Keithley 237 Service Manual

SECTION 4
Principles of Operation
U35)
to provide
the
interface
function.
All of these
pe
ripheral chips are interfaced to
the
6809
via a
program
mable
logic
device
(U26; PAL
16L8).
A NOR
gate
package
(U32)
is used
to provide
protection
for
calibration constants
stored in
non-volatile
memory
(U33)
during power-up and power-down.
A power-up
reset
control
circuit
(U36)
guarantees a de
Iayed reset after
power-up
and
a prompt
reset
on power
down
by sensing power supply threshold.
442
SourcelMeasure
Controller
The
schematic diagram for the
source/measure
control
1er
is
provided
on page 2
of drawing 236-106.
This
controller
System
also
uses
a
68B09 microprocessor
(U15).
As
with
the I/O
controller
(see
previous para
graph),
it
derives
its 2MHz operating
frequency
from the
8M1-Iz
crystal
(Y1).
The peripherals
for
this
6809
include
32K
of ROM
(U17),
32K
of static
RAM
(U13), and a
versa
tile
interface adapter
(U14) which
provides interface to
the analog circuitry
as weil
as
a
few
other miscellaneous
functions.
Devices
U18, U19
and
U58 are
binary
counters
that
form
the
core
of
a software watch-dog
timer.
This timer moni
tors software
execution of both
CPUs
via limes
that
are
regularly toggled.
If
any line
fails
to
toggie within ap
proximateiy
4msec the
entire
system
will
be
reset.
As
with
the
I/O controller,
all the peripheral
chips
of
the
source/measure
controlier
are
decoded
and
controlled
by
a programmabie iogic
device
(U16;
PAL
16L8).
A/D
controi
as weil as
A/D counts
accumuiation are
per-
formed
by
the VIA (U14) with
a prescaie
counter
(U21).
Inverters
(U23)
provide
a software selectabie enable for
the high
voltage converter in
the
Model 237
only.
443
Communications
The schematic
diagram
for
the
communications
circuitry
is
iocated
on
page
4
of
drawing
236-106.
Communications
are
provided
by
opto-coupiers that
serve as
an
interface
between
the
source/measure con
troller
and the
analog
circuitry.
This
particuiar circuit is
fairiy unique
in
that
control
signais
are sent through two
optos
that are
driven
180°
out of phase
with one another
to
provide
immunity from
false giitching.
The outputs
of
the compiimentary
opto-couplers are de
coded
by
programmabie
iogic
device Ui
(PAL 20L10).
From
Ui,
the
signais
are
sent to
various piaces
on
the ana
log
board
by
decoder
U39
which
determines
one of
four
ciock paths
as
set by the
bank
select
signals 0 and 1
which
are
also derived
from Ui.
This circuit
topology
provides
for
a very reliabie
communications
scheme
such
that
the
analog
circuitry can
never
receive
faise
data.
Opto-coupiers
AT1, AT2,
AT3,
AT7 and
AT8 are
all
in-
volved
in the transmission
of
ciock,
data,
strobe,
and the
two bank
seiect
signais from
the
VIA of the
source/meas
ire
controller
to the analog
board.
Opto-coupler AT4 is
the high voltage
enable
coupier which
enables
the high
voltage resonant
converter
in the Modei
237. Opto-cou
pler
AT5 returns a
CV/CC
constant voltage
or constant
current level signal to
the source/measure
controlier
so
that
the
present
state
of
the
analog
output can
be
deter
mined.
The A/D
converter
counts
are returned to the mi-
croprocessor
of the source/measure
controller by
AT6
which is
a
high speed
opto-coupier.
44.4 Display
Circuitry
The
schematic
diagram for
the display
circuit is provided
on
drawing 236-116.
The display,
which
is controlied by
the I/O
controller,
is
basically an 18-digit
aipha-numeric
LED
display.
lt con
sists of nine
two-digit
display
elements
(DS1
through
DS9).
Each
of these display
elements
are muitiplexed
in
a
matrix
fashion,
Each display
digit has
its common
cath
ode
connected
to
sink drivers
(U4,
U7 and
U8).
These
sink
drivers
are switched
on in
a
rotating
fashion.
The
display
is divided
into two
banks of ten
columns
such that the
DS1 left
digit
and DS6 left
digit
are
on
simul
taneously,
as
can
be evidence
by the
common connec
tions
of the inputs
of
U4
pin 3
and
U7
pin
8.
This
allows
the
display
to be
refreshed faster,
thus eiiminating
ificker
associated
with
line
frequency
beating with
lighting and
such. The
front
panel
indicator
LEDs
(DSIO
through
DS29)
are
driven
in
a similar manner by
sink
drivers
of
U8. The
anodes of
the
two
separate
banks of
display
ele
ments are
driven by
source drivers U5
and
U6
(left
bank),
and
U9 and
U10 (right
bank).
These
are
serial input,
par
4-10

Keithley 237 Specifications

General

Brand	Keithley
Model	237
Category	Measuring Instruments
Language	English