Digital Functional Automatic Test (FATE)
Digital functional testers, often referred to as FATE systems are not as
widely used as they used to be some years ago. These systems apply a signal
pattern to the input of the board. This may often simulate or nearly simulate
the live input to the board, and then the system monitors the outputs looking
for the correct output pattern. The advantage of this type of tester is that it
is able to provide a very fast test of a board. This enables it to be passed on
to the next stage of the equipment assembly with a very high degree of certainty
that it will function to its specification in the unit or system into which it
is incorporated.
Interface
Like in-circuit testers the interface to the board is usually effected using a
bed of nails. These may be virtually the same in construction as one used for in
circuit testing and it enables fast connection to be made to the board. Whilst
connection through the connectors would often be possible, this takes additional
time, and in view of the cost of many of these testers and the throughput
required, this would not be an acceptable solution. Instead a bed of nails
fixture that is operated either using a vacuum or mechanically is used. In this
way the board is simply placed onto the fixture and the connections are made as
it is pulled onto the pins. This operation is completed in a matter of a few
seconds rather than tens of seconds or even a minute or more if connectors were
used.
The fixture need not be nearly as complicated as one used for an in circuit
test. The reason for this is that connection is only required to the input and
output nodes rather than all the circuit nodes in the case of an in circuit
tester. Indeed if pins were applied to all the nodes, the stray capacitance
introduced may impeded the operation of the board.
Programme generation
FATE systems are most widely used for testing digital boards. Much of the
programme can be generated automatically by entering the circuit data into the
tester. Once this has been done the computer within the tester builds up its own
picture of the circuit and then with a knowledge of the pin connections it can
then build up a test programme for the board. The simulations run by programming
stations have been known to reveal design problems such as race states or even
circuitry that is not required.
Unfortunately programme generation is never as straight forward as might be
liked and the programmes generated in this way usually need a lot of finishing
which is generally very time consuming. In addition to this any analogue areas
need to be programmed manually and often require analogue measuring instruments
to be used. This can be very time consuming.
In view of the significant level of manual programming required for
functional test programmes they can be very expensive to implement.
Guided probe
FATE systems are very fast at finding functional faults with a board. They are
not always so fast in finding a problem. In many instances the tester will be
able to deduce the problem from its knowledge of the circuit. In most cases they
are unable to locate a problem because they do not have "sight" of the internal
areas of the board. To overcome this it is necessary to gain a view of the
circuit at intermediate points in the board. This is generally achieved using
what is called a guided probe. This is a probe connected to the tester that can
be manually applied to different points on the circuit under programme control.
In this way it is possible to check the points on the board that are not
accessible via the bed of nails.
Some of the routines that are required to fault finding using a guided probe
may be generated automatically, but often they need to be programmed manually,
especially for any analogue areas. This programming can be particularly time
consuming although very necessary if a large number of reject boards is not to
be the result.
Advantages and disadvantages
The main advantage of an FATE system is that it tests boards very quickly. Speed
is reduced quite considerably, though, when analogue testing is required. Part
of the reason for this is that the analogue instruments may take time to settle.
Another contribution arises from the fact that they may be controlled via a GPIB
port, although some may use VXI instrumentation.
The disadvantages with large FATE systems are generally the cost. The system
itself may cost several hundred thousand pounds. On top of this, there are the
fixture costs for each board as well as the programming costs. A further
disadvantage is that the lead lengths to some points on the circuit have
significant levels of capacitance and these restrict the testing to speeds much
slower than the full speed of the board. This is becoming more of a problem with
many boards today
Today an option that many people are opting for is a low cost bench-top
combination tester that combines in circuit testing with boundary scan and
functional testing. In this way a very high degree of confidence can be reached
whilst still being able to locate faults quickly. However for high speed digital
boards other solutions often need to be devised.
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