Service
Manual
OP-1
01-15
Onloff control
The "main switch" of the instrument is U1.2. Q1 is an.auxiliary on-switch.
When the ONIOFF key is pressed, Q1 is turned on, the DCIDC converter U2 receives current
and starts, the
5
V
voltage supply builds up and the gate array U11 and microcontroller U10
receive a reset. When a reset is detected, the gate array causes line INT2 (U11 PIN 20) to go
high, turning on
Q10 and Q9 and so also turning on U1.2. The instrument is now switched on.
If the ONIOFF key is held down, capacitor C3 charges via R7 and Q2. From a certain level (ON1
OFF pressed
>
approx. 2 seconds), the gate array detects PERM-IN low at its input (U11 pin
10) and the instrument goes into permanent mode. This means: the PERM LED is turned on
and the instrument's power cut-off facility disabled.
If the PERM-IN input remains. high (ONIOFF
<
approx. 2 s), the instrument will turn itself off
automatically after 20 minutes to save the batteries.
Turning off the instrument: If the ONIOFF key is pressed, the gate array receives a pulse via Q2
and
C4
at input CPFF (U11 pin 11). The INT2 line goes LOW, Q10 turns off after a delay
(provided by C21), so turning off Q9 and U1.2 too. The delay gives the microcontroller time to
store the current device settings in EEPROM before the instrument turns off.
Battery monitoring
Comparator U6.2 is used for battery monitoring. The LOW-EAT signal (pin 5) goes low if the
battery voltage goes below 2.2
V.
BAT is displayed as a warning.
+5
V
power supply
The DCIDC converter U2 goes into operation if the battery voltage goes above approx. 2
V
and
is designed to operate up to about 3.5
V.
Q8 and U1.l help the DCIDC converter to start.
-5
V
power supply
The DCIDC converter U3 generates tha -5
V
supply from the
+5
V
supply.
Calibration
EEPROM
All alignments are carried out using corrections which are stored in EEPROM U13.
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