Chapter 1 Introduction To Vl-Bus - American Megatrends Enterprise-III User Manual

Chapter 1
Introduction to VL-Bus
The Series 68 Enterprise-III EISA Local Bus Motherboard conforms to the VESA® (Video Electronics
Standards Association) VL-Bus® specifications as well as the EISA (Extended Industry Standard
Architecture) specification.
VL-Bus Slots are Buffered
The American Megatrends Enterprise-III VL-Bus implementation is unique. The American Megatrends VL-
Bus design fully buffers both VL-Bus expansion slots. This buffering is totally transparent to the end user
and preserves complete VL-Bus compatibility.
Advantages of Buffered VL-Bus
Because VL-Bus Adapter Cards sit directly on the CPU bus, the additional loading on the CPU signals can
cause unreliable CPU operation. At 33 MHz, most CPU signals have Setup/Hold times in the order of 10
seconds. It is easy to see how timing can be thrown off by the additional load from devices on the VL-Bus.
The American Megatrends VL-Bus design includes full buffering for all VL-Bus expansion slots. Each VL-
Bus expansion slot always appears as a fixed non-capacitive load on the CPU bus. Therefore, the end user
can install any type of VL-Bus-compatible Adapter Card on the system and still be certain of totally reliable
operation.
Enterprise-III EISA VLB Motherboard User's Guide
-9
1