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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.
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.
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.
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.