An open-drain output actively pulls a line low but releases it for high. A pull-up resistor raises the released line. Several compatible open-drain devices can share a line: any one can pull it low, while all must release it for the line to become high.
Worked example
An illustrative 3.3 V line with a 4.7 kΩ pull-up draws roughly 3.3/4700 = 0.70 mA when held near zero. A larger resistor reduces this current but generally makes the capacitive line rise more slowly. Choose its value from the devices' limits and required timing.
What to watch for
A pull-up does not automatically make arbitrary voltage domains compatible. Check voltage tolerance, low-level sink capability and the permitted rise time.
Try it yourself
If two open-drain devices share a line and one pulls low while the other releases, what is the line's logic level?
Answer: Low, provided the pulling device meets the load and voltage specifications. Releasing is not the same as actively driving high.
How to study this topic
Use the numerical values here as teaching examples. For a physical design, consult the actual component data sheet at the intended supply, load and temperature. Logical equivalence alone does not establish electrical compatibility.
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