VESA provided a standardized connector and protocol for local bus expansion system for PCs. VESA also announced a second generation VL Bus standard (Version 2) in 1992. This revision redefined the maximum number of VL Bus slots from 3 in the original specification to 3 slots at 40MHz and 2 slots at 50MHz in a circuit.

Its speed was originally set at up to 50MHz, and is a 32-bit card. VESA cards have two sets of connectors, one set is based on the 16 bit ISA slot, and a second set of 36 pairs of connectors that carry local bus information.

Let me make a few points about the design objectives of the VL Bus before we discuss how it actually works:

1. First, high resolution graphics revealed the inability of the ISA bus to transfer data quickly enough.
2. Early design of local buses were propriatory, and were incompatible.
3. The VESA Local Bus's design allowed other high volume devices such as hard disk drives to use the bus.
4. The VL Bus was designed around the Intel 486 processor's needs.
5. The VL Bus's high speed was its primary selling point. It's maximum speed is 66MHz.
6. At the 33MHz level, with the 32-bit design, the original VL Bus speed was a theoretical 132 mb/s! It had burst mode or non-burst mode. Burst mode means that the bus uses a single address cycle followed by four data cycles. This means it takes 5 clock cycles to transfer four double-words of 32-bits each. So you transfer 4x32bits (128bits) each 5 clock cycles, which translates to about 105 MB/s of real data. The VL Bus is slower in non-burst mode and maxed out at about 66 mb/s because it required an address cycle for each data transfer of 32 bits, which takes 2 clock cycles.
7. VL Bus Version 2.0 allowed 3 40MHz slots and 2 50MHz slots if low capacitance is maintained.
8. VL Bus Version 2.0 defined a 64 bit interface. The wider bus and the higher clock speed will allow for total theoretical throughput of 400MB/s. Of course there are overhead, etc.
9. Boards designed for VL Bus Version 2.0 will work in the original VL Bus slots by specification requirement.
10. The VESA Local Bus is ideal for 486 machines. Why? Because it can handle about everything the 33MHz buses in the 486 machines can throw at it. VESA is much slower than PCI and has trouble with the 66 MHz and higher speed buses in Pentium and P6 type machines.

    The VESA operates as follows:

    1. The CPU sends signals to the I/O Controller that handles VL-Bus operations. The signals contains code and destination addresses for all the signals to follow.
    2. The I/O controller decodes the addresses to determine if the signals are for any of the local-bus adapters.
    3. Signals going to non-local bus adapters are sent to the ISA I/O controller. While the signals were passed at 33MHz and 32bits, the ISA controller will convert them to 16 bit, 8 MHz for the ISA BUS.
    4. If a signal is for the VESA local bus, the VESA controller sends a signal control signal to the adapter in that slot, telling the adapter to execute the operation requested.
    5. Once the adapter starts the operation, the VL controller lets the data for that operation pass directly from the processor to the local bus slot over a 32 bit data path, and at speeds up to 50MHz.

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