has caused the interrupt. Depending on the selected mode or
application, actions will be taken or the interrupt will be ignored.
Keyboard Control
The keyboard is scanned by the ROWO to ROW3 and the COLO
to COL3 lines. Any changes on the keyboard will be recognized by
U601 and the inforrnation will be forwarded to the microprocessor.
Display Data and Brightness
The current contents of the display are stored in the display pattern
RAM, U602. The data are serialized by U601 and sent as signal
DSHlFT to the shift registers on the display board (schernatic 4).
Each display refresh is carried out over 8 cycles. Each of the 8
cycles can be enabled or disabled. li ali 8 cycles are enabled, full
display brightness is reached. Depending of the number of enabled
cycles different brightness settings can be achieved. The brightness
inforrnation is stored in RAM U602.
Display Voltage Supply.
The supply voltages for the display are
generated by U603 and T601. U603 works as an DC-AC converter
and is enabled by signal LVFDEN from the gate array U601. T601
generates the DC supply voltage VDlSP (+38 Volt) and the heating
current for the display,
Power Supply Board
Primary regulation
1-4 Theory of Operation
This information deals with schematics 3A and 3B.
The power supply in the HP 8153A is a switched supply with a line
input voltage range from 100 to 240 Volt AC.
Regulation is controlied by a switching regulator lC (U102) which
determines the on-time of switching transistor Q101. This on-time is
dependent on the line voltage and the DC load conditions.
During the power-on phase the supply voltage for UI02 is derived
via RI07 and RI08 until the start level of UI02 is reached. Then
the supply voltage is generated from TIOI, rectifìed by CRI05 and
applied to pin 8 of UI02. CI09 at UI02 pin 7 forces the switching
regulator to start with a reduced duty cycle (soft start).
The duty cycle of the switching circuit UI02 and QIOI is mainly
determined by the line voltage. This is sensed at UI02 pin 2 via
R111 and R112. Decreasing current from Rl l l and R112 increases
the duty cycle at U102 pin 5 and therefore the on-time of Q101.
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