What is the difference between a push-pull output and an open-drain output?

First, let me introduce what is push-pull output and open-drain output.

Push-Pull Output (Push-Pull Output)

The push-pull output structure is controlled by two MOS or transistors receiving complementary control signals. One of the two tube clocks is on and the other is off. The figure below is a push-pull output circuit diagram using a triode.
When the input is positive, the upper N-type transistor is turned on, and the current is supplied to the load from top to bottom, which is called "push";
when the input is negative, the lower P-type transistor is turned on, and the current is provided from bottom to top. For the load, it is called "pull".
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Advantages : Strong driving ability
Disadvantages : The push-pull output cannot be connected, otherwise it will cause a short circuit.
For example: if there are two circuits, one of which outputs high level, the output should be connected to Vcc, and the other outputs low level, the output should be connected to GND.
Then now connecting the outputs of the two circuits together creates a short circuit.

Open Drain Output (Open Drain Output)

Commonly referred to as open-drain output actually includes open-drain output and open-collector output .
The principles and characteristics of these two outputs are basically similar, the difference is that one uses a MOS tube, where the "drain" refers to the drain of the MOS tube; the other uses a triode.
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The NOP MOS tube used here, when the input is high, the MOS tube is turned on, the IO output is pulled to GND, and the output is low; when the input is low, the NPN transistor is closed, and the output is equivalent to an open circuit (output high impedance) .
It can be seen that the open-drain output is inverted.
When the input is low, the output is high;
when the input is high, the output is low.
The non-inverting output is realized by connecting an additional MOS tube.

The main characteristic of open-drain output is that high level has no drive capability, and an external pull-up resistor is needed to truly output high level.

Advantages: It is very convenient to adjust the output level, because the output level is completely determined by the power supply level connected to the pull-up resistor. So it is very suitable to use open-drain output where level shifting is required.
Another advantage is that the "wire-AND" function can be realized. The so-called "wire-AND" means that multiple signal lines are directly connected together. Only when all the signals are at high level, the combined bus is at high level; As long as any one or more signals are low, the bus is low.

compare the two

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This article refers to an article on Kevin Zhang Zhihu, link: https://zhuanlan.zhihu.com/p/41942876