Motherboard; Power Supply; Processor Stack; Fans - HP 9020 Service Manual

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Theory of Operation
Motherboard
The motherboard is positioned in the base of the computer. The motherboard has six layers, with
one layer used for the ground plane. Although the motherboard has no active components, it is the
primary element of the power and lOP bus distribution network.
The motherboard has 14 shrouded pin headers. Modules and cables that plug into the mother-
board have receptacle headers.
Power Supply
The power supply plugs directly into the motherboard. The motherboard interconnects the power
supply boards, distributes power and voltage sense lines from the supply, and provides logic and
sequencing signals to the supply.
The power supply interconnects with the motherboard via six motherboard connectors. Each of the
three power supply boards plugs into two motherboard connectors. The boards and the associated
motherboard connectors are as follows:
Primary Board: J42 (12-pin)
J43 (70-pin)
Secondary Board: J46 (36-pin)
J47 (50-pin)
Supervisor Board: J44 (24-pin)
J45 (50-pin)
Processor Stack
The processor stack plugs directly into the motherboard via two 25-pin motherboard connec-
tors, J48 and J49. The motherboard distributes power and power-up sequencing signals to the
stack.
Fans
A 7 -pin motherboard connector, J52, distributes power and ground signals to the three cooling
fans.
Mass Storage
The motherboard has two 8-pin connectors, J53 and J54, to send power and ground signals to
the flexible disc drive and/or fixed disc drive.
I/O Backplane
The
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backplane plugs directly into the motherboard connector J40. This 96-pin connector
ties the lOP bus signals and power and ground signals to the backplane for distribution to
various modules.