Open-Drain vs Open-Collector
Both are output types that can pull LOW but cannot drive HIGH.
A pull-up resistor is required to generate the HIGH level.
1️⃣ Open-Collector (OC)
- Uses an NPN bipolar transistor
- Can pull output LOW
- Cannot drive output HIGH → needs external pull-up resistor
How it works
- Transistor ON → output pulled to GND → LOW
- Transistor OFF → output floats → pull-up resistor → HIGH
Common uses
- I²C
- Interrupt lines
- Wired-OR / wired-AND logic
- Level shifting (pull-up to different voltage)
2️⃣ Open-Drain (OD)
- Same behavior as open-collector
- Uses an NMOS FET instead of an NPN transistor
How it works
- FET ON → output pulled LOW
- FET OFF → pull-up resistor → HIGH
Common uses
- Modern low-voltage ICs
- I²C bus
- Shared interrupt lines
3️⃣ Why use open-drain / open-collector?
Because multiple devices can safely share one wire:
- Any device can pull the line LOW
- No device ever drives HIGH → no short-circuit conflict
This enables wired-AND / wired-OR logic.
4️⃣ Comparison Table
| Feature | Open-Collector | Open-Drain |
|---|---|---|
| Device type | NPN transistor | NMOS FET |
| Pulls LOW? | ✔️ Yes | ✔️ Yes |
| Drives HIGH? | ❌ No | ❌ No |
| Needs pull-up resistor? | ✔️ Yes | ✔️ Yes |
| Common in | Older TTL/CMOS | Modern ICs |
| Functional behavior | Same | Same |
5️⃣ Key Point
Open-drain and open-collector behave the same.
The name only depends on the type of transistor used.