Seca 727 Series Service Manual page 4

When the start button is pressed, the following
steps are executed:
Starting
The µC is started with the reset logics (see below)
and the program is executed.
Self-test
A test digit is written into all of the RAM cells in
succession, then read out and compared.
The main CPU commands are checked via
computer operations to see that they function
correctly.
The sum of the digits of those memory values
which are important for the weighing result is
continuously calculated and checked for
correctness.
In the event of fault, "EEEEE" is written to the
display.
Zero point determination
After starting, "SECA" is displayed for approx. 1
second. During this period, the zero point is
determined. The measured value M o obtained is
saved and subtracted from the relevant measured
values.
For incubator scale 748 only:
After "SECA", "-UP-" flashes in the display. The
scale must be relieved by at least 0.5 kg in order
that the zero point can be determined. Once the
zero point has been determined, a sound is
emitted for approx. 1 second which signals that
the scale is ready for weighing.
Zero point follow-up
If the current measured value M i changes only
slightly in relation to the zero point value M o within
a given time (C = 0.5 d/sec), the current measured
value is regarded as the zero point (M i = M o ).
Weight calculation
The weight is calculated from the current
measured value M i minus the zero point value M o ,
divided by n e n e is the internal step count per step
displayed.
−
M M
( )
=
i 0
F
n
e
Here n e = 10.
Display
The current weight F is displayed on a 7-segment
LED display. The display is controlled via multiplex
operation. Consequently, actuation faults affect all
segments and are detected immediately.
08.02.98 Law
Circuit description
Model 717,727,728,737,748,757
The µC outputs the processed 7-segment
information to the segment port. Via Darlington
driver 419 (8 transistors) and resistor network 317
(8 resistors), the cathodes of the LED's are set to
0 V. The shared anodes are connected to + 5V via
the relevant digit transistor.
Overload detection
The current measured value M i is checked with
respect to two limit values:
a) Overranging
If F = F max + 9d, "STOP" is displayed.
(d = graduation on the display)
b) Overshooting the limit
If F = F lim (ca. F max + 20 %), "EEEEE" is
displayed.
C) A/D converter limit values
If the bottom or upper limit of the A/D converter is
overshot, "EEEEE" is displayed.
Range switch-over for model 717
By pressing the weighing range switch-over
button, where normally the tare button is fitted, the
scale can be switched over between weighing
range 1 and 2:
Weighing range 1: 15kg / 5g
Weighing range 2: 6kg / 2g
Hold and tare function for model 727
The Hold/Tare button has two functions.
Tare range: up to 0.4 kg
Hold range: from 0.4 kg
Taring function for models 737 and 757
The taring function is activated by pressing the
tare button.
Hold function for models 717,737,748 and 757
The hold function is activated by pressing the hold
button.
Taring
If the taring function is activated, this is detected
by the µC, tare indicator 27 on the display board is
switched on and the weight on the scale is tared
off.
The zero point M o is subtracted from the
measured value M i and the result is saved as M t .
The weight now results from:
− −
( M M M
=
i 0
F
n
e
Blatt 2(3)
)
t
25-01-02-467 Index E