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An open collector output can onliy drive the output pin to GND. That is: when the output is logic zero the pin is connected to GND and when logic one the output pin is floating (high impedance state).
Normal TTL outputs can drive the output pin to GND (logic zero) or to VCC (logic one)
OC outputs have only one transistor (it may be a FET in CMOS logic) . The collector is connected to the output pin.
Normal TTL outputs have 2 transistors (one to VCC and the other to GND) which is known as a push-pull or totem pole output.
An open collector output can onliy drive the output pin to GND. That is: when the output is logic zero the pin is connected to GND and when logic one the output pin is floating (high impedance state).
Normal TTL outputs can drive the output pin to GND (logic zero) or to VCC (logic one)
OC outputs have only one transistor (it may be a FET in CMOS logic) . The collector is connected to the output pin.
Normal TTL outputs have 2 transistors (one to VCC and the other to GND) which is known as a push-pull or totem pole output.
There are many applications, for example:
OC permits to wire several outputs together to make what is called a wired OR. You cannot do this with normal TTL outputs.
To get a logic one from an OC output you must place a pullup resistor. As this resistor could be connected to +5v, +12v or more (depending on the maximum voltage allowed by the output transistor) you can use the OC to shift levels or to drive a stage that requires only connection to GND as a high side driver with a PNP transistor.
It can even be used in analog circuit. As it is just as a switch to GND.
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