HDMI connection - straight, cross, odd? - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: October 02, 2017, 01:44:48 am
content: It's probably an obvious-answer question, but i couldn't google a definite answer to it.How do you connect an HDMI output and input together, on signal level?I got on a board an HDMI switch chip and a HDMI->LCD chip. Normally each of them are connected to n HDMI socket and they are linked with a cable.But being on the same board, i'm just linking them directly.That is, TMDS Data2+ to TMDS Data2+, TMDS Data2- to TMDS Data2-, and so on.Is that the right way to connect them?

author: Artlav
author_rank: Frequent Contributor
timestamp: October 02, 2017, 01:52:43 am
content: Ok, i've realized i can just plug in the cable and probe it.Apparently all the lines are connected one-to-one.

author: jbb
author_rank: Super Contributor
timestamp: October 02, 2017, 06:59:36 am
content: All hail the continuity tester!(It has saved my ass many times.)

author: wraper
author_rank: Unknown
timestamp: October 02, 2017, 08:31:44 am
content: HDMI is always straight.

author: langwadt
author_rank: Super Contributor
timestamp: October 02, 2017, 08:39:01 am
content: Quote from: blueskull on October 02, 2017, 01:52:10 amHDMI/DVI is meant to be DC coupled

author: wraper
author_rank: Unknown
timestamp: October 02, 2017, 09:01:57 am
content: Quote from: langwadt on October 02, 2017, 08:39:01 amYep, if you add capacitors, it will stop working. HDMI transmitter is only able to push down, pull up is provided by HDMI receiver.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 03, 2017, 07:09:57 pm
content: Quote from: wraper on October 02, 2017, 09:01:57 amThis diagram has no 50R termination on TX side. How is it going to work with Gbps signals?Leo

author: wraper
author_rank: Unknown
timestamp: October 04, 2017, 07:18:16 pm
content: Quote from: Leo Bodnar on October 03, 2017, 07:09:57 pmHDMI/DVI normally has no termination on TX side according to specification. Now recalling you sell HDMI lag tester, it's really strange to hear from you.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 04, 2017, 07:39:32 pm
content: Quote from: wraper on October 04, 2017, 07:18:16 pmIt's complicated... We have an AC-coupled design that we use for 4K HDMI signal. There is also an offical thing called"AC-coupled HDMI". I have not really researched in details how it works but I will now. I have a suspicion that HDMI market quietly moved on to have TX-terminated side too.This diagram is "conceptual" and has been dragging along since DVI and 25MHz dot clock days.With 600MHz rate and rubbish cables I don't believe the termination is now purely on RX side. Leaving TX high impedance (essentially open circuit if drivers are true current source switches) would be suicidal for HDMI source manufacturers. (This is my opinion - not a fact.)The first thing I am going to do tomorrow is throw all HDMI sources I can find on the signal integrity rig and see if there is any signal present with no Rx side pull-ups. This will show whether this diagram is "conceptual" or still used as shown. It is still there in HDMI 2.1.Leo

author: wraper
author_rank: Unknown
timestamp: October 04, 2017, 08:23:05 pm
content: Quote from: Leo Bodnar on October 04, 2017, 07:39:32 pmAs I understand from briefly reading that datasheet, it's basically HDMI over DP++ to HDMI level shifter.From the datasheet:Quote

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 11, 2017, 06:45:43 pm
content: Sorry, I have not yet had time to investigate thoroughly - my test jig does not have easily updatable EDID memory to make sources output various video modes. I have seen both signal and no signal with no sink pull-ups. It's inconclusive at best at the moment.This is of academic interest so not too high on my priorities list.Leo