interconnecting device. Ordinarily, this is not
VAST was designed with ease of maintenance
required if the avionics equipment has been
as a primary objective. In addition to the
designed with the requirements for VAST. Very
modularized design of VAST building blocks,
often, you may obtain passive circuit functions
there are three levels of fault detection that ensures
through the use of standard plug-in modules.
The last element of the test program is the
The three levels of detection are auto-check,
instruction booklet or microfilm strip. These
self-check, and self-test.
instructions give the details for all the steps to
follow in testing any given unit. The steps include
You may initially uncover a fault through
initial procedures, such as hook up and clearing
auto-check. The auto-check is inherent in the logic
and control design of the VAST station. It
operations, and proceed to the final steps of
includes verification of instructions and fault
disconnect and UUT closeout.
monitoring. Auto-check works on a continuous
OPERATION OF A VAST STATION
basis during station operation and, when a fault
occurs, interrupts testing.
In a typical VAST procedure, ease in opera-
The second level of VAST fault detection is
tion of the actual testing becomes apparent. You
may make the initial setup of the weapon
self-check. This is a programmed sequence that
replaceable assembly (WRA), including removal
the VAST operator starts through the DTU
of dust covers, cooling provisions, and con-
keyboard. Self-check may be either internal or at
nections to the interface device, off station to
a system level. Internal self-check measures the
minimize disruptions of station operators. Final
ability of a building block to perform against its
connections between the VAST stations interface
own internal standards. System self-check requires
panel and the UUT take only a few moments
the use of two or more building blocks in a test
directly at the station.
configuration selected to isolate faults within the
The operator begins testing by selecting the
test set up.
The self-check philosophy you use to verify
code that starts the test program. Before applying
power or stimulus to the UUT, you should
VAST operation is based upon confirmation of
key system elements first. It then uses these
program selection. This will also ensure no
elements to check the remaining building blocks.
conditions exist that will damage the VAST
Checks of basic core building blocks are by
station or the UUT once active tests start. If
internal standards. Once satisfactory performance
is assured, the system uses their capabilities to
everything checks out OK, the testing proceeds
automatically. The operator only has to respond
check the remaining building blocks. The system
to instructions that appear on the CRT display.
accomplishes check-out of noncore building
The program will not stop until it encounters a
blocks by using any combination of core measure-
fault or it reaches a program halt.
ment or stimulus building blocks.
The purpose of programmed halts is to allow
manual intervention during testing to make
The final level of VAST fault detection is
adjustments and observations. When the identi-
self-test. This is a series of test programs used to
locate faults within a building block. If a building
fication of faults and the operator's instructions
are required--such as interpreting a complex
block contains a malfunction as a result of the
waveform--the operator may refer to the test
self-check routine, remove the faulty building
program instructions. Upon completion of the test
block and connect to VAST as a unit under test.
program, the CRT display shows any closeout
The system will then conduct self-test programs.
A VAST station is completely autonomous
and normally operated under computer control
in a fully automatic mode. Of course, the operator
In its simplest form, the interconnecting device
can select any one of three semiautomatic modes
consists of an adapter cable, which connects the
or a manual mode.
unit under test (UUT) to the VAST interface. In
The semiautomatic modes include a one-
some cases, passive and active circuits are
group, a one-test, and a one-step mode. These
auxiliary modes permit detailed observation of
amplify low signals. These circuits are introduced
various test sequences. They are also useful in
as part of the electrical interface in the