The 5 volt earth referenced supply (+5V_ER) is produced by rectifier
CR101, CR102, CR103, CR104, and regulator U101. This supply is
earth referenced by the screw which mounts the PC board to the
instrument chassis.
The unswitched +5V_ER supplies U305, U303, U320, and U302.
The rear-panel interfaces (GPIB, RS-232, Alarms, and external
triggers) are powered from the switched +5V_BP supply to ensure
that when power is turned off no voltages are present at the interfaces.
The +5V_ER supply is switched by Q101A and Q101B to create the
+5V_BP (backplane) and fan power supplies.
Undervoltage sensor U102 provides the earth reference controller
reset at initial power on.
Chapter 5 Theory of Operation
Power Supplies
126
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