Digital Electronics: Principles, Devices and Applications

Troubleshooting Digital Circuits and Test Equipment 655Shorting of input or output pins to V CC puts a permanent HIGH on those pins. If it is the outputpin, it again fails to respond to any changes on the corresponding input pins, and, if it is the input pin,it affects the output response of the IC depending upon the nature of the IC. The following examplesillustrate this point further:1. If it is the input of the NAND gate, a permanent HIGH on the input permanently transforms it intoan inverter circuit, which means that the NAND gate no longer performs its intended function.2. If it happens to be the input terminal of an OR gate, it drives its corresponding output to a permanentlogic HIGH state.16.1.1.2 Open Circuiting of Input or Output PinsOpen circuiting of input and output pins occurs for reasons internal to the IC when the fine wire thatconnects the IC pin to the relevant location on the chip breaks. The effects of open circuiting can beserious too. For instance, an open on the input or output pin makes it a floating terminal, and, if theIC belongs to the TTL logic family, it will be treated as logic HIGH. It could even lead to overheatingand subsequent damage to the IC. An open on the input pin also prohibits any genuine changes onthe pin from reaching the input on the chip, with the result that the output fails to respond to thosechanges. Similarly, an open on the output pin affects the response of the subsequent IC to whose inputthis particular output is connected.16.1.1.3 Shorting of Two Pins Other than GND and V CC PinsThis fault forces the affected pins to have the same logic status at all times. For obvious reasons, theoutput responds incorrectly. Such a situation also leads to shorting of the two pins from where theseaffected (internally shorted) pins are being fed. The ultimate effect on the performance depends uponthe nature of the ICs involved.16.1.1.4 Failure of the Internal Circuitry of the ICFailure of the internal circuitry could be anything from damage to a certain active device to increasein the resistance value of a certain on-chip resistor. Bearing in mind the complexity of the internalcircuitry of the present-day digital ICs, there could be numerous possibilities. However, the occurrenceof such a fault is not very common.16.1.2 Faults External to Digital Integrated CircuitsThe commonly observed faults external to digital ICs include the following:1. Open circuits.2. Short circuits.3. Power supply faults.