DisplayPort 20-Pin: Why One Contact Is Left Open
Quick answer / TL;DR: Short answer: a compliant passive DisplayPort cable leaves pin 20 unconnected at both ends. If pin 20 is wired through, two sources can feed each other's 3.3 V rail, which may prevent boot or damage a port. Test a suspect cable with a continuity meter between pin 20 at each end; it should read open.
The A/V family at a glance: four connector families, two signal worlds.A DisplayPort connector has 20 pins, but a standard cable does not connect all of them. Pin 20 carries DP_PWR, a 3.3 V supply intended for powered adapters, and wiring it through a passive cable creates a back-feed path between two devices.
This is one of the few cable faults that can damage hardware rather than just fail.
What Pin 20 Is For
| Pin | Signal | Cable practice |
|---|
| 18 | Hot plug detect | Connected |
| 19 | Return | Connected |
| 20 | DP_PWR 3.3 V | Not connected in passive cables |
| 13 | Return | Connected |
Powered adapters and some active cables use DP_PWR legitimately. The problem is a passive cable that copies a full 20-pin layout without omitting the supply.
Symptoms of a Wired-Through Pin 20
- A device that fails to power on when the cable is attached
- Intermittent detection on one specific cable
- A port that runs warm
- A laptop that boots only when unplugged from the dock
Because the symptom looks like a device fault, the cable is rarely suspected first. That is what makes the check worth doing.
Real-World Applications
Service benches keep a known-good cable and a meter. When a docking station misbehaves, the cable test takes a minute and eliminates one variable before anything is replaced.
Standards and Verification
| Reference | Relevance |
|---|
| VESA DisplayPort connector definition | Pin assignment and DP_PWR |
| VESA DisplayPort cable guidance | Passive cable practice |
| Safety and EMC | Back-feed considerations |
Pin-out for a specific cable assembly is not provided in the knowledge base. Verify with a continuity test rather than trusting the drawing.
Practical Notes
Mark any cable that fails the pin 20 test and remove it from service. A cable that back-feeds is not repairable and not worth the risk.
Related reading: displayport cable — blogs/news/displayport-cable
Related reading: displayport 2 1 cable — blogs/news/displayport-2-1-cable
Related reading: displayport 1 4 cable — blogs/news/displayport-1-4-cable
Frequently Asked Questions
Is pin 20 used in all cables?
No. In compliant passive cables it is deliberately left open.
Can a bad pin 20 damage a device?
It can back-feed a supply rail, which may prevent boot or damage a port.
How do I test it?
Continuity between pin 20 at both ends should read open.
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