Keithley 236 Service Manual

SECTION 4
Principles of
Operation
new
chargeba1ance phase, and starts
the
final-siope
phase of the
A/D
converter.
The final-siope
phase essentially removes all charge re
maining
on the integrator
by
ramping
the output
of
U50
to 0V. Counts are accumulated
during
the final-slope and
used
for calculation of the
effective input signal. The
A/D
converter is run through
a
number of phases that
are
used
to
null any error
signals
present
in the
A/D
or
analog front end.
The time
base
for
the A/D converter is provided
by
an
os
cifiator that
consists of inverter U57 and crystal Y2. The
7,68MHz time
base, which is
a
multiple of 60Hz, is di-
vided
by
counters U41 and
U40
to
provide system timing
functions for charge
balance control. The 7,68MHz is fed
directly
to
NOR
gate
U47
where it is gated on
and
off by
the rest of the A/D
function. The A/D power supply ref
erence voltages, -40V, and
+5V are derived from opera
tional amplifier
U56
and associated circuitry. These are
reference voltages used in
generating currents
and
thresholds for the A/D
converter.
4,4
DIGITAL CIRCUITRY
The schematic diagrams for the
digital circuitry are lo
cated
in
last section
of this manual, drawing number
236-106.
The Source Measure
Unit is controlled
by two 6809 8-bit
2MHz N-MOS microprocessors.
These
two microproces
sors
coordinate all instrument functions. These include
running the display
and keyboard, servicing the
IEEE-488 interface, and control
of all analog functions.
4.4.1 I/O Controller
The schematic
diagram for the I/O controller
is provided
on
page
1 of drawing 236-106.
The I/O controller
controls
the
display and front
panel
controls, the IEEE-488
interface,
test
fixture lid closure
detection,
and TRIGGER IN and TRIGGER OUT.
lt also
maintains calibration constants, and contains
the major
portion of the operating system for the
instrument.
At
the heart of the I/O controller system
is an 8-bit 68B09
microprocessor
running at 2MHz.
The system master os
dillator
Y1 is an 8MHz canned
crystal osdillator.
I/O
con
troller peripherals
include
a
64K X 8
ROM
(U31)
which
contains
the program
memory,
a
32K X
8
static
CMOS
RAM
(U30)
which is battery
backed up via U29 and
the
battery
(BT1). Calibration
constants
are
stored
in non
volatile memory (U33;
E 2 ROM).
A
veratile interface
adapter
(U28)
provides peripheral
interface to
the
6809
microprocessor.
lt is primarily
used to
interface with
the
display
and keyboard.
lt is also used to
interface
TRIG
GER OUT
and TRIGGER IN, as
well as monitor
the
test
fixture
lid
detect
interface.
A 9914
IEEE-488 standard
in-
terface (U27)
is combined
with two
transceivers
(U34
and
+v
-v
Figure4-7.
Simplified
Schematic
ofResonant
Converter
4-9

Keithley 236 Specifications

General

Brand	Keithley
Model	236
Category	Measuring Instruments
Language	English