S277HK monitor drops hotplug signal after reading EDID

gloacker
gloacker Member Posts: 1 New User

My S277HK monitor does not apply a stable hotplug detect (HPD) signal, although the +5V signal is verified to be stable. I've seen this behavior on all three S277HK monitors available at my location.

First, everything looks fine, but after the first block of the EDID is successfully read the HPD signal looks like to float and drops down to 3-4V. When switching TMDS on HPD even falls down to 1-2V. It's not the source device pulling the line down, because the HPD looks exactly the same if I disconnect the HPD from the source device.

What makes everything working fine is just to make the edges of the DDC lines softer, for example by only connecting a BEAGLE I2C protocol analyzer or by adding some 100pF load.

Strange, isn't it? Has someone else seen a behavior like this or even knows how this can be avoided?

 

Yellow: HPD signal, Green: one TMDS signal line, Red/Blue: DDC lines

S277HK-drops-hpd.png

 

Exactly same setup as above, only I2C protocol analyzer connected to DDC lines - reduces high voltage on DDC lines. Now everything is fine. Why?

Yellow: HPD signal, Green: one TMDS signal line, Red/Blue: DDC lines

S277HK-drops-hpd-ok.png