[dbcurtis]

Consider a I/O pin set to open-drain drive. It either actively pulls the output pin to the ground rail, or the I/O floats. Externally, you have a pull-up resistor.

If there are multiple chips connected in open-drain, you write the I/O pin to 0 or 1 to either ground or float the pin. You read the I/O pin to see if anyone else has grounded it if you haven't. This was called "wired-OR" in the old days. (It's really AND, but was commonly used in DeMorgan equivalent form for communication buses before tri-state drivers became a thing. I'll get my cane and hobble back to my rocking chair now..)

[posterboy]

I think in the old days, before CMOS, this was called open-collector. I had learned about it in first semester digital logic not too long ago playing with the "high speed" 7400 series. If I remember correctly, the latch needs to support the open collector capability, but the reason escapes me.

[dbcurtis]

Open collector if you have a bipolar technology. Modern microcontrollers are CMOS, so the pull-down transistor has a drain, not a collector. TTL is bipolar, so the transistor has a collector.