Hardware Reference
In-Depth Information
potential solutions. With the exception of a front panel display controls, separate
PC instrument modules usually resemble the conventional stand-alone instru-
ments which they replace. However, in a relatively recent development, USB
instruments provide a low-cost solution and several manufacturers are actively
developing test instruments for budget conscious application that make use of
the PC's USB port.
Several manufacturers have risen to the challenge of producing PC instru-
ments such that the range of equipment currently available includes oscillo-
scopes, digital multimeters, universal counters, timers and frequency meters,
spectrum analysers, function generators, pulse generators, voltage/current gen-
erators, and logic analysers. Individual instruments can be combined to provide
more complex instrumentation facilities. A data logger, for example, can be
assembled from a scanner and multimeter, and controlled flexibly from the PC.
PC instruments are ideal for making repetitive measurements during which
data must be accumulated over a period time. The PC allows such measure-
ments to be automated with the data acquired being sent to a file for future
analysis.
As an example of the use of a PC instrument, consider an application in
which the output frequency of an oscillator has to be monitored accurately over
a long period of time. This task can be accomplished by means of a dedicated
digital frequency meter with readings taken at appropriate intervals, logged
on paper, and a graph showing the long-term variation of frequency can then
be drawn. The alternative approach using a 'PC instrument' simply involves
fitting a digital frequency meter expansion card (such as the Guide Technology
GT200) to a standard PC compatible and using simple software (in conjunction
with the driver(s) supplied with the card) to automate the measurement, and
store the results in a data file for import into an analysis package (such as
DADiSP). A typical application is discussed later in this chapter.
Industrial PC systems
Ruggedized PCs are the obvious choice for use in the harsh environment found
in most industrial plants. Industrial PCs usually offer the same range of facilities
associated with conventional PCs and compatibles, and invariably support the
industry standard bus architecture. Hence an industrial PC will generally accept
the same range of expansion cards as mentioned under the previous heading.
Alternatively, where additional expansion beyond the limit imposed by the
available free slots, industrial PCs may be fitted with bus extenders which are
normally based on an external rack assembly.
In difficult environments it is possible to implement a completely
diskless
system
using solid-state read/write memory devices (e.g. Flash, SD, or XD
cards) inserted into IDE slots in order to provide a bootable operating system
together with one or more application programs.
Backplane bus-based systems
A backplane bus system offers a reasonable compromise between a standard
PC-based system at one extreme and a specialized industrial PC system at the
other. Backplane bus systems are inherently flexible and reliable, and can simply
