When using a sink open drain pin as an input, why should the output of this pin be set to 1 ?

Microcomputer Frequently Asked Questions (FAQs) NO.EB9901301

Answer

When using a sink open drain pin as an input, the output of this pin should be set to 1. This is because setting output data to 1 turns off the output transistor, enabling external input to be captured.
The following shows an example of a port equivalent circuit.

Example: TMP86C807