Open Drain GPIO

The DS2413 outputs are "Open Drain". What that means is that the output is the "Drain" of an N-channel FET:
Internally, the "Source" of the FET is connected to Ground. But the Drain is left open. When switched on, the FET provides a path for current to flow from the output pin to ground.
Since there is no internal connection to VCC, the open drain output does not put out a voltage like an Arduino GPIO pin. You have to provide your own external pullup. While this may seem inconvenient, the open drain configuration allows for a lot of flexibility. Since the output voltage is not dependent on the operating voltage of the breakout, you can use a variety of power sources. The FETs in the DS2413 are capable of switching voltages up to 28v and up to 20mA.

The diagram below from the DS2413 data sheet shows how input and output connections should be made - with an external pullup to a local power source.
For more information, refer to the DS2413 data sheet:
This guide was first published on Mar 28, 2014. It was last updated on Mar 28, 2014. This page (Open Drain GPIO) was last updated on Aug 20, 2019.