Model 8903B Service
The Remote Interface receives inputs from the external interface bus
(HP-IB),
processes the information, and
interrupts the Controller in a manner similar
to
the Keyboard.
It
also processes the measurement information
and outputs
it
on the HP-IB
if
requested. The Remote Interface
is
designed
to
make operation
from
an external
computing controller
as
similar as possible
to
operation from the front panel.
Instrument Software Supervisor
Flowchart
The instrument’s software is structured in a form called the supervisor. See Figure
8A-1.
The supervisor
is
a loop
that is continually traversed with displays made near the end
after
checks for oscillator tuning, input and output
leveling, notch tuning, and measurements. Arithmetic manipulation (for example, for the ratio function) follows
the measurement, and the programs loop back to the beginning after outputting
to
the display.
The various level and tune blocks verify that the instrument is adjusted
to
make an accurate measurement.
A
measurement is not made until all of the
tests
are passed in succession.
If
a
test
is not passed, corrective action
is taken. The decision after that block then forces the program back
to
the top of the supervisor, bypassing the
measurement for that loop
if
corrective action
is
unsuccessful. The software interface with the hardware makes
use of
two
concepts called software
state
and hardware
state.
The software
state
is
located in the
RAM
and
totally describes the state of the instrument.
On
power-up, the initialization procedure loads the sohare
state
from ROM. Keyboard and
HP-IB
entry routines modify only the software state and do not effect the hardware
immediately. The setup block in the supervisor is where the hardware
state
is
made
to
conform with the software
state. Setup is not the only place where hardware
is
affected; tune, level, and measurement blocks manipulate the
hardware
as
well.
The Keyboard and
HP-IB
interrupt the flow around the loop, forcing the Microprocessor
to
execute a short
program and then return to the loop as shown in the diagram. Since the supervisor can be interrupted at any
point but always returns to a single location, Keyboard
and
HP-IB interrupts must abort the current measurement
and
start
a new measurement cycle. The Keyboard and
HP-IB
can be thought of
as
a medium through which the
user requests a certain setup.
The microprocessor-based Controller interacts closely with the hardware of the instrument. Many circuits are
used by the Controller for different functions at different times. Thus, a specific failure in one circuit can show
up as
a
collection
of
symptoms that superficially seem unrelated. The appearance of several symptoms can often
be used to advantage as they provide many avenues to pursue when tracking down a problem.
A
clear line is drawn between Service Special Functions and normal instrument operation. When most Service
Special Fbnctions are used, normal instrument functions are suspended. When the Service Special F’unction mode
is left
to
resume normal measurements, all effects of the Service Special Function on hardware are lost.
As
an
example, a Service Special Function can be used
to
display the oscillator frequency. But once normal measurements
are resumed, the display will revert back
to
what
it
was
before the Service Special hction was invoked.
Power
Supplies
The instrument
is
run from four regulated supplies: +15V, -15V, +12V, and +5V. All supplies are independent
except the +5V supply which is dependent on the +15V and -15V supplies.
rev.20
JUNB
7
8A-5
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