Open collector

An open collector is a common type of output found on many integrated circuits (IC), which behaves like a switch that is either connected to ground or disconnected.

Instead of outputting a signal of a specific voltage or current, the output signal is applied to the base of an internal NPN transistor whose collector is externalized (open) on a pin of the IC. The emitter of the transistor is connected internally to the ground pin. If the output device is a MOSFET the output is called open drain and it functions in a similar way.

In the picture, the transistor base is labeled "IC output". This is a signal from the internal IC logic to the transistor. This signal controls the transistor switching. The external output is the transistor collector; the transistor forms an interface between the internal IC logic and parts external to the IC.

On schematic component symbols, the open output is indicated with these symbols:

The output forms either an open circuit or a connection to ground. The output usually consists of an external pull-up resistor, which raises the output voltage when the transistor is turned off. When the transistor connected to this resistor is turned on, the output is forced to nearly 0 volts. Open-collector outputs can be useful for analog weighting, summing, limiting, etc., but such applications are not discussed here.

A three-state logic device is unlike an open collector device, because it comprises transistors to source and sink current in both logic states, as well as a control to turn off both transistors and isolate the output.

Because the pull-up resistor is external and does not need to be connected to the chip supply voltage, a lower or higher voltage than the chip supply voltage can be used instead. Open collector circuits are therefore sometimes used to interface different families of devices that have different operating voltage levels. The open-collector transistor can be rated to withstand a higher voltage than the chip supply voltage. Such devices are commonly used to drive devices such as Nixie tubes, and vacuum fluorescent displays, relays or motors which require higher operating voltages than the usual 5-volt logic supply.

Another advantage is that more than one open-collector output can be connect to a single line. If all outputs attached to the line are in the high-impedance state, the pull-up resistor will hold the wire in a high voltage (logic 1) state. If one or more device outputs are in the logic 0 (ground) state, they will sink current and pull the line voltage toward ground. This wired logic connection has several uses.

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Wikipedia