Fluke PM3082 Service Manual page 303

5.3-2
Diagram 2
The resistance value of the indication ring in the probe at channels 1,2, 3 and 4 is present between
ground and pin 13, 14, 15 and 12 of multiplexer D1008 respectively. The analog probe resistance
value results in a certain DC voltage that is switched to ADC input PTEST-XA of the processor
D1001.
The circuit part indicated with 'RS232-C INTERFACE' consists of a buffer D1302 that converts the
microprocessor's TTL signals (0 and 5 volt) into -12 and +12 volt RS232 output levels. The buffer
D1301 does the same in the opposite direction.
The buzzer circuit incorporates an oscillator built around the buffer D1301 and amplifier D1302.
D1302 is enabled by flipflop D1102. The buzzer H1001 is a piezo ceramic transducer.
The +5 V reference voltage for the A.D.C.'s inside D1001 is derived from the instrument's +10 V
reference voltage via operational amplifier N1801.
Diagram 3
This diagram shows the "DAC-POT" IC D1112 that converts the digital data from the microprocessor
into 16 analog voltages. Each of these voltages is independently adjustable between 1 V and 4 V.
The reference voltage for this D/A converter comes from N1101.
This diagram also shows the IEEE interface that is a factory-installed option. The heart of the
interface is lEEE-controller D1104. The registers inside D1104 can be controlled by microprocessor
D1001 via the buffer D1103. The clock for D1104 is generated by crystal oscillator G1111, D1101 and
divider stage D1102. The bidirectional buffers D1107, D1108, D1109, D1111 form an interface
between D1004 and the IEEE bus devices outside the oscilloscope. D1106 controls the bidirectional
buffers.
Diagram 4
This diagram comprises the generation of CRT text and cursors. The heart of the system is formed
by the text generator IC D1402. This device is controlled by the microprocessor via the 8 data bus
lines CPA00 through CPA07. The text that has to be displayed is stored in RAM D1401. Every 19 ms
the input TXTCS-LT at pin 35 of D1402 becomes active and the actual text information is written into
the text generator.
The vertical component of the text display is represented by the 10 bits code Y0 through Y9. This is
converted into analog via DAC N1401. The balanced current output signals are YTXT0 and YTXT1.
These signals are in opposite phase.
The horizontal component of the text display is represented by the 10 bits code X0 through X9.
Conversion into analog occurs via DAC N1402. The balanced current output signals are XTXT0 and
XTXT1. These signals are in opposite phase.
The switching of the X and Y deflection between text and normal signal is controlled by the current
signals XYSW0 and XYSW1 that are derived from pin 64 and 65 of D1402 and are in opposite phase.
Text is displayed if XYSW1 is at 1 mA and XYSW0 is at 0mA. Text display is on during the autocal
procedure if TXTOPT-HT is high.
The current signals ZSW0 and ZSW1 switch the Z amplifier between signal display and text display
and are derived from pin 80 and 1 of D1401. At time base sweep speeds of 0.1 ms/div and faster,
parts of the text are displayed during the time base hold-off period. The presence of the hold off period
is indicated to D1402 via V1401 by signal TBHOTXT. At sweep speeds of 0.2 ms/div and slower, the
text is displayed at a random basis during the normal signal display that is interrupted for a while. This
is done so that it is invisible to the user.
UNITS DESCRIPTION