Fluke 93 Service Manual page 42

3-8
CIRCUIT
DESCRIPTIONS
Analog
ASIC bus
The
Analog
ASIC(A-ASIC
D2301, see
circuit
diagram A2aJA2b,
figure 10.5/10.6)
orA-ASIC,
as
used
In
the following
text, is
controlled
by
tine
rncroprocessor.
The
mtcrc^rocessor
uses
the signals
CDAT,
CCLK
and DTAEa,b,c
to set
the A-
ASIC
and
the attenuator sections
on
the
analog
A2 PCB. These
signals
toge^er form
Ihe
CONTROL
bus.
Flash
ROM
type
selection
The ScopeMeter hardware
allows the
usage
of different
types
of
Rash
RO^/s.
The
actual Flash
ROM
configuration
Is
indicated
by
resistors
R1222
and R1224.
FLASH
ROM
CONFIGURATION
F51211X)
The
resulting
voltage
levels
(0 volt,
2.5
vott
or 5
volt)
are read
directly
by the
microprocessor A/D
converter
inputs.
ON/OFF
circuit
The
ON^OFF
circuit
operates almost
like
a
thyristor.
When
the
ON/OFF
key
Is
pressed, a current
1$
drawn
from
the base
of
VI
503, via
R1
503 and VI 501
.
Transistor
VI
503
will
now
start to
conduct. This
results
in
a
current through
R1507, R1504,
V1502 and
R1506.
The
signal
POWER.ON
will
now
become
"high*.
Also
transistor
V1 506
will
conduct, supplying
base
current
to
VI 503
after
the
ON/OFF
key
Is
released.
The
POWER-ON
signal
will
latch "high*.
The
ON/OFF
signal
will
go
high, turning
off
VI 506 and
VI 503, the next time the
ON/OFF
hey
Is
depressed.
The
POWER_ON
signal
will
become
"low"
and
the
ScopeMeter power
turns
0
IF.
RESBTelreuH
The
RESET
circuit
consists
of
VI 203,
VI
205,
VI 21
5,
VI
201,
D1205
and
related
components.
When
the
ScopeMeter
power
Is
switched
on, the -fSV supply
voltage
starts
to
rise.
This
causes
the
zener diode VI 202
to
conduct.
After
some
time
transistor
V1 203
also
starts to
conduct.
R1204 and Cl203form atime
delay (see
figure 33).
switch
The
RESET
signal
now
Is
buffered by
^
momem
D1 206 and connected
with
the
RESET
Inputs of
the microprocessor
and
the
Figure 3.3
RESET
signal timing
D-ASIC
circuitry.
After
a
reset,
the voltage
on
the
EA
(External
Address)
input of the
microprocessor
(pin 14)
is
'high
".
The
Microprocessor
starts
up
using the internal
Mask
ROM
software.
Rrsithe
Flash
ROMs
are
checked
to
see
If
they contain
valid
software,
if
this
Is true,
ou^ut
pin 6
of
^ip'flop
D1202
is
set "low
*.
Now
the
microprocessor invokes a software
reset.
Because
of the
"low**
voltage
on
the
EA
input
of
the microprocessor, the microprocessor
will
"start
up" again, using the external Flash
ROM
software.
The
reset pulse
is
blocked by
translstcw
VI 201
to
prevent the
RESET
signal
from performing a
"hard-
reset"
on
the
microprocessor
agair>,
At
this
software
reset,
the
microprocessor enables
the
LCD
by
means
of
the signal
LCDPWR.
Then
the
buffers that control
the
LCD
contain
valid data.
Resistor(s)
R1222
R1224