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January 14, 2019

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HDMI to DisplayPort and DisplayPort to HDMI Conversion Explained

February 7, 2019

Have you ever tried to connect a DisplayPort cable in your conference or training room to your display or projector through a DisplayPort table jack and it won’t work? What if the only available port on your display is DisplayPort, and your computer is HDMI – will that work? To avoid frustration, we must carefully consider DisplayPort signal path conversion early on, during AV system design.

 

An example of a relatively simple AV system could include: A display on the wall with only HDMI inputs, and a wall-plate directly below that has HDMI and DisplayPort jacks that connect to the HDMI inputs on the display. A user connects to the wall-plate using DisplayPort, selects the correct Input on the display, and they are off and running – right? Not so fast; there needs to be a conversion from DisplayPort to HDMI somewhere, and if that converter is not in the right place it will not work. This often causes frustration and misplaced blame. You might think why not just put that converter behind the display, right at the HDMI input port so the user does not see it or behind the wall-plate, but you would be wrong.

 

You should not use an adapter to convert DisplayPort to HDMI anywhere in the signal chain other than right at the source. Here’s why:

  • The DisplayPort provides 3.3v DC power on pin 20. Most DisplayPort cables do not have this pin (or it is not connected inside the cable) because it will cause a short-circuit when connecting source to sink.

  • DisplayPort natively outputs in a LVDS signal type that is not compatible with HDMI (HDMI uses TMDS). It does have a dual-mode version that will support TMDS in compatibility mode.

  • DisplayPort signals operate at 3.3v, even when using TMDS in compatibility mode. HDMI TMDS signals operate at 5v.

  • An external adapter allows a dual-mode DisplayPort output to send TMDS signal, and also bumps the voltage up to the standard HDMI 5v.

  • These adapters are powered by pin 20 in the DisplayPort receptacle, and can only be used right at the source device – if they are used with a DisplayPort cable in front of them, the signal does not get converted because no power is available on the DisplayPort cable pin 20.

  • This type of DisplayPort to HDMI adapter is one-way only and MUST be used at the DisplayPort source receptacle. 

 

You cannot use the above adapter in reverse – to adapt an HDMI signal to a monitor’s DisplayPort receptacle even though the gender is a perfect match. A different type of adapter must be used when going from HDMI to DisplayPort:

 

  • The HDMI specification does not support the DisplayPort LVDS signal type, and if the HDMI TMDS is plugged into a DP monitor, it will not work.

  • The DisplayPort receptacle on a monitor or display will only accept the LVDS 3.3v DisplayPort signal type.

  • A different active adapter that converts the HDMI TMDS signal at 5v to DisplayPort LVDS at 3.3v is required.

  • This conversion process uses more current than the power pin on the receptacle can provide and must be an externally powered adapter.

 

The DisplayPort and HDMI device receptacles both provide limited power on a single pin to power some external devices such as adapters, converters, active cables, and basic switches. A wall-plate, patch cable, or table cubby jack breaks this power signal chain. You cannot use an HDMI cable with built-in extender in most situations because they draw power from the missing pin.

 

The HDMI specification for source and sink devices provides 5v power on pin 18. DisplayPort specification is 3.3v on pin 20. Most cables do not have this pin – it is missing. And if it wasn’t, it could result in a short. Look at any reputable HDMI or DisplayPort cable and see for yourself – there is either no contact on one of the corner pins or that pin is not electrically connected.

 

 

Damage can occur to the connected devices if this pin is electrically connected and some of the off-brand cables do just that. Any time a reputable patch cable is used in the signal path this power connection is broken as it should be.

 

If you need help resolving connectivity issues in your meeting spaces, contact Tempest Technologies, LLC today for a solution.