Technical Description; Introduction - Datron 1062 Service 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. 3.1).
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
Drawing No. 430328)
The Analog
assembly
is
split into three distinct
sections:
(i)
the
Analog
Interface,
(ii)
the
DC
Isolator
and
(ill)
the
Analog
to Digital
(A
-
D) Converter.
The Analog
Interface receives data
from
the Digital
assembly
to control the selection, range scaling
and other
features of the analog circuitry.
Messages between
the
Analog and
Digital
assemblies are passed
via
opto-isolators,
electrically isolating
one from
the other.
The
DC
Isolator
includes the preamplifier,
range
scaling
circuits
and bootstrapped
supplies.
The
A
-
D
section converts the scaled input
signal to a
time period
proportional to the
signal
using
a
modified
triple slope
technique.
3.2.1
Analog
Interface
(430328
sheet
5)
3.2.1.
1
Introduction
The Analog
Interface
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
function, 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 5kHz.
3.
2.
1.2
Power-On
At power-on
the
A
-
D
converter
is
placed
into the
RESET
condition
(See Section
3. 2. 3. 8).
The
analog
cir-
cuitry
is
then interrogated to discern
which
options
(if
any)
are fitted.
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 procedure,
in
more
detail,
the
Analog
Interface Data
(ID) lines are
all
set to a logic
'1'except one,
which
is
set to
a logic 'O',
depending on
the
option being
interrogated (See
Fig. 3.2).
As
an
example
we
will
check
to
see
if
the
AC
option
is
fitted.
ID1
is
set low, the
rest
of
the ID
lines set
high
and
the
Analog
Interface
Address
lines,
IA0 and
IA1
set
low.
The
opto-isolators invert
all
signals,
thus
M17-3
is
low and
M19
pins 10,
4 and
11 are
high.
If
the
AC
option
is
not
fitted
M19-2
is
driven low
via
R55 from
Ml
7-3,
causing
M19-3
to
be
high,
producing
a
logic
'0'
(—15
volts)
on M18-4.
If
the
AC
option
/s
fitted a
33kI2
resistor
on the
AC
assembly (R14) overrides
R55 and
and
a
high
is
placed
on
Ml
9-2.
The
effect
is
to
produce
a
f
\
y
FIG.
3.1
PRINTED
CIRCUIT
BOARDS BLOCK DIAGRAM
J