TEST EQUIPMENT

Microelectronic developments have had a great impact on the test equipment, tools, and facilities necessary to maintain systems using this technology. This section discusses, in general terms, the importance of these developments.

Early electronic systems could be completely checked-out with general-purpose electronic test equipment (GPETE), such as multimeters, oscilloscopes, and signal generators. Using this equipment to individually test the microelectronics components in one of today's very complex electronic systems would be extremely difficult if not impossible. Therefore, improvements in system testing procedures have been necessary.

One such improvement in system testing is the design of a method that can test systems at various functional levels. This allows groups of components to be tested as a whole and reduces the time required to test components individually. One advantage of this method is that complete test plans can be written to provide the best sequencing of tests for wave shape or voltage outputs for each functional level. This method of testing has led to the development of special test sets, called AUTOMATED TEST EQUIPMENT (ATE). These test sets are capable of simulating actual operating conditions of the system being tested. Appropriate signal voltages are applied by the test set to the various functional levels of the system, and the output of each level is monitored. Testing sequences are prewritten and steps may be switched-in manually or automatically. The limits for each functional level are preprogrammed to give either a "go/no-go" indication or diagnose a fault to a component. A go/no-go indication means that a functional level either meets the test specifications (go) or fails to meet the specifications (no-go).

If a no-go indication is observed for a given function, the area of the system in which it occurs is then further tested. You can test the trouble area by using general purpose electronic test equipment and the troubleshooting manual for the system. General purpose electronic test equipment (GPETE) will be discussed later in this topic. (Effective fault isolation at this point depends on the experience of the technician and the quality of the troubleshooting manual.) After the fault is located, the defective part is then replaced or repaired, depending on the nature of the defect. At this stage, the defective part is usually a circuit card, a module, or a discrete part, such as a switch, relay, transistor, or resistor.

BUILT-IN TEST EQUIPMENT

One type of fault isolation that can be either on-line or off-line is BUILT-IN TEST EQUIPMENT (BITE). BITE is any device that is permanently mounted in the prime equipment (system); it is used only for testing the equipment or system in which it is installed either independently or in association with external test equipment. The specific types of BITE are too varied to discuss here, but may be as simple as a set of meters and switches or as complex as a computer-controlled diagnostic system.

ON-LINE TEST EQUIPMENT

Functional-level testing and modular design have been successfully applied to most electronic systems in use today; however, the trend toward increasing the number of subassemblies within a module by incorporating microelectronics will make this method of testing less and less effective.

The increased circuit density and packaging possible with microelectronic components makes troubleshooting and fault location difficult or, in some cases, impossible. The technician's efforts must be aided if timely repairs to microelectronic systems are to be achieved. These repairs are particularly significant when considered in the light of the very stringent availability requirements for today's systems. This dilemma has led to the present trend of developing both ON-LINE and OFF-LINE automatic test systems. The on-line systems are designed to continuously monitor performance and to automatically isolate faults to removable assemblies. Off-line systems automatically check removable assemblies and isolate faults to the component level.

Two on-line systems, the TEST EVALUATION AND MONITORING SYSTEM (TEAMS) and the CENTRALIZED AUTOMATIC TEST SYSTEM (CATS), are presently in production or under development by the Navy.

Test Evaluation and Monitoring System (TEAMS)

TEAMS is an on-line system that continuously monitors the performance of electronic systems and isolates faults to a removable assembly. This system is controlled by a computer using a test program on perforated or magnetic tape, cassettes, or disks. Displays are used to present the status of the equipment and to provide data with instructions for fault localization. Lights, usually an LED, are used to indicate which equipments are being tested and also which equipments are in an out-of-tolerance condition. A printer provides a read out copy of the test results. These results are used by maintenance personnel to isolate the fault in a removable assembly to a replaceable part.

Centralized Automatic Test System (CATS)

CATS is an on-line system that continuously monitors the performance of electronic systems, predicts system performance trends, and isolates faults to removable assemblies. CATS, however, is computer controlled and the instructions are preprogrammed in the computer memory. The status of the electronic system being monitored by CATS is presented in various forms. Information concerning a failed module is presented on a status- and fault-isolation indicator to alert the maintenance technician of the need for a replacement module. If equipment design does not permit module replacement, complete electrical schematics and fault-isolation procedures will be made available to the maintenance technician.

OFF-LINE TEST EQUIPMENT

The Navy has under development an advanced assembly tester designated

Naval Electronics Laboratory Assembly Tester (NELAT). This tester is an off-line, general-purpose test system designed to check-out and isolate faults in electronic plug-in assemblies, modules, and printed circuit boards. Equipped with a complete range of instrumentation, the system allows testing to be accomplished automatically, semiautomatically, or manually. In the automatic mode, a complete range of stimuli generators and monitors are connected and switched by means of a microfilmed test program.

The NELAT incorporates modular electronic assemblies that will facilitate updating of the system. The system is designed for use aboard ship. When put into service, this tester will greatly improve the technician's capability in the checkout and fault isolation of microelectronic assemblies.

Another important system for off-line testing is the Versatile

Avionic Shop Test System (VAST). VAST is used in the aviation community for fault isolation in aviation electronics (avionics) equipment on ships and shore commands with aircraft INTERMEDIATE MAINTENANCE DEPARTMENTS (AIMDs). It is an automatic, high-speed, computer controlled, general-purpose test set that will isolate faults to the component level.

GENERAL-PURPOSE ELECTRONIC TEST EQUIPMENT (GPETE)

When no automatic means of accomplishing fault isolation is available, general-purpose electronic test equipment and good troubleshooting procedures is used; however, such fault diagnosis should be attempted only by experienced technicians. Misuse of electrical probes and test equipment may permanently damage boards or microelectronic devices attached to them. The proximity of leads to one another and the effects of interconnecting the wiring make the testing of boards extremely difficult; these factors also make drift or current leakage measurements practically impossible.

Boards that have been conformally coated are difficult to probe because the coating is often too thick to penetrate for a good electrical contact. These boards must be removed for electrical probe testing. Many boards, however, are designed with test points that can be monitored either with special test sets or general-purpose test equipment. Another method of obtaining access to a greater number of test points is to use extender cards or cables. The use of extender cards or cables makes these test points easier to check.

Special care should be exercised when probing integrated circuits; they are easily damaged by excessive voltages or currents, and component leads may be physically damaged. Precautions concerning the use of test equipment for troubleshooting equipments containing integrated circuits are similar to those that should be observed when troubleshooting equipment containing semiconductor or other voltage and current-sensitive devices.

Voltage and resistance tests of resistors, transistors, inductors, and so forth, are usually effective in locating complete failures or defects that exhibit large changes from normal circuit characteristics; however, these methods are time-consuming and sometimes unsuccessful. The suspect device often must be desoldered, removed from the circuit, and then retested to verify the fault. If the defect is not verified, the device must be resoldered to the board again. If this procedure has to be repeated several times, or if the board is conformally coated, the defect may never be located. In fact, the circuit may be further damaged by the attempt to locate the fault. For these reasons, the device should never be desoldered until all possible in-circuit tests are performed and the defect verified.

Q.7 List the three groups of test equipment used for fault isolation in 2M repair.
Q.8 What test equipment continuously monitors electronic systems?
Q.9 NELAT and VAST are examples of what type of test equipment?