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front panel display, in addition to the "LOW
BATTERY" annunciator.
4.25.5 CHARGE MONITOR CIRCUIT
NOTE:
Previous 4802 scales used rechargeable
nickel cadmium batteries and had a front panel LED
to indicate when the battery was being charged. On
this new model, the LED is removed; however, we still
use some of the former charge circuitry as described
next.
An additional circuit is included on the instrument
board. Transistor Q4, along with resistors R56 and
R57, is used to monitor the "CHARGE" annunciator
line which illuminates the front panel indicator. Q4's
collector is connected to I/O pin "P2.5" of U10 to tell
the microcomputer 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.
An option in the scale's set-up software (see section
2.40, "AC con") is also provided to disable/enable
this continuous "on" feature.
4.26
MICROCOMPUTER
CIRCUITS
To attain various additional features such as
automatic zero tare, pounds/kilograms conversion,
weight lock-in, previous weight memory, etc., a
microcomputer is employed to additionally process
the data supplied by the A/D converter.
microcomputer
system
microcomputer; U9, a non-volatile memory which
stores the previous weight reading; and U11, a device
to generate reset conditions for the microcomputer.
During the operation of the scale, the microcomputer
continually receives the weight readings from the A/D
converter. This data is received in a "multiplexed"
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 P1.1 through
P1.7) The microcomputer also continually scans the
keyboard (using U4 on the display board) 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 it for viewing and transfers
it to the front panel display.
AND
SUPPORT
This
consists
of
U10,
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
internal timing of the microcomputer.
4.26.1 THE I
The SCALE-TRONIX® model 4802 PEDIATRIC
SCALE makes extensive use of serial data
transmission to send data to the display, read the
keyboard status, read/write information to the
non-volatile memory, etc. This particular serial data
format is referred to as "I
"Inner-integrated circuit". This bus consists of only
two wires, which are labeled "SDA" (serial data), and
"SCL" (serial clock).
Multiple integrated circuit devices can be attached to
2
the same I
C serial bus. A device will only activate
when its specific address is sent. Each device has a
unique pre-assigned address (inherent to the specific
type of integrated circuit) plus additional pins to set a
unique address for multiple devices of the same type.
By manipulation of the SCL and SDA lines, the
master device controlling the I
microcomputer U10) can send and receive data to all
the other devices on the bus. Specific timing of the
SDA and SCL lines can also reset, start and stop
transmission to devices attached to the bus.
4.26.2 NON-VOLATILE MEMORY
The internal memory of microcomputer U10 does not
a
retain data when the power is switched off. Because
some features of the scale require lasting data
retention (such as last weight recall) integrated circuit
U9 is included. This device, called an "electrically
erasable programmable read-only memory", or
"EPROM" will store selected information for periods
of up to 100 years.
Information needed to be stored to or retrieved from
U9 is sent in serial form using the "I
(serial clock) and SDA (serial data).
controlled by microcomputer U10. A data bit (a high
or low level) is sent and received on SDA when the
SCL line provides a clock pulse. U9 shares these
lines with other devices within the scale by having a
unique address which is controlled by the set of pins
"A0", "A1" and "A2".
15
2
C SERIAL DATA BUS
2
C", which stands for
2
C bus (in this case
2
C" bus, lines SCL
These are
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