What are the open-collector gates?
Open collector gates are a type of logic gates whose output is externalized, that is, open or without resistance in the collector of the output transistor. By using this type of integrated circuit, the possibility of using the appropriate resistance value according to your needs is left.
open collector

Open collector outputs are useful for:
Set high and low voltage values as needed. Furthermore, this allows for the coupling between logic gates with different high levels.

Guarantee the output current necessary to connect several logic gates to the output of this one.

Connection of several gates with open collector output to the same data bus. By sharing the external resistance in the collector, a “wired” gate is created, that is, the logical function between the connected gates occurs in the cable.

Open collector gates can operate without the external resistor when connected to the inputs of other gates, although this is not recommended due to the low noise immunity found. Without an external resistance, the gate output will be an open circuit when the output is in the low state.

Open collector gates are basically used in three main applications:
-drive a lamp or relay.
– perform wired logic
– for the construction of a common bus system.

An open collector output can drive a lamp placed at its output through a tender resistor.

Wired logic.
If the outputs of multiple TTL open collector gates are linked with a single external resistor, wired-AND logic is performed.
wire logics

Common bus.
Open collector gates can be linked to form a common bus. At any time, all gate exits linked to the bus, except one, must be kept in their high state. The selected gate can be in either the high or low state, depending on whether you want to transmit a 1 or a 0 on the bus.

commune bus