Scale-Tronix 5002 - Auto Zero Phase (A;D); Signal Integrate Phase (A;D); Ref. Integrate, Signal De-Integrate Phase (A;D); Clock Circuitry

4.23.1 PHASE 1, AUTO ZERO
During auto zero, the errors in the analog
components (offset voltages of buffers, comparators,
etc.) will be automatically nulled out. This is
performed by internal logic that disconnects the input
pins (9 & 10) from the applied analog signal,
connects them to ground, then closes an internal
feedback loop such that offset error information is
stored in the "auto zero" capacitor, C21. Also during
this phase, “reference capacitor" C22 is charged to
the voltage present on "Vref" (pin 2 of U6).
4.23.2 PHASE 2, SIGNAL INTEGRATE
The input signal is reconnected and then
integrated for exactly 10,000 clock pulses. On
completion of the integration period, the voitage V is
directly proportional to the input voltage,
corresponding to the weight applied to the scale,
Capacitor C20 is the integration capacitor, with
resistor R32 setting the integration current. At the
end of this phase the input signal polarity is
determined.
4.23.3 PHASE 3, REF. INTEGRATE, SIGNAL DE-
INTEGRATE
The input to the integrator is switched from
the input signal to reference capacitor C22. Internal
switches connect capacitor C22 to the integrator input
so that its polarity is opposite that of the previously
applied input signal. This causes the integrator to
discharge back towards zero, The number of clock
pulses counted between the beginning of this cycle
and the time when the integrator output passes
through zero is a digital measure of the magnitude of
the input signal. This count is stored in an internal
latch on U6 for output to the microcomputer.
4.23.4 ZERO INTEGRATOR PHASE
One minor additional phase is included to
insure that the integration capacitor C20 is fully
discharged to zero volts. This typically lasts 100-200
counts.)
4.23.5 CLOCK CIRCUIT
A clock is required for the A/D converter, integrated
circuit 6.
23005 R01 board; Integrated circuit U2 is a dual
timer circuit. One half (pins 8,9,12,13) is connected as
an astable oscillator and used to generate a system
clock for the A/D converter. Resistors R6, R7 and
capacitor C2 set the clack frequency to approximately
150 KHz.
23005 R02 board: A clock signal is generated
internally in microcomputer U? and appears on port
pin "P1.0". The frequency is internally set by the
microcomputer's software and is nominally 120 KHz.
4.24 POWER SWITCHING, VOLTAGE
REGULATION, AND SUPPORT CIRCUITRY
Depending upon their configuration, SCALE-TRONIX®
second generation scales may contain disposable
primary cells (models 5002, 5602, 6002, 6502, 6702),
an AC power supply (models 5102, 6202), or an AC
power supply with rechargeable batteries (model
6102). (Note that some models may be specially
ordered to utilize a different power supply than is
normally provided; refer to the technical description
that best matches the particular scale.) Additional
circuitry is included to switch the battery supply,
provide voltage regulation, and detect low battery
voltage conditions.
4.24.1 BATTERY SWITCHING
In order to conserve battery life the battery supply is
switched on and off as needed by the scale.
23005 R01 board: Transistor Q1 is a series switch
which applies battery voltage to the remainder of the
circuitry. Q1 is controlled by transistor Q2 which, in
turn, is controlled by one-half of dual timer UT. This
half of U1 (pins 1,2,4,5,8,10) is configured as a bi-
stable latch to keep the battery power "on" once set,
To initiate power-on Q1 is turned on through
momentary closure of membrane switch S9 CON)
and diode CR1. The switch closure is also coupled
through diode CR2 to set latch U1, thus keeping O1
and Q2 "on" when S9 is released. The switch closure
is also coupled through diode CR10 to signal input pin
"PB.7" of port expander U7 that the "ON" switch is
pressed. (Because the model 5102 remains on
constantly, transistor Q1 is permanently wired "on" by
use of a jumper across J2 or JTEST.)
5002, 5102, 5602, 8002, 6102, 8202, 6502, 6702 19