Page 107 - Building A Succesful Board-Test Strategy

Test Methods 93

Receiving
optional

To
System
Assembly

Figure 2-30 A typical disk-drive test strategy,

inside out. A hot-mockup test executes a read cycle on a fragmented file, stops in
the middle, moves the heads, and reads the next segment. A data discontinuity
between file segments indicates faulty board logic.
Hot-mockup is attractive because, in most cases, the self-tests that it runs
already exist. Engineering teams develop self-tests with new products. Therefore,
this approach minimizes test's impact on frantic development schedules. The
engine, in this case a conventional PC, already exists, as does the target system, so
hardware and software development costs are very low.
On the minus side, hot-mockup is very labor-intensive. Results are qualita-
tive, not quantitative, and fault diagnosis depends more on operator experience
than on the test itself. Little information is available on fault coverage. Perhaps
most inconvenient is the problem of mockup-system wearout. Again referring to
the disk-drive manufacturer, consider the logistics of managing 400 sets of test
apparatus. Replacing them only when they die is probably the least expensive
option, but possibly the most disruptive to the manufacturing cycle. Replacing
them all on a schedule, either a few at a time or all at once, minimizes unantici-
pated failures but increases hardware costs.
Because commercially available functional testers cope better with the chal-
lenges of certain products, some manufacturers are again selecting to abandon or
considering abandoning hot-mockup in favor of that option.

2.3.14 Architectural Models

Dividing testers into MDAs, in-circuit testers, and so on categorizes them by
test method. Within each group, it is possible to separate members by tester archi-
tecture into monolithic, rack-and-stack, and hybrid systems.