Agilent Technologies 16900A, Service Manual

Page: 111 / 130

Theory of Operation
8
16900A, 16902A, and 16903A Service Guide 111
16900A/16902A MIB (Module Interface Board)
Top level function
The MIB is primarily an interconnect and power distribution
board. It performs some low level system monitoring and
control functions such as power ON/OFF.
PLD
The PLD is the low level system control element powered
from the AUX +5 V rail and continuously ON. It logically
controls the soft power ON/OFF through signal lines to the
CPU and power switch monitoring. It physically turns
power ON/OFF via power on/off signals on the power supply
sense cables. In addition to the power switch starting a
power down sequence, two additional monitoring circuits
(overtemp and voltage rail) may cause the PLD to initiate
power down. Both of these conditions are "latched" faults
and require removing the power cord for 15 seconds to clear
the condition.
Voltage rail monitor
All six voltage rails are continuously monitored for an under
voltage fault condition. This could be caused by such things
as a failed power supply or a short circuit within the
instrument. When detected, both power supplies are
immediately shut down. At power up, there is a one second
time delay to allow the power to stabilize before the
monitoring circuit is enabled.
Overtemp monitor
A temperature monitoring circuit protects the instrument
from thermal damage due to things such as failed fans or
blocked frame vents. The circuit first notifies the CPU of a
problem which then prompts the user with a warning
message and initiates a soft power down. If the soft power
down fails, a forced power down is initiated after about two
minutes.
Fan control
The instrument uses a pair of 24 V fans driven from a
DC/DC boost converter circuit. This circuit creates a linear
voltage ramp from 12 V to 24 V across the operating