Fluke ScopeMeter 123 Service Manual page 54

123
Service Manual
Each 14 ms the LCD picture is refreshed during a frame. The frame pulse (FRAME)
indicates that the concurrent LINECLK pulse is for the first column. The column drivers
must have been filled with data for the first column. Data nibbles (4 bit) are supplied via
lines LCDAT0-LCDAT3. During 20 data clock pulses (DATACLK0) the driver for
Y161..240 is filled. When it is full, it generates a carry to enable the driver above it,
which is filled now. When a column is full, the LINECLK signal transfers the data to the
column driver outputs. Via the common drivers, the LINECLK also selects the next
column to be filled. So after 240 column clocks a full screen image is built up on the
LCD.
The LCD unit generates various voltage levels for the LCD drivers outputs to drive the
LCD. The various levels are supplied to the driver outputs, depending on the supplied
data and the M(ultiplex) signal. The M signal (back plane modulation) is used by the
LCD drivers to supply the various DC voltages in such an order, that the average voltage
does not contain a DC component. A DC component in the LCD drive voltage may
cause memory effects in the LCD.
The LCD contrast is controlled by the CONTRAST voltage. This voltage is controlled
by the D-ASIC, which supplies a PWM signal (pin 37 CONTR-D) to PWM filter
R436/C436. The voltage REFPWM1 is used as bias voltage for the contrast adjustment
circuit on the LCD unit. To compensate for contrast variations due to temperature
variations, a temperature dependent resistor is mounted in the LCD unit. It is connected
to the LCDTEMP1 line. The resistance change, which represents the LCD temperature,
is measured by the D-ASIC via the S-ADC on the POWER part.
The back light lamp is located at the left side of the LCD, so this side becomes warmer
than the right side. As a result the contrast changes from left to right. To eliminate this
unwanted effect, the CONTRAST control voltage is increased during building up a
screen image. A FRAME pulse starts the new screen image. The FRAME pulse is also
used to discharge C404. After the FRAME pulse, the voltage on C404 increases during
building up a sreen image.
Keyboard Control, ON/OFF Control
3-28
FRAME
Column
Driver
Din
DCl
LnCl
M
Carry
Column
Driver
Din
DCl
LnCl
M
Carry
Column
Driver
LCDAT0-3 Din
DATACLK0 DCl
LINECLK LnCl
M M
Figure 3-13. LCD Control
Do Di
Common Driver Common Driver
LnCl LnCl
M M
X1..80 X81..160
TOP
Y1..80
LEFT
Y81..160
FRONTVIEW
LCD
PIXEL (0,0)
Y161..240
Do Di
Common Driver
LnCl
M
X161..240