Logic circuits
When several open-drain outputs are connected to one signal line with a pull-up resistor, how does the line behave?
- AThe outputs conflict and are always damaged
- BIf any one outputs 0, the line becomes 0, acting as a wired AND
- CIf any one outputs 1, the line becomes 1
Show answer and explanation
Answer: B. If any one outputs 0, the line becomes 0, acting as a wired AND
An open-drain output either pulls the line to ground with a transistor or releases it; the 1 level is provided by the pull-up resistor. As a result, the line goes to 0 if any output drives 0 and is 1 only when all release it, acting as a wired AND. This is used for I²C and interrupt lines, among others.
- A. The outputs only pull down, so connecting them together does not damage them.
- B. Correct. Any single 0 makes the line 0, a wired AND.
- C. Open-drain outputs never actively drive 1, so a 1 cannot take precedence.