX2-50 or 486DX4-75, the local bus will run at 25 MHz x 32 bits (100 MB/s) and the VLB slots will run at this rate.

VLB slots were short-lived exactly because they are connected directly to the CPU external bus: whenever a new local bus standard was released, VLB had to be redesigned. That is exactly what happened when Pentium processor was released, since it runs externally at 64 bits and not at 32 bits as occurred with 486 and 386.

Three kinds of peripheral boards benefited from VLB: video cards, LAN cards and hard disk drive host cards (IDE and SCSI).

This bus was quickly replaced by PCI when the later was released.

click to enlarge
Figure 1: Motherboard with VLB slots.