HPD stands for Hot Plug Detect. It is an HDMI signal a connected device uses to let a source know that the connection or capability status has changed.
Why is HPD important?
When a display, receiver or switch disappears and returns, the source may read the information again and reconfigure its output. In complex chains this can show up as a black screen, a resolution change or a new display detection on a PC.
How HPD works technically
The source puts 5 volts on pin 18 of the HDMI connector. As soon as the display or receiver sees that voltage and has its EDID ready, it pulls pin 19 (Hot Plug Detect) high. For the source, that is the signal to read the EDID over the Display Data Channel (DDC) and then send video.
If the EDID changes, the display signals this with a short low pulse on the HPD line. According to the HDMI specification, that pulse lasts at least 100 milliseconds. The source then reads the EDID again and reconfigures its output.
HPD on the Dr HDMI
After you choose an EDID bank, the Dr HDMI activates the selected EDID and starts an HPD event. This gives the source a clear reason to read the selected profile again. According to HDfury, the whole event takes about two seconds; you cannot change the EDID during that time. So after a bank change, wait about two seconds before switching again.
Because the Dr HDMI presents the EDID itself, the source keeps seeing a valid EDID even when the display is temporarily switched off. HDfury describes this as keeping the source's HDMI/DVI-D output active. The Dr HDMI can also extend the +5V signal through the HDMI/DVI-D chain.
When is an HPD reset useful?
If a source no longer shows a picture after standby, an input change or switching on a receiver, a bank change forces a new EDID read. Select the same bank again, or switch briefly to another bank and back. If the picture stays away, follow the troubleshooting guide.