Nomally the output of a logic chip is a one or a zero and this is done by
making the pin on the chip present a high or low voltage. This is push-pull.
Depending on what you connect to the pin, current could flow either
into or out of the pin for either a one or a zero.
Also, you would never want to connect two outputs together because
they could fight each other.
A tri-state output can be "opened-up" under logic control so it looks
like the pin has been disconnected from the chip. This lets you connect
several outputs together in a way where the logic can decide which of
them is doing the outputing. It's a push-pull with a disable feature.
Open collector is when it's only pull, not push. The pin can only pull
current into it, it's can't push current out. This lets you connect
several outputs and they won't fight but you don't have to have
any disable logic. There is usually something called a "pull-up resistor"
to replace the missing transistor in a push-pull arrangement.
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