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Why do all 845 and 845E Design Guides tie the SMBus to the System Management Link (SMLink) interface, and why are they isolated by FETs?

April 26, 2017design guides link Management smbus system tie

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Why do all 845 and 845E Design Guides tie the SMBus to the System Management Link (SMLink) interface, and why are they isolated by FETs?

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SMLink is an optional SMBus link for external system management ASIC or LAN controllers. Typically, SMBus is powered by the 3.3V power plane, while SMLink is powered by the 3.3V Standby power plane. Systems that implement standby voltages use the isolation to separate devices that can wake up a sleeping system from those that cannot. For example, a LAN controller with Wake On LAN capabilities is isolated from a DIMM SPD that does not have such capability. Because of the FET separation, each power plane requires a pull-up resistor. If your system does not implement standby voltage, FET isolation is not necessary and SMLink can be connected directly with SMBus.