Scale-Tronix 5002 - Microcomputer and Support Circuits

switches high when the battery is low, itis coupled
to the busy signal of US (pin 21) through resistor R36
to the driver for the front panel "LO BATT” indicator.
This causes the "LO BATT" indicator to flash when
ihe battery is low.
The remaining half of U3, pins 5, 6, and 7, are
connected to resistors R11, R12, and R13 to form the
“law-low battery" detector. The output on pin 7,
which is normally "high" with a good battery, goes
"low" when the battery is too weak to reliably operate
the scale. This output is connected to an WO pin on
microcomputer U10 to signal the microcomputer that
"low-low" battery has been recognized.
The microcomputer U10 will process the ‘low-low”
battery signal and cause "bAttrY“ to appear on the
scale's front panel display, in addition to the flashing
“LO BATT” annunciator.
4.24.5 CHARGE MONITOR CIRCUIT
An additional circuit is included on the instrument
board for the model 6102 built-in scale. Transistor
Q4, along with resistors R56, R57, and R58, is used
to monitor the "CHARGE" annunciator line which
iluminates the front panel indicator. Q4’s collector is
connected to an YO pin of U7 to tell microcomputer
U10 when the front panel "CHARGE" indicator is on,
Software contained in U10 recognizes this and
prevents the scale from automatically shutting-off
when AC power is present. When AC power is not
present, the "CHARGE" indicator is not energized,
and the scale will shut off after some period of
inactivity to prevent discharging the battery.
4.25 MICROCOMPUTER AND SUPPORT
CIRCUITS
To attain various additional features such as
aufomatic zero tare, pounds/kilograms conversion,
weight lock-in, previous weight memory, etc. а
microcomputer is employed to additionally process
the data supplied by the A/D converter, This
microcomputer system consists of U10, a
microcomputer, U7, a peripheral port expander to
furnish additional input/output lines; U9, a non-volatile
memory which stores the previous weight reading,
and UTI (23005 R02 only), a device to generate
reset conditions for the microcomputer,
During operation of the scale the microcomputer
continually receives the weight readings from the A/D
converter, This data is received in a "multiplexed"
format (one digit at a time) from the output of the A/D
converter (microcomputer input lines P1.0 through
P 1,8). The microcomputer also continually scans the
keyboard (using U7} looking for closed switches. If a
key press is sensed the microcomputer executes
whatever action is called for in its program. After
processing the A/D data the microcomputer assembles
ìt for viewing and transfers it to the front panel display.
U10 is a complete microcomputer, containing a
software program stored in read-only memory,
read/write memory for temporary storage of program
variables, an arithmetic logic unit, input/output and
other control lines, etc. Crystal XTAL1 and capacitors
C29/C30 form the clock oscillator which controls the
internat timing of the microcomputer.
4.25.1 PORT EXPANDER/KEYBOARD/BEEPER
Integrated circuit U7 is included to supplement
the I/O (input/output) of the microcemputer system.
U? contains additional I/O lines (referred to as "ports“).
Microcomputer U10 reads (from input lines) or writes
(to output lines) data to U7 periodically by use of the
data bus (lines DBO through DB7) and the /RD and
ANR lines.
The front panel keyboard is attached to some of U7's
input port pins (PB.O through PB.7, and PC.4).
Resistor networks R42/R43 serve as “pull-ups” and
keep the input pins at a "high" state (+5V) until a key
switch is pressed; this pulls the respective input pin
“low” (04). The microcomputer will recognize this key
press when it reads the input pins from U7 and if the
key remains closed for a number of milliseconds the
microcomputer will execute that key's function.
A four position DIP“ (dual in-line package) switch,
SW1, may be optionally included. It is connected to
U7's "C" port, lines PC.0 through PC.3. It is used to
select software contained in U7.
A small audio annunciator is driven by transistor Q3,
which in turn is controlled by U7's output pin PA.7.
The annunciator gives a short beep as audible
recognition of a key being pressed. The length of the
beep and its various sequences are controlled by
U10's software. Additional output lines of U7 are used
to control the driver for annunciator lamps on the
5002, 5102, 5602, 6002, 6102, 6202, 6502, 6702 21