Bayer HealthCare RapidPoint 400 Series, Service Manual, Page: 81 / 458

Rev. C Electronic System Overview 81
C45, C46, and U22, and U21 on-board oscillator circuit, make up the
system clock. The nominal frequency (11.0592 MHz) facilitates the
generation of the 9600-baud serial signal. U27 translates the serial signal
from TTL to RS-232 signal levels and vice versa.
A µC monitor IC, U11, provides a reset pulse of at least 140 mS on
power-up, power-down, and low-voltage brown-out conditions. In this
configuration, it holds µC in reset when the +5 V power supply goes below
approximately 4.65 V. U12 also uses this signal to hold its output latches in
a known state until the µC has established control of the circuit.
U13, a 32K x 8 static RAM device, provides memory beyond what is
available on the µC. This additional memory is intended for the
downloadable font feature. U14, a 128K x 8 flash EEPROM device, is for
program memory.
U10 is a power logic, octal, D-type latch.
Touch Screen Switches
Transistors Q2 to 9, along with their associated diodes, resistors, and
capacitors, are the switches that interface with the touch screen.
DC-DC Converter Circuit
U19 is a synchronous-switching regulator controller that converts the
+5 V power supply input to the display board to the +7 V power supply that
powers the thermal printer. U20, R48 to 51, C29, and C39 form a
+12 V regulator to power U19.
LCD Backlight Inverter Circuit
U15 is the module that provides the power to run the LCD backlight. The
backlight typically requires a starting voltage of ~1100 V
RMS
and a steady
state voltage of ~500 V
RMS
at a supply current of ~6-7 mA
RMS
. U24 and its
associated drive circuitry provide the ability to turn the backlight on and off.