I. Voltage Method for Determining the Quality of Digital Integrated Circuits
1. If the power supply voltage of the digital integrated circuit is normal and the soldering is good, but the measured voltage value of its power supply pin is too low, then the digital integrated circuit under test is damaged.

2. If the measured voltage value of the power supply pin of the digital integrated circuit is normal, but the voltage values of most other pins are abnormal, then it indicates that the ground pin has a cold solder joint, while the integrated circuit itself is mostly normal.

3. If the measured voltage value of one or a few pins of the digital integrated circuit deviates significantly from the normal value, then the corresponding peripheral components should be checked for faults, such as short circuits, open circuits, leakage, or breakdown of capacitors. If the peripheral components are not faulty, then the digital integrated circuit under test is damaged.

4. If the measured voltage values of most pins of the digital integrated circuit deviate significantly from the normal value, and its power supply voltage is normal, and there are no cold solder joints on the power supply and ground pins, then the digital integrated circuit is damaged.

II. Testing the Quality and Performance of TTL Circuits
Carefully observe the model number of the TTL integrated circuit (IC), consult the relevant datasheet, and locate the ground terminal of the IC. Ideally, find its internal circuit diagram or wiring diagram.

Set the multimeter to the R×1K range. Connect the black probe to the ground terminal of the IC, and use the red probe to test the DC resistance to ground of each input and output terminal in sequence. Under normal conditions, the resistance to ground of each pin of the IC should be 3~10kΩ. If the resistance to ground of any pin is less than 1 kΩ or greater than 12 kΩ, the IC is damaged.

Ground the multimeter to ground with the red probe, and use the black probe to test each input and output terminal of the IC in sequence. Under normal conditions, the reverse resistance to ground of each terminal should be greater than 40 kΩ. The resistance to ground of each pin of a damaged IC is less than 1 kΩ. In a normal TTL circuit, the forward and reverse resistance values of the power supply positive and negative pins are smaller than the resistance to ground of other pins, with a maximum of no more than 10 kΩ. If this value is zero or infinite, it indicates that the power supply pin of this integrated circuit is damaged.

III. Differentiating TTL and CMOS Circuits by Voltage Method
Based on their model numbers, such as CC4000, CD4006, and MC14021, they are CMOS circuits, while CT3020 and the 74 series are TTL circuits.

Based on their power supply voltage, if a digital integrated circuit can operate normally at 3~4.5V or 5.5~18V without knowing its model number, it is definitely a CMOS circuit. Alternatively, a multimeter can be used to test the output level of the integrated circuit. When the power supply voltage is 5V, connect the circuit's input terminals to high and low levels, then use a multimeter to test the output terminals. If the difference between the high and low levels is close to 5V, it is a CMOS integrated circuit; if it is close to 3.5V, it is a TTL integrated circuit.

Distinguishing between CMOS and high-speed CMOS circuits can be done by analyzing their output voltage levels. Using a simple gate circuit as an example, with a 5V power supply and a multimeter set to the 10V DC range, connect the input terminals of the integrated circuit to high and low voltage levels sequentially, and measure the corresponding voltage values at the output terminals. If the difference between these values is close to 5V, it is a CMOS circuit; if the difference is close to 3.5V, it is a TTL circuit.

IV. Differentiating between CMOS and High-Speed CMOS Circuits
Since the power supply voltage of CMOS circuits is 3~18V, while that of high-speed CMOS circuits is 2~6V, if the integrated circuit operates normally after applying 2~2.5V, it is a high-speed CMOS circuit; otherwise, it is a CMOS circuit.