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Understanding Uni-Directional vs. Bi-Directional Video Cables

Last Update: November 13th, 2025
Article ID: 3586988

When connecting a laptop/dock to a monitor, the right direction matters. Many “converter” cables only work one way. If the cable’s intended direction doesn’t match your source (video output) and display (video input), you’ll get no signal. Even if the connectors fit.

Key terms

  • Source (Output): Laptop, dock, GPU port sending video
  • Display (Input): Monitor/TV/projector receiving video
  • Uni-directional: Works only one way (e.g., DP to HDMI)
  • Bi-directional: Works both ways (common with HDMI to/from DVI, DP to/from mDP)
  • Active adapter: Has electronics to convert formats/directions (e.g., HDMI to DP)
  • Passive adapter/cable: No electronics; relies on source capabilities (e.g., DP++ to HDMI)

What typically works and what doesn’t

Scenario

Directionality

Requirement

Notes

DisplayPort (DP) source to HDMI display

Often uni-directional (DP to HDMI)

Passive may work with DP++; otherwise use an active DP to HDMI

Many docks/GPUs support DP++, but some do not so please check specification

HDMI source to DisplayPort display

Uni-directional (HDMI to DP)

Active HDMI to DP adapter

A DP to HDMI cable won’t work in reverse.

USB-C (DP Alt Mode) source to HDMI/DP display

Uni-directional

Alt-Mode USB-C to video cable/adapter

Not reversible (you can’t feed HDMI into USB-C).

Thunderbolt 3/4 source to HDMI/DP display

Uni-directional

TB/USB-C video adapter

Behaves like USB-C Alt Mode for displays.

HDMI to/from DVI

Bi-directional (passive)

Simple adapter/cable

Same TMDS signaling.

DP to/from Mini-DP

Bi-directional (passive)

Simple cable

Same protocol.

Digital to/from VGA (analog)

Direction varies

Active converter

Digital/analog conversion required.

Rule of thumb: For DP to/from HDMI, assume DP to HDMI unless the product explicitly says HDMI to DP (Active).

Docking stations: quick notes

  • USB-C/Thunderbolt docks output standard HDMI/DP signals (direction rules above still apply)
  • DisplayLink-based docks still output HDMI/DP at the port (same direction rules)
  • DP MST/daisy-chain works only in DP domain; converting to HDMI at the first hop ends DP-specific features downstream

Resolution, refresh, and features

  • Achievable resolution/refresh (e.g., 4K60) depends on source + cable/adapter + display
  • Features like HDR, HDCP, and VRR may require active adapters and sufficient bandwidth
  • For long runs or flaky links, prefer active solutions

Shopping checklist (esp. on Amazon)

  • Check the pictures: Listings often show Source to Display with icons/arrows (match this to your setup)
  • Read the title/bullets: Look for explicit direction (“DP to HDMI,” “HDMI to DP (Active)”)
  • Scan specs: “Active” vs. “Passive,” DP++, supported resolutions, and any power needs

Quick examples

  • Dock (DP out) to Monitor (HDMI in): DPtoHDMI cable/adapter; use active if DP++ isn’t supported or for higher reliability
  • Laptop (HDMI out) to Monitor (DP in): Active HDMItoDP adapter required
  • USB-C laptop (Alt Mode) to HDMI monitor: USB-CtoHDMI cable/adapter; not reversible

FAQs

Why doesn’t my “DP to HDMI” cable work from an HDMI laptop to a DP monitor?
It’s uni-directional (DP to HDMI). Your path is the reverse and needs an active HDMI to DP adapter.

Are “bi-directional” cables truly both ways?
Commonly for HDMI to/from DVI and DP to/from mDP. For DP to/from HDMI, true bi-directional products are uncommon—verify the exact directions in the listing images/text.