push-pull vs open drain

[rookie]

Hi !

I'm trying to understand the difference between push-pull and open-drain in the context of multiple inputs connected on the i/o line. For example, I know open drain allows multiple inputs to drive the line low, something that is not possible in push-pull configuration. But I don't know the technical (electronics) reason behind this. Something something p-mos/n-mos... Can someone enlighten?

[ataradov]

Because in an open drain configuration an output transistor can do two things:
1. Not drive the output.
2. Drive the output to ground.

The high level is ensured by the outside pull-up resistor.

There will never be a conflict of levels. If either of the devices drives the line low, it will be low no matter what others are doing.

In a push-pull configuration the output is actively driven, so if one device decided to drive high level and another one decided to drive low level, there will be a conflict. The end result is unpredictable and depends on the exact design of the output stages.

[MarkF]

Here is a crude visualization using relays and switches.
The resistor is NOT part of any gate.