Digital Electronics: Principles, Devices and Applications

654 Digital ElectronicsFault detection means knowing the nature of the fault, which could be done by comparing the actualor present performance of the circuit with the ideal or desired performance. Complete knowledge aboutthe nature of the fault often gives an idea about the nature of tests and measurements to be performedto isolate the fault. It is therefore important that the nature of the fault is properly understood andappreciated in terms of the functions performed by various parts of the overall digital circuit or system.Fault isolation means performing tests and making measurements with the available diagnostic toolsto know precisely where the fault lies. This could be in the form of a faulty component or a shorted oropen track and so on. The level of documentation that is available plays an important role in decidingabout the type of measurements to be made to isolate the fault. Comprehensive documentation helpsin significantly reducing the time period required to actually latch on to the faulty component or area.Again, the faults could either be internal to the components and devices, digital integrated circuits,for instance, or external to the components. These two types of fault are discussed in the followingparagraphs.Remedial measures follow the fault isolation. This could mean repairing of tracks or replacement ofone or more components.16.1.1 Faults Internal to Digital Integrated CircuitsDigital circuits and systems are dominated by the use of digital integrated circuits (ICs). The numberof discrete devices is usually much smaller than the number of ICs used. Therefore, the knowledgeof typical faults that can occur in digital ICs is central to fault isolation in digital systems. The mostcommonly observed defects or failures in digital ICs are as follows:1. Shorting of input or output pins to V CC or ground terminals or shorting of tracks.2. Open circuiting of input or output pins.3. Shorting of two pins other than ground and V CC pins.4. Failure of the internal circuitry of the IC.16.1.1.1 Internal Shorting of Input or Output Pins to GND or V CCThis is one of the commonly observed faults internal to digital ICs. Shorting of one or more of theinput or output pins internally to GND puts a permanent LOW on the pin(s). This could have severalmanifestations depending upon the nature of the IC and also upon the nature of the component drivingthese pins. Some of these manifestations are given in the following examples:1. If an input pin that is internally shorted to GND is being driven from an output pin of another IC,that particular output pin will face a permanent ground and will be affected accordingly. A pulsatingsignal, if originally present at that pin, will vanish.2. If the shorted input terminal happens to be that of a NAND gate, the output of the gate willpermanently go to the logic HIGH state and will not respond to any changes on the other input.3. If the shorted input pin is the PRESET input of a presettable, clearable J–K flip-flop with activeLOW PRESET and CLEAR inputs, the output of this particular flip-flop will always be in the logicHIGH state irrespective of the status of the J and K inputs.4. Shorting of the output pin to GND puts a permanent logic LOW on that pin, and this particularoutput does not respond to changes on the corresponding input pins.