Datron 1061 - Analog Lnput Signals; Display Signals; Keyboard Data Encode

..;.,...
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.