Dip-in-line, opposite type, with semi-circular notch at the top, 1 foot on the left, and the largest foot on the right, such as 14, 16, 20, etc.

Smaller packaging patches, such as SO, SSOP, etc. There is a dot or triangle near the 1 pin.

Or, put the chamfered side (the other side is not chamfered) on the left, and the first position on the upper left is 1.

The lowest corner on the left of most TTLs is VSS, which is usually grounded, and the highest corner on the right is VCC, which is usually connected to 5V or other positive voltage.

When the connection is reversed, it just rotates, and the ground and power supply are reversed. Usually, the circuit current will increase a lot, which may increase by more than 100 mA. The reverse chip is hot, but if the power supply is turned off in time, most chips will not be damaged (bad quality or bad luck), and can still be used after correction. Sometimes multiple chips are connected in parallel between power supply and ground. I don't know which chip is connected backwards. You can even determine which one is hot by hand. Your hands are delicate and tender, and your skin is rough and unresponsive (use with caution! )。

When debugging a new circuit, if you are worried about reverse connection, you can adjust the current limit of the power supply to a lower level. Once the current is abnormal, the power supply will limit the current and step down to protect the chip from being directly damaged by large current.