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An open collector output processes an IC's output through the base of an internal bipolar junction transistor (BJT), whose collector is exposed as the external output pin. For NPN open collector outputs, the emitter of the NPN transistor is internally connected to ground, [1] so the NPN open collector internally forms either a short-circuit ...
Open-collector buffers connected as wired AND. The wired AND connection is a form of AND gate. When using open collector or similar outputs (which can be identified by the ⎐ symbol in schematics), wired AND only requires a pull up resistor on the shared output wire. In this example, 5V is considered HIGH (true), and 0V is LOW (false).
The open collector input/output is a popular alternative to three-state logic. For example, the I²C bus protocol (a bi-directional communication bus protocol often used between devices) specifies the use of pull-up resistors on the two communication lines. When devices are inactive, they "release" the communication lines and tri-state their ...
A common variation omits the collector resistor of the output transistor, making an open-collector output. This allows the designer to fabricate wired logic by connecting the open-collector outputs of several logic gates together and providing a single external pull-up resistor. If any of the logic gates becomes logic low (transistor conducting ...
The output of an inverter is at the collector. Likewise, it is either a current sink (low logic level) or a high-z floating condition (high logic level). Like direct-coupled transistor logic, there is no resistor between the output (collector) of one NPN transistor and the input (base) of the following transistor.
A 555 timer can act as an active-low SR latch (though without an inverted Q output) with two outputs: output pin is a push-pull output, discharge pin is an open-collector output (requires a pull-up resistor). For the schematic on the right, a Reset input signal connects to the RESET pin and connecting a Set input signal to the TR pin.
If you've been having trouble with any of the connections or words in Wednesday's puzzle, you're not alone and these hints should definitely help you out. Plus, I'll reveal the answers further ...
An alternative to push–pull output is a single switch that disconnects or connects the load to ground (called an open collector or open drain output), or a single switch that disconnects or connects the load to the power supply (called an open-emitter or open-source output).