Hardware Functional Description; Front Unit - Fluke PM6685 Service Manual

Hardware Functional Description

Front Unit

LCD Drivers
U 20 1
Dr iv er 1
P C F8 576
S CL
S y nc .
S D A
U2 02
Dr iver 2
PC F 8576
Fig. 4-2
Front panel LCD drivers.
An LCD and two LEDs are used as indicators. The LCD is used to
show both the measurement result and the state indicators of the in-
strument setting. The LEDs show standby and gating.
The LCD has 158 segments that are multiplexed with a ratio of 2:1.
Two parallel and synchronized LCD drivers (U201 and U202) are
used. They are connected with a serial I
on the main board. The clock frequency of the drivers is approxi-
mately 140 kHz, set by R201. The VLCD pin is connected to GND
on the main board.
The LCD is provided with a backlight, an LED array integrated into
one component. Its current consumption is set by the resistors
R204-R207. The backlight dissipates approximately 1.5 W .
4-4 Hardware Functional Description
B ac k pl. 0
Bac k pl. 1
LC D
1 58 s egm ent s
2:1 M ultiplex
2
C bus to the microcomputer
Keyboard
U13A
Main Board
Latch
U11
AD0-AD7
CPU
U14A
Latch
Fig. 3
Keyboard scanning.
The front panel pushbuttons are connected in a matrix. The scanning
signals H0 to H3 come from the main board. If a push button is
pressed and H0 to H3 is high, one of the output signals V0 to V7 will
be high. The STAND-BY/ON and LOCAL-PRESET buttons are not
part of the scanning but are connected directly to the main board.
The front unit is fixed to the main board unit with three screws. The
electrical connection is made with a 40-lead flat cable to the main
board.
Keyboard & Display
Board
H0-H3
V0-V7
LOCAL/PRESET
SCL
SDA