1. PCB Maintenance Example
important hint:
When the chip pins are short-circuited to ground or power, the short-circuit resistance value is between 10-25 milliohms.
A one-inch-long attached copper wire has a resistance of approximately 40-50 milliohms.
For example: during an online function test of a 74640, pin 6 shows a resistance to ground of 1 ohm. However, with QT50 measurement, the resistance to ground on the 6th pin is about 160 milliohms. This suggests a potential short-circuit fault due to other connected devices. Upon inspection with QT50, it is discovered that the chip is linked to a resistor bank and a setting switch. The resistance to ground measured at the switch contact is approximately 40 milliohms, lower than that measured on pin 6. This indicates that the chip’s displayed short-circuit fault to ground on pin 6 is actually caused by the setting switch connecting to ground, rather than a fault within the chip itself.
Under normal circumstances, bus device pins in the design should not have (or should have very minimal) grounding or power supply. If a pin shows grounding or power supply, retesting the device is recommended.
In the absence of a good board for reference and comparison, analyzing pin status and actual measured waveforms can facilitate repair of faulty boards. For instance, many chips utilize only a portion of the logic cells in circuit design, and the input pins of unused parts are typically grounded to prevent erratic operation and interference with the circuit. Analyzing the actual output waveform based on the chip’s logic function can greatly aid in determining whether the tested chip is indeed faulty.
Example 1: 74123 (monostable resonator) pin appears floating (FLT)
During online functional testing, the input pin of the device typically shows a high-impedance state (resistance greater than 1 megaohm). This occurs when the TTL or CMOS load is not connected during offline testing. In online testing, the input pin of the chip is usually connected to the output pin of another chip to ensure it drives the fan-out load with low impedance.
If an input pin displays “FLT” in the pin status window during online testing, it indicates the pin is floating. This could be due to it being connected to the boundary connection of the circuit board, a tri-state device, or an open circuit on the PCB.
Comparing this pin’s state with other input pins helps determine if it’s operating normally. In this example, the 6th pin connects to the input of an RC circuit. Since this pin needs to discharge the capacitor, it cannot remain in a high-impedance state; otherwise, it would fail to discharge the capacitor, leading to an ICFT test error. For instance, the 7th pin showed “FLT” while the corresponding pin 15 exhibited a normal logic level (about 550 ohms to ground).
Even though QT200 reported a “test failure,” observing the flipped output pin suggests a timing issue. If overlooked, this might be mistaken as a timing problem.
Carefully monitoring and analyzing pin status information is crucial for accurately identifying faults. An input pin with 550 ohms impedance to ground cannot be in a floating state (“FLT”). In this example, the malfunctioning monostable resonator prevented proper capacitor discharge due to functional damage.
Similarly, an output pin cannot float (“FLT”) because it must either source or sink current to drive any fan-out load. Users should also note that any node’s impedance to ground shouldn’t drop below 5-10 ohms unless there’s an actual short circuit (where impedance might measure around 2 ohms). Typical buffer drivers in a logic low state have an impedance of approximately 15-17 ohms.
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important hint:
When the chip pins are short-circuited to ground or power, the short-circuit resistance value is between 10-25 milliohms.
A one-inch-long attached copper wire has a resistance of approximately 40-50 milliohms.
For example: during an online function test of a 74640, pin 6 shows a resistance to ground of 1 ohm. However, with QT50 measurement, the resistance to ground on the 6th pin is about 160 milliohms. This suggests a potential short-circuit fault due to other connected devices. Upon inspection with QT50, it is discovered that the chip is linked to a resistor bank and a setting switch. The resistance to ground measured at the switch contact is approximately 40 milliohms, lower than that measured on pin 6. This indicates that the chip’s displayed short-circuit fault to ground on pin 6 is actually caused by the setting switch connecting to ground, rather than a fault within the chip itself.
Under normal circumstances, bus device pins in the design should not have (or should have very minimal) grounding or power supply. If a pin shows grounding or power supply, retesting the device is recommended.
In the absence of a good board for reference and comparison, analyzing pin status and actual measured waveforms can facilitate repair of faulty boards. For instance, many chips utilize only a portion of the logic cells in circuit design, and the input pins of unused parts are typically grounded to prevent erratic operation and interference with the circuit. Analyzing the actual output waveform based on the chip’s logic function can greatly aid in determining whether the tested chip is indeed faulty.
Example 1: 74123 (monostable resonator) pin appears floating (FLT)
During online functional testing, the input pin of the device typically shows a high-impedance state (resistance greater than 1 megaohm). This occurs when the TTL or CMOS load is not connected during offline testing. In online testing, the input pin of the chip is usually connected to the output pin of another chip to ensure it drives the fan-out load with low impedance.
If an input pin displays “FLT” in the pin status window during online testing, it indicates the pin is floating. This could be due to it being connected to the boundary connection of the circuit board, a tri-state device, or an open circuit on the PCB.
Comparing this pin’s state with other input pins helps determine if it’s operating normally. In this example, the 6th pin connects to the input of an RC circuit. Since this pin needs to discharge the capacitor, it cannot remain in a high-impedance state; otherwise, it would fail to discharge the capacitor, leading to an ICFT test error. For instance, the 7th pin showed “FLT” while the corresponding pin 15 exhibited a normal logic level (about 550 ohms to ground).
Even though QT200 reported a “test failure,” observing the flipped output pin suggests a timing issue. If overlooked, this might be mistaken as a timing problem.
Carefully monitoring and analyzing pin status information is crucial for accurately identifying faults. An input pin with 550 ohms impedance to ground cannot be in a floating state (“FLT”). In this example, the malfunctioning monostable resonator prevented proper capacitor discharge due to functional damage.
Similarly, an output pin cannot float (“FLT”) because it must either source or sink current to drive any fan-out load. Users should also note that any node’s impedance to ground shouldn’t drop below 5-10 ohms unless there’s an actual short circuit (where impedance might measure around 2 ohms). Typical buffer drivers in a logic low state have an impedance of approximately 15-17 ohms.
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