AT&T 3B2/300 Technical Reference Manual page 560

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Version 2 3B2 Computer Power
Figures 3-98 and 3-99 are functional block diagrams of the 3B2/300 and 310 computers Power
Supply and the 3B2/400 computer Power Supply. Color codes and power connector pin identification
for the power cables are shown in these figures. The 3B2 computer requires +5 volt (VCC), + 12 volt,
and -12 volt power. Logic power is +5 volt. The power supply upper trip point for VCC is between
+6.00 volt and +6.75 volt. The DUART uses +12 volt and -12 volt. The various drives (hard disk,
floppy, and cartridge tape) require +5 volt and +12 volt.
The commercial AC line is fused and filtered at the Power Supply Set input. The input AC line
voltage is selected for either 115 volt AC or 220 to 240 volt AC operation. The Voltage Select Jumper is
a factory installed jumper. Different solution packages are provided to support the different input AC
lines.
Cooling for the power supply and the 3B2 computer is provided by a current load sensitive fan in
the power supply assembly. As additional equipment is added to the configuration, the fan speed
increases in response to the increased load. Unfiltered air is pulled from the front and left side of the
cabinet and is exhausted at the back of the power supply.
FUNCTIONAL DESCRIPTION 3-307