Application of rear drive technology in electronic testing and maintenance

l Device failure forms and traditional testing and maintenance methods
In the testing and maintenance of equipment, there are many failure factors on the circuit board. Practical statistics show that device failure accounts for 94%, timing failure accounts for 3%, solder joints account for 2%, and others account for 1%. Therefore, quickly and accurately finding faulty devices is the key to equipment maintenance.
Device failures are generally manifested as functional failures and parameter failures. Functional failure means that the device cannot realize its basic function, for example, the inverter cannot invert, the amplifier cannot amplify, etc. Parameter failure means that the device cannot perform its function well, for example, the inverter can invert, but the fan-out capacity decreases, the amplifier can amplify, but the accuracy decreases, etc.
In general, device failures will be reflected on the input and output ports of the device. If the device on the faulty circuit board is functionally tested and the faulty device is found, the circuit board can be repaired. The traditional testing method is to use an oscilloscope and a multimeter to manually test the device. This method has many disadvantages: the testing process is slow and the repair time is long; it requires the tester to be familiar with the principle of the circuit board and have rich experience in testing and maintenance; the test results will be different depending on the level of the tester, and there are many cases of misjudgment.
With the increasing complexity of circuit boards and the high degree of integration of devices, traditional testing methods have encountered many difficulties, as shown in the following:
(1) When the equipment leaves the factory, it is only equipped with an instruction manual, lacking the schematic diagram and PCB diagram of the electrical part, which brings certain difficulties to the testing and maintenance personnel.
(2) Modern equipment has a high degree of integration, and traditional discrete component circuit knowledge and detection methods are inefficient and ineffective, and are no longer applicable.
(3) Device function testing requires online testing. How to isolate the device from other devices when powered on without damaging the device is a technical problem.
In order to improve the efficiency of modern electronic testing, shorten the testing cycle and cost, and reduce the professional requirements for testers, a new type of online testing technology has been rapidly developed and applied.

2 Back-drive technology
2.1 Principles of back-drive technology
In 1968, Fractron of Schlumberger Company in the United States first proposed the concept of "back-drive technology", which is mainly used for online testing of digital circuits. The literature discusses the safety of back-drive technology in the maintenance of military equipment and formulates strict technical specifications. The military standard defines back-drive as follows:
"The occurrence of back-drive is a direct result of node focing. Node focing means that when a certain step of the test is performed, the input pin of the device under test (DUT) is forced to a specific logic state, regardless of the logic state of the other device pins connected to it. This technology is called back-drive."

The core of the post-drive technology is to use the characteristics of semiconductor devices that allow instantaneous overload to inject a large instantaneous current into the output stage of the front-stage driver chip of the device under test, forcing it to become high or low according to the test requirements, so as to achieve the purpose of logically isolating the influence of the front-stage device on the device under test.
In digital circuits, the output of the front-stage driver rather than the input of the driven device determines the potential of the circuit node. According to the principle of post-drive technology, the output voltage can be changed by changing the size of the node output current. Figure 1 shows the current flowing out of the output terminal when the output is high and the current flowing in from the outside when the output is low.
Post-drive technology can realize the electrical isolation of digital IC devices online without destructive physical isolation, making it possible to test the function of digital IC devices one by one online.
2.2 Application analysis of post-drive technology
Figure 2 is a typical TTL output stage circuit structure. When T4 is cut off and T3 is deeply saturated, the output is low level. The output low level is the saturation voltage drop of T3, which is about 0.2~0.3 V. When T3 is saturated, β3*Ib3》Ic3. At this time, if a sufficiently large current is injected into T3 from the output end, T3 will be out of saturation and the output level will be increased. That is, driving a large current after the transient state is injected into the line can force the node to be set from a low level to a high level. Conversely, it can force the node to be set from a high level to a low level.

2.3 Impact of post-drive technology on devices and safety margin analysis

The literature provides a typical current-voltage characteristic curve for post-driving of a TTL circuit, as shown in FIG3 .

The literature also makes strict regulations on the current and test time when the circuit is tested online using the back-drive technology. It stipulates that the absorption/output current of the pin driver should not exceed 750 mA, and the action time should not exceed 65 ms.
At present, the current of online testers based on the back-drive technology at home and abroad is set within 500 mA, and the test time is within 16 ms to ensure the safety of the instrument during online testing.

3 Application of post-drive technology
The development of post-drive technology has promoted the development of testing technology and instruments, opened up a new way for modern electronic testing and maintenance, and changed the traditional equipment testing and maintenance methods. The specific application of post-drive technology is the circuit in-circuit tester. Its principle is shown in Figure 4. The tester is based on the test program library. It instantly and forcibly applies a driving signal to the input end of the device under test, tests its actual output signal, and compares it with the standard code. If the results are consistent, it is considered that the device under test functions normally. If the results are inconsistent, it is considered that the device under test functions abnormally.

The tester based on post-drive technology has a powerful test library and can test a variety of devices, including TTL75/54, CMOS40/45, various drivers, Russian device libraries, various RAM/EPPROM, PLD, integrated operational amplifiers, comparators, voltage regulators, etc. The online tester can not only realize the functional test of the device, but also perform the VI characteristic curve test, model identification, memory test, circuit board network diagram extraction, etc.

4 Conclusion
Practice has proved that the online circuit tester based on post-drive technology has broad application prospects in the civil and military fields. It can greatly shorten the time for equipment fault testing and maintenance, quickly restore the combat effectiveness of weapons and equipment, improve work efficiency, and save maintenance expenses. If you want to conduct a deeper test on electronic devices, you need to improve the test library of the tester on the basis of existing technology, improve the test speed and accuracy, and add multi-voltage output to support the test of more devices. This is also the next development direction of the online tester.