Service Model 8901B
Instrument Software Supervisor
Flowchart
The instrument’s software is structured in a form called the supervisor. See Figure 8D-5.
It
is
a loop
that
is
continuously traversed, with measurements made near the end, after checks for proper frequency
tuning, proper RF and
IF
level, and correct audio range. Arithmetic manipulation (for example, for the
ratio function) follows the measurement, and the program then loops back up
to
the display.
The frequency, level, and audio 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 immediate succession.
If
a test is not passed, corrective action
is
taken. The decision after that block forces the program back
to the top of the supervisor, bypassing the measurement for that loop.
The software interface with the hardware makes use of
two
concepts called sohare state and hardware
state. The software state
is
stored in 38 bytes of RAM and totally describes the state of the instrument.
On power-up, the initialization procedure loads
the
software state from
ROM.
Keyboard and HP-IB
entry routines modify only the software state and do not affect 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; the frequency tuning, leveling, audio ranging, and
measurement blocks manipulate the hardware as well.
In a normal, stable measurement cycle, the program takes the measurement display branch at the top
of the supervisor and
so
avoids the time overhead associated with the setup block. However,
if
the
program loops back before taking a measurement,
or
if an error condition exists, the non-measurement
display branch will be traversed, thus lighting an appropriate display and going through the setup block.
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
instrument setup.
It
is important to note that the actual instrument setup is guaranteed to conform
to the Keyboard request only at the moment a measurement is taken. The Controller may change
the instrument hardware at other times to optimize
its
tuning, leveling, and ranging functions.
For
example, in troubleshooting, 3.1 SPCL may be keyed in
to
check if the
455
kHz
IF
filter is being
selected properly.
If
there is no RF input signal and the instrument is trying
to
auto-tune, it would
be discovered that both
IF
filters (wide
455
kHz and 1.5 MHz) are being used. The proper test would
have been to use a Direct Control Special Function
(0.031
SPCL).
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. For example, a
failure of the squelch detector in the FM Demodulator can result in frequency errors when tuning to
an
RF
signal with large amounts of
AM.
The appearance of several symptoms can often be used as an
advantage since they provide many avenues to pursue when tracking down a problem.
A
distinct difference exists between special functions used for service (that
is,
Direct Control Special
Functions and Service Special Functions) and those used
for
normal instrument operation. When
service special functions are used, normal instrument functions are suspended. When the special
function mode is left to resume normal measurements, all effects of these special functions on hardware
are lost (with some exceptions such as AM
or
FM calibration and enabling of service errors). (Refer to
paragraph
8-7
for details.) As an example, a Direct Control Special Function can be used to activate
a particular Input Attenuator to check its operation. But once normal measurements are resumed,
the attenuator setting will revert back to what
it
was before the Direct Control Special Function was
invoked.
8D-10
Service Sheet BD1
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