Analog Lnput Signals; Display Signals; Keyboard Data Encode - Datron 1061 Calibration And Servicing Handbook

..;.,...
42
F!c.3.44
LINE LOCKING
T¡MING DIAGRAM
(50Hzl
Count
Down
Period
l-|-
Count Up
Period
2OmS
40mS
M26-1
M
M23-1A
R
M39-8
F
M'23-20
C
l,lt23-21
O
€>
UP
crocK
CLOCK
i_
l_
CLOCK
CLOCK
Latches
M42/51
are updated
with this
new
count
during
the A-D
RESET period
and apqlies
the count to
resistor
network AN4 which forms a
D'A
converter.
Chang-
ing
the voltage applied
to
varicap
D9
alters
its
capacitance,
thus adjusting
the
LC
of
the Colpitts oscillator.
Therefore
the frequency
of
the
Master
Clock
is increased
or
decreased
to
be
an
exact
multiple of
the mains frequency.
3.9
FRONT PCB ASSEMBLY
(Circuit Drawins
No.
4302541
The Front
pcb
assembly accepts
the
measurement
sígnals,
digitally
displays the value,
provides manual
control
of
the
measurement
circuits and data conditioning,
and
gives a
vísual status indication
of
the
selectable
instrument
states.
3.9.1
Analog
lnput
Signals (430294
sheet 2)
The Front pcb
connects
thç
terminals
to
the
2/4-
wire
Ohms and
Local-Remote
Guard
switches.
Thus
in
'2-wire':
Hi is
connected
to
J+, and Lo to
I-,
through
thermistors
R1 and R2. ln 'Local':
Guard
is not
directly
linked
to
the front
panel
Lo
terminal, as
this
becomes
active
in
4-wire Ohms. lnstead,
'Local' links
Guard to
Ohms Guard,
which
is
permanently connected to
DC
lsolator
Lo.
Signals applied
to
the six
front
panel
terminals
are
routed
through
to the
Rear
pcb
(to the
Rear lnput/Ratio
pcb
or
Rear lnput pcb
if
Option 40 or 41 is fitted)
via
ifre
Signal Cable
assembly. Each
of
the terminal
leads
passes
through its own HF
choke,
all six inductors
being
wound
in
the
same
direction
on the
same
core.
This
'Common Mode'choke presents
high impedance
to
transient
common
mode currents,
but
low
impedance
to
normal
mode
differential input
currents.
Two
screened cables
are
used
to
transfer
the
signals
to the rear:
I+
and
Hi
are carried in one;
I-,
Lo and Ohms
Guard
in the
other. Guard is carried
via the cable
screens,
thus
guarding the
signals
during transfer'
3.9.2
Display
Signals
1430294 sheet
1l
The front
panel
assembly
routes
the display
signals
from
the Display Driver board
to
the gas
discharge
display'
3.9.3
Keyboard Data
Encode
1430294
sheet 1)
Selection
of
a
front
panel keyswitch causes
one of
the
two
16-key encoders (M7 or
M10)
to
send a
data
avail-
able
message
to
M2 (a
data
latchl and.to
remember which
key
was pressed.
The
output
ol
M2,
(pin
1
or
13)
signals
the interrupt circuitry of the
D¡gital
Board
(lROKt
or
tRoK2).
When
the
microprocessor accepts
the interrupt
and
has
located
the
source,
the
XKY
BRD
line
to pin
13
of
M7
and
M10 is taken low,
enabling
the data
outputs
of
the
encoders
to
be placed on
to the
CMOS
data bus
(See
Fig.
3.45
for
the key
select
coding). This signal
also resets M2
'ready
for
the
next key
selection.