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It is a referance to the output of the component. If you can emagine the connection to the device goes directly to the collector of the component within the device. Very usfull for interfacing and matching loads.
The open collector output of a TTL device (for example) is the collector of a NPN transistor. The emiter is connected to ground. And the base is driven by the internal circuit.
This output can have two states:
OFF - No current passes from the collector to GND.
ON - The transistor conducts to gnd. (transistor in saturation).
So basically you have a switch to ground. to obtain a voltage level from this circuit you must provide a pullup resistor. That is, a resistor to VCC or any other voltage.
Other applications include:
Wired OR - You can parallel several Open collector outputs.
Level conversion
Low side driving.
K
In the fallowing ckt, what is your output with out the external resister conncted. Zer/floating. This type of output is Open Collector-Nothing conncted to collector of output transister. You can't drive anything with open collector outputs
If you use a pull up, the output changes between Vcc and Ground.
So you can connect any value of Vcc say 5v/24V. To give you an option to select the output level, it was left unconnected to a fixed value during manufacturing stage
In the old days of 40 years ago this served a useful purpose. One was making the OR function by putting all of the open collectors to one node with a resistor pullup. Back then a DIP logic IC cost one hour of salary of an average factory worker, so saving one DIP in a design was economically rewarding. The open collectors could also be used to control relays.
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