16
Resistors R54/R55 are provided as pull-ups on the
SCL/SDA lines to insure the data and clock pulses
are properly shaped. Capacitor C36 improves power
supply bypassing.
4.26.3 RESET GENERATION
In order for microcomputer U10 to properly execute
its software instructions, it must be initialized to the
start of the program when power is first turned on.
Rest pin 9 of U10 will accomplish this when it is set
“high”.
A reset pulse of approximately 200 mS is
automatically generated by “watchdog time” U11
when the VCC level rises above 4.65 volts. If VCC is
below 4.65 volts, the reset line stays “high”, keeping
the microcomputer U10 in an inactive state.
4.27 BEEPER
A small audio annunciator is driven by transistor Q5,
which in turn is controlled by U10’s output pin P2.3.
The annunciator gives a (short/long) beep(s) as
audible recognition of a key being pressed, a zero or
weight lock-in, a warning of low-low battery, etc. The
length of the beep and its various sequences are
controlled by U10’s software.
4.28 DISPLAY BOARD AND KEYBOARD
Presentation of the weight information is performed
by the display board assembly. It incorporates LED
(light-emitting diode) digits and annunciators to
provide a clear, bright, easy-to-read display. The
hardware for detecting key presses on the front panel
is also contained on this board. The scale may
incorporate a version 48091 or 50043 printed circuit
board; both are equivalent and interchangeable.
4.28.1 DIGIT DISPLAY
The weight value is displayed on six, 0.43” high
common anode digits. These are driven in a
multiplexed fashion (one digit on at a time) by LED
drivers U1 and U3. U1/U3 receive the digit display
information from microcomputer U10, by connecting
to the “I
2
C” serial data bus consisting of line SCL
(Serial Clock) and SDA (Serial Data). A data bit (a
high or low level) is sent and received on SDA when
the SCL line provides a clock pulse. U1 and U3 share
these lines with other devices within the scale by
having a unique address which is controlled by the
setting of pin “ADDR”.
U1 and U3 automatically perform the multiplexing
necessary to properly illuminate each LED digit.
Once the correct segment data is loaded into U1/U3,
the display will continue without further intervention
from U10. An internal clock to control the multiplexing
is generated by capacitors C1 and C3 on pin “C EXT”.
Transistors Q1, Q2, Q3 and Q4 are used as digit
drivers to increase the current from U1/U3. Segment
current is internally limited within U1/U3. Later
versions of the display board (p/n 500043)
incorporate resistor networks RN7/RN8/RN9 to
provide additional current limiting. Capacitors C2 and
C4 provide high-frequency bypassing of noise on the
VCC power supply lines.
4.28.2 ANNUNCIATOR DISPLAY LAMPS
A variety of LED annunciator lamps are contained on
the front panel to indicate “POUNDS”, “OUNCES”,
“KILOS”, “RECALL PRIOR WEIGHT” and “LOW
BATTERY”. (Scales equipped with the RS-232 data
port may also have the “DATA” annunciator, ANN2.)
These annunciator lamps contain multiple LED’s to
provide an evenly illuminated surface. They are
driven by integrated circuit U2. U2 receives the on/off
information for the annunciators from microcomputer
U10 via the “I
2
C” serial data port lines “SCL” (Serial
Clock) and “SDA” (Serial Data). The address for U2
is controlled by pins “A0”, “A1”, and “A2”. Resistor
packs RN1 through RN6 provide current limiting for
annunciators ANN2 through ANN7.
4.28.3 KEYBOARD INPUT
The front panel keyboard is attached to the display
board assembly by connector J3, and connected
directly to integrated circuit U4, an I/O expander port,
U4 communicates with microcomputer U10 via the
“I
2
C” serial data port lines “SCL” (Serial Clock) and
“SDA” (Serial Data). The address for U4 is controlled
by pins “A0”, “A1” and “A2”.
The front panel keyboard consists of five (5) normally
open switches. The common side of the keyboard is
connected to ground. The port pins of U4 are set to a
high (+5V) level by communication with
microcomputer U10. If a key is pressed, it pulls its
respective port pin of U4 to ground.
Microcomputer U10 periodically communicates with
U4 to look at the condition of the switches on the front
panel. A closed switch will be detected by U10’s
software. If the switch remains closed for a period of
time, it will be validated by U10 (“debounced”) and the
appropriate action called for will be executed.
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