1. **Introduction**
In the reverse engineering or maintenance of electronic equipment, engineers must first understand the interconnections between unknown PCB components. This requires measuring and recording the relationships between the component pins. Each component on the PCB must be considered. The simplest approach is to use a multimeter in “short circuit buzzer” mode to check the connectivity between pin pairs by touching the two probes, manually recording the status of each “pin pair” (connected or disconnected). To obtain a complete set of connections for all “pin pairs,” the data must be organized based on the principle of “pin pairs.” As the number of components and pins on the PCB increases, the number of required measurements also grows significantly. Clearly, performing this task manually would involve a large volume of measurement, recording, and verification, making it very time-consuming and prone to human error.
Additionally, the accuracy of such measurements is limited. It is well known that the resistance value displayed on a typical multimeter between two probes can reach 20 ohms, and the buzzer may still emit noise, indicating a connection path. To improve the efficiency of measurements, it is essential to automate the process of measuring, recording, and verifying the component “pin pairs.” To achieve this, the author has designed a path detector controlled by a microcontroller as a front-end detection device, coupled with a robust back-end processing software system that enables automatic measurement and recording of component connections on the PCB.
This article primarily discusses the design concepts and technologies used in the path detection circuit, enabling automatic measurements. The fundamental requirement for automatic measurement is to connect the pins of the component under test to the detection circuit. To achieve this, the testing equipment is equipped with multiple measuring heads. These heads are connected to various test fixtures and component pins via cables. The number of heads determines how many pins of the detection circuit are connected simultaneously in a given batch. Then, based on the program control of the detector, the pins are measured one by one, following the combination principle, to form the measurement path.
In the measurement path, the pass/break status between “pin pairs” is represented by a resistor between the pins. The detection circuit converts this resistor value into a voltage, which is used to determine the pass/break condition and record the result.
The PCB path detection circuit based on this concept should primarily achieve three key functions:
• Automatically select test “pin pairs” and perform measurements;
• Automatically determine the path relationship between “pin pairs”;
• Automatically record the measurement results.
### 2 Automatically Select and Measure a Pair of Pins
#### 2.1 Automatic Switching of Measurement Pin Pairs
To enable the detection circuit to select different pins for measurement from multiple measuring heads connected to component pins based on the combination principle, a corresponding switch array can be configured through programming. By turning off different switches, the component pins can be inserted into the measurement path, allowing their pass/break relationships to be obtained.
Since the measurement is done via an analog voltage, an analog multiplexer should be used to form the switch array.
#### 2.2 Measurement of Pass/Break Relationship
The design principle of the detection circuit allows for the simultaneous measurement of the pass/break relationship between all the pins connected to the measuring head, following the combination principle.
Since this measurement is performed between the pins of a component held in the test fixture, we refer to it as “in-fixture” measurement. If the component’s pin is not clamped, it must be measured using a test pen. As shown in Figure 2, the pen is connected to one analog channel, while the other is grounded. By turning off the appropriate switches, the control switch can be measured, which is called “stylus measurement.”
#### 3.1 Measurement Voltage (VA)
If VA is used as the measurement voltage, theoretically, when VA = 0, it indicates a path, while VA > 0 suggests an interruption. The value of VA will vary depending on the resistance between the two measurement channels. However, due to the non-negligible conductive resistance (RON) of the analog multiplexer itself, after the measurement path is formed, if VA is a path, VA will not be zero, but equal to the voltage drop across RON. Since the goal of the measurement is only to determine the pass/break relationship, there is no need to measure the exact value of VA. Therefore, only a voltage comparator is necessary to check if VA exceeds the voltage drop across RON.
Set the threshold voltage of the voltage comparator to match the voltage drop across RON. The output of the voltage comparator will then represent the measurement result, a digital value that can be directly read by the microcontroller.
#### 3.2 Determination of Threshold Voltage
It has been observed that RON varies with temperature and from one device to another. Therefore, it is essential to set the load threshold voltage individually by closing the analog switch channels, which can be accomplished using a D/A converter. By cycling through switch pairs (e.g., 1-1, 2-1, 1-2, 2-2, etc.), as shown in Figure 2, the threshold data can be easily determined. After each pair of switches is closed, the corresponding number is sent to the D/A converter, incrementing from small to large. The output of the voltage comparator is then measured. When the output switches from 1 to 0, the corresponding data represents VA at that moment. This process allows the voltage drop across RON for each channel path to be measured, which is the value when a pair of switches is closed.
#### 3.3 Dynamic Setting of Threshold Voltage
Using the threshold data collected above, a table can be constructed. During in-fixture measurements, the corresponding data can be retrieved from the table based on the numbers of the closed switches, and the threshold voltage can be sent to the D/A converter.
For pen-clamp and pen measurements, since the measurement path only passes through the analog switch, only the threshold data for that switch needs to be loaded. Additionally, because the circuit itself (D/A converter, voltage comparator, etc.) has inherent errors, and the contact resistance between the test fixture and the component pin may also affect the measurement, the actual load threshold voltage should be corrected. This ensures the system does not mistakenly classify a path as open. However, the corrected threshold voltage will suppress the impact of small resistances, so that even small resistances between the two pins are correctly identified as a path. The threshold voltage correction should be set reasonably based on actual conditions.
Through multiple PCB testing experiments, the testing circuit can accurately identify resistances greater than 5 ohms between pins, with accuracy levels that significantly surpass those of a typical multimeter.
In the reverse engineering or maintenance of electronic equipment, engineers must first understand the interconnections between unknown PCB components. This requires measuring and recording the relationships between the component pins. Each component on the PCB must be considered. The simplest approach is to use a multimeter in “short circuit buzzer” mode to check the connectivity between pin pairs by touching the two probes, manually recording the status of each “pin pair” (connected or disconnected). To obtain a complete set of connections for all “pin pairs,” the data must be organized based on the principle of “pin pairs.” As the number of components and pins on the PCB increases, the number of required measurements also grows significantly. Clearly, performing this task manually would involve a large volume of measurement, recording, and verification, making it very time-consuming and prone to human error.
Additionally, the accuracy of such measurements is limited. It is well known that the resistance value displayed on a typical multimeter between two probes can reach 20 ohms, and the buzzer may still emit noise, indicating a connection path. To improve the efficiency of measurements, it is essential to automate the process of measuring, recording, and verifying the component “pin pairs.” To achieve this, the author has designed a path detector controlled by a microcontroller as a front-end detection device, coupled with a robust back-end processing software system that enables automatic measurement and recording of component connections on the PCB.
This article primarily discusses the design concepts and technologies used in the path detection circuit, enabling automatic measurements. The fundamental requirement for automatic measurement is to connect the pins of the component under test to the detection circuit. To achieve this, the testing equipment is equipped with multiple measuring heads. These heads are connected to various test fixtures and component pins via cables. The number of heads determines how many pins of the detection circuit are connected simultaneously in a given batch. Then, based on the program control of the detector, the pins are measured one by one, following the combination principle, to form the measurement path.
In the measurement path, the pass/break status between “pin pairs” is represented by a resistor between the pins. The detection circuit converts this resistor value into a voltage, which is used to determine the pass/break condition and record the result.
The PCB path detection circuit based on this concept should primarily achieve three key functions:
• Automatically select test “pin pairs” and perform measurements;
• Automatically determine the path relationship between “pin pairs”;
• Automatically record the measurement results.
### 2 Automatically Select and Measure a Pair of Pins
#### 2.1 Automatic Switching of Measurement Pin Pairs
To enable the detection circuit to select different pins for measurement from multiple measuring heads connected to component pins based on the combination principle, a corresponding switch array can be configured through programming. By turning off different switches, the component pins can be inserted into the measurement path, allowing their pass/break relationships to be obtained.
Since the measurement is done via an analog voltage, an analog multiplexer should be used to form the switch array.
#### 2.2 Measurement of Pass/Break Relationship
The design principle of the detection circuit allows for the simultaneous measurement of the pass/break relationship between all the pins connected to the measuring head, following the combination principle.
Since this measurement is performed between the pins of a component held in the test fixture, we refer to it as “in-fixture” measurement. If the component’s pin is not clamped, it must be measured using a test pen. As shown in Figure 2, the pen is connected to one analog channel, while the other is grounded. By turning off the appropriate switches, the control switch can be measured, which is called “stylus measurement.”
#### 3.1 Measurement Voltage (VA)
If VA is used as the measurement voltage, theoretically, when VA = 0, it indicates a path, while VA > 0 suggests an interruption. The value of VA will vary depending on the resistance between the two measurement channels. However, due to the non-negligible conductive resistance (RON) of the analog multiplexer itself, after the measurement path is formed, if VA is a path, VA will not be zero, but equal to the voltage drop across RON. Since the goal of the measurement is only to determine the pass/break relationship, there is no need to measure the exact value of VA. Therefore, only a voltage comparator is necessary to check if VA exceeds the voltage drop across RON.
Set the threshold voltage of the voltage comparator to match the voltage drop across RON. The output of the voltage comparator will then represent the measurement result, a digital value that can be directly read by the microcontroller.
#### 3.2 Determination of Threshold Voltage
It has been observed that RON varies with temperature and from one device to another. Therefore, it is essential to set the load threshold voltage individually by closing the analog switch channels, which can be accomplished using a D/A converter. By cycling through switch pairs (e.g., 1-1, 2-1, 1-2, 2-2, etc.), as shown in Figure 2, the threshold data can be easily determined. After each pair of switches is closed, the corresponding number is sent to the D/A converter, incrementing from small to large. The output of the voltage comparator is then measured. When the output switches from 1 to 0, the corresponding data represents VA at that moment. This process allows the voltage drop across RON for each channel path to be measured, which is the value when a pair of switches is closed.
#### 3.3 Dynamic Setting of Threshold Voltage
Using the threshold data collected above, a table can be constructed. During in-fixture measurements, the corresponding data can be retrieved from the table based on the numbers of the closed switches, and the threshold voltage can be sent to the D/A converter.
For pen-clamp and pen measurements, since the measurement path only passes through the analog switch, only the threshold data for that switch needs to be loaded. Additionally, because the circuit itself (D/A converter, voltage comparator, etc.) has inherent errors, and the contact resistance between the test fixture and the component pin may also affect the measurement, the actual load threshold voltage should be corrected. This ensures the system does not mistakenly classify a path as open. However, the corrected threshold voltage will suppress the impact of small resistances, so that even small resistances between the two pins are correctly identified as a path. The threshold voltage correction should be set reasonably based on actual conditions.
Through multiple PCB testing experiments, the testing circuit can accurately identify resistances greater than 5 ohms between pins, with accuracy levels that significantly surpass those of a typical multimeter.
