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I have a PC with a DisplayPort monitor. When the monitor sleeps or is powered off, the OS loses track of the current display resolution. My research indicates this is probably due to the Hot Plug Detect (HPD) pin being brought low when the monitor enters sleep mode or is powered off. To remedy this, I'd like to have DisplayPort's (DP) hot plug detect (HPD) always report that it is connected.

For HDMI, there's a device that puts a 1K resister between the power and HPD pins. I think something similar would work for DP. I can't find any easy-to-open DP to DP adapters but maybe I can hack a cable.

Further research indicates that in DP 1.1 there is a power line but it's not available in DP 1.2. (The specs seem to say that the power is connected at the port but cables should not pass it through in DP 1.2 like they would in DP 1.1.)

USB seems a viable alternative for power but HPD uses a 3.3V supply while USB is 5V. Since it's a cable hack, I think keeping the component count down is necessary. Maybe a zener diode based regulator would do the trick, sort of like this:

zener-based votage regulator

Spice simulations seem to indicate the voltage would fall within what is necessary to pull up the HPD.

So far this is all theoretical. I don't have access to the official DP specs (or the official USB specs for that matter) and I'd rather not fry my video cards or monitor.

I'm hoping there's someone familiar with DisplayPort here that can either tell me if this is likely to work or can point me in the right direction.

(Using other video connection types is not an option.)

Kevin Reid
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Ouroborus
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2 Answers2

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Displayport 1.1 and 1.2 both provide for a 3.3V power pin. Standard Displayport Cables for 1.2 do not wire the pin, but there are cables that do. Regardless of the wiring, if the PIN is present, you can hack the cable:

DP_PWR 3.3V+/-10% / 500mA max, available both on upstream/downstream device connectors

Mainly intended for powering cable adaptors

Standard DisplayPort cable does not have wire connected on DP_PWR pin

Cut the HDP from the downstream device (Monitor), and wire a typical pull-up resistor (4.7k ~ 60k, try experimenting) between the upstream DP_Pwr pin and the upstream HDP pin will work. Of course, this does break some features, as the HDP pin is also used as a signal from the downstream device to the upstream device:

Asserted by a downstream device when it is ready for AUX transaction

HPD line also used for an IRQ assertion by a downstream device

Info gathered from http://www.vesa.org/wp-content/uploads/2010/12/DisplayPort-DevCon-Presentation-DP-1.2-Dec-2010-rev-2b.pdf

Community
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Passerby
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  • Accessing the wire side of the connector isn't feasible as most (all?) cheap DP cables use cast-in-place or welded covers rather than snap-together covers. Even so, the connector itself includes a metal shell for strain relief and shielding. This is why I thought about using USB power instead. That aside, any idea what features this might break? – Ouroborus Jul 19 '15 at 12:23
  • @Ouroborus the Vesa specs state Aux transactions. No extra details. – Passerby Jul 19 '15 at 20:20
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I could find a congatec Application Note (AN17 HDMI DP Implementation) published on Congatec AG company site.

Their implementation seems to offer both options active low and high. So that could be a mirrored info from DP standard or something to support HDMI & DVI too.

But I think this is enough to show if your circuit gonna work.

schematic

simulate this circuit – Schematic created using CircuitLab

user.dz
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