But sometimes your application may require the simpler open collector output. When in the high impedance state the pins can usually be used as inputs. Instead, they are more generally useful totem-pole, three-state outputs - they have transistors to actively source current for a logic high, and to sink current for logic low, as well as the ability to turn off both transistors to isolate the output in a high impedance state. The digital outputs of the PDQ Board, QCard Controller, and other Wildcards are not open-collector. In their high state, they are open circuits, and in their low state they pull the output pin to ground, sinking as much current as needed, up to a limit. Essentially, they act as simple switches to ground. Open-collector or open-drain outputs do not source current, they can only sink current. When driving external devices, it is sometimes necessary to use open-collector or open-drain outputs.
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