Introduction; Analog Assembly; Analog - Datron 1061 Calibration And Servicing Handbook

15
SECTION 3
TECHNICAL
DESCRIPTION
3.1
INTRODUCTION
The internal
circuits of the
basic DC
only instrument
are divided
between
five printed circuit
board
assemblies
(shown
in bold
outline in
Fig.3..l).
For the
purpose of
explanation, each assembly
will
be
described separately
and
each
assembly
further
sub-
divided
according to
the various functions
involved.
3.2
ANALOG ASSEMBLY
(Circuit
Drawins
No.430328)
The Analog
assembly
is
split into three distinct
sections:
(i)
the Analog lnterface,
(ii) the
DC lsolator
and
(iii)
the
Analog
ro
Digiral
(A - D)
Converter.
The Analog lnterface
receives data
from the
Digital
assembly
to
control the
selection,
range
scaling
and other
features
of
the
analog
círcuitry.
Messages
between
the
Analog and Digital
assemblies
are passed
via
opto-isolators,
electrically
isolating one
from
the other.
The DC lsolator
includes
the
preamplifier,
range
scaling
circuits and bootstrapped
supplies.
The
A
D
sect¡on converts
the
scaled
input
signal
to
a
time period
proportional
to
the
signal using
a
modified
triple
slope
technique.
3.2.1
Analog
lnterface
(430328
sheet
5)
3.2.1
.1
lntroduction
The Analog lnterface provides electrical
isolation
between
the
Digital and Analog circuitry.
Latched
data
from the
microprocessor
is
passed through
opto-isolators,
decoded
and
latched again
on
an
analog assembly
to
select
functíon,
range.
test,
average and
the D
- A
converter
set
up
conditions.
A
line
is also
provided
to
instruct the micro-
processor
which options
are
present
and
if
the
AC assembly
is measuring
a signal above
SkHz.
3.2.1.2
Power-On
At
power-on
the
A -
D
converter is placed
into
the
RESET
conclition
(See
Section
3.2.3.8). The
analog cir-
cuitry is then
interrogated
to
discern
which options (if
anyl
are
f
itted. Finally the
analog
circuitry
is placed
into
the
DC, 1000V
range
until a different
range
or
function
is
selected
(See
Fig. 3.3).
To
determine which options are
fitted the
Digital
assembly sends
a
series
of
messages
across
the
isolation
barrier,
decodes
them
on the
analog side and gates them
with
lines
from the option
assemblies
to
feed a
signal
back
across
the
isolation barrier
to
the
micro-processor.
Looking at
the
proceduruin
more
detail, the
Analog
lnterface Data
(l
D)
lines are
all
set
to
a
logic
'1'except
one,
whích is set
to a logic'0',
depending
on the option
being
interrogated (See Fig.
3.2).
As
an
example
we
will
check
to
see
if
the AC option
¡s
f¡tted. lD1
is set low, the
rest
of
the lD
lines set high
and the Analog lnterface
Address
lines,
lAØ and lA1 set low. The
opto-isolators ¡nvert
all
signals,
thus
M17-3
is low
and
M19 pins 10,4
and
11
are
high.
lf
the AC option
is
not fitted
M19-2
is
driven
lowvia
R55
from
M17-3, causing
M19-3
to
be
high, producing
a
logic
'0' (-15
volts)
on M18-4.
lf
the AC optíon
rb
fitted
a
33kO
resistor on the
AC assembly
(R14)
overrides R55
and
arrd a
high
is
placed
on
M19-2. The
effect
¡s
to
produce
a
DISPLAY
DRIVER
ASSEMBLY
FRONT PCB
ASSEMBLY
IEEE
OR
BCD
ASSEMBLY
REAR
PCB
ASSEMBLY
DIGITAL
¡\SSEMBLY
ANALOG
OUTPUT
AC
ASSEMBLY
ANALOG
ASSEMBLY
o,s
ASSEMBLY
CURRENT
ASSEMBLY
RATIO REAR
INPUT
FIG.3.1
PRINTED
C¡RCUIT BOARDS BLOCK DIAGRAM
RATIO
INPUT
FRONT PANEL
INPUT
REAR
INPUT
:
.
. .:
.'