Page 109 - Building A Succesful Board-Test Strategy

Test Methods 95

individual instruments. For an application that requires them, there is no other
choice.
Disadvantages to this approach include test-program generation, which
remains primarily a manual process. A user teaches the "tester" about instruments
in the system and loads vendor-supplied or in-house-developed instrument drivers.
Automatic software tools then guide the programmer through code creation.
Although these tools can analyze information from computer-aided design and
other CAE equipment, people must still create and select the final tests.
Bus-based test systems tend to be noisy, limiting measurement precision and
possibly compromising functional board performance. Users must generally design
their own fixtures or other interfaces between tester and board under test.
Hybrid systems offer some features of monolithic and rack-and-stack alter-
natives. They consist of an embedded or stand-alone computer engine, again
usually a PC-type, and a collection of printed-circuit-board-based instrument
modules connected through a standard I/O bus designed specifically for this
purpose. The current frontrunner for this arrangement is the VME extension for
Instrumentation—-VXI—along with its more recent siblings, such as MXI and PXI.
VXI provides what the IEEE-488 bus would call the "switching matrix" as a
special card cage. The standard specifies how to connect the modules, and the
signals that allow them to communicate. This architecture permits I/O speeds of
about 10 MHz and as many as 255 individual instruments, all of which may be
talkers. Board and system manufacturers are beginning to create in-house-built
testers around this design. In addition, traditional "monolithic" tester vendors are
adopting VXI to create modular systems that increase the flexibility and expand-
ability of the monolithic option.
The hybrid-system approach incorporates the best compromise between
monolithic and rack-and-stack alternatives. Ideally, hardware and software inte-
gration resembles the monoliths, as do programming and data-analysis features.
Because instrument modules are available from numerous manufacturers, test engi-
neers can select capabilities that best match their needs. As with rack-and-stack
choices, users can adopt new instrument products that improve system perfor-
mance with a minimum of effort.
Disadvantages include the technique's relative immaturity compared to
IEEE-488. Instrument choices are, at present, still more limited. Also, this option
is still slower than many monolithic products, and users must generally create
system-level and other high-level software.
The VXI standard represents a compromise among costs, features, and ease
of implementation. The same can be said of de facto standards for MXI and PXI.
They cannot accommodate absolutely every function that every instrument vendor
can conceive of. Lead lengths in the cage and other architectural limitations mean
that noise and timing can be a concern in critical situations. Board manufacturers
requiring low noise and very precise timing may not be able to adopt this solution.
In addition, system developers still have to design test fixtures, and users must con-
struct them. Chapter 6 will explore VXI in more detail.