Hardware Reference
In-Depth Information
which may arise, not just those typical of normal operation. The following are
particularly important:
Will the system initialize itself in a safe state? Will there be momentary
unwanted outputs during start-up?
What will happen if the user defaults an input or if an input sensor becomes
disconnected?
What will happen if the power fails? Will the system shut down safely?
What input validation checks are required? What steps should be taken if an
'out-of-range' input is detected?
Applications
The remainder of this chapter provides details of eight representative PC-based
applications. These applications are not particularly novel but they do address
problems that are typical of those which face the instrumentation and control
engineer. The applications have been chosen to illustrate contrasting aspects of
design and, while it would be impossible to describe any of these applications in
their entirety, they should provide a feel for various aspects within the process
of designing and implementing a PC-based system.
Monitoring oscillator stability
The client is a manufacturer of synthesized HF radio transceivers and wishes
to develop a prototype voltage-controlled oscillator (VCO) which operates in
the range 40-60 MHz for use within the frequency-generating circuitry. Several
circuits have been constructed and the client wishes to ascertain the short- and
long-term frequency stability of each unit.
Specification
The manufacturer requires that the output frequency is measured at appropri-
ate intervals (e.g. every 100 ms for the short-term stability measurement and
every 10 s for the long-term stability measurement). The results of each set of
measurements are to be stored in an ASCII file for later graphical analysis. The
software is, however, required to determine a number of simple performance
indicators for each prototype unit including:
Maximum frequency during the measurement period.
Minimum frequency during the measurement period.
Mean frequency over the measurement period.
Total frequency drift during the measurement period.
The manufacturer also requires that the entire set of measurements and stat-
istical calculations should be repeated at ambient temperatures of 0 C, 10 C,
20 C, 30 C, and 40 C.
This task would require considerable manual effort if it were to be car-
ried out using a conventional digital frequency meter. It is, however, an ideal
candidate for automated measurement using a PC and appropriate expan-
sion card.
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