PU/PD on AUX of the displayport sink both wired to AUX_P

[Yantrio]

Hey, I'm fairly new to electronics so I could be misunderstanding something here.

I was looking at the reference schematic for the hd3ss215 and your schematic as reference for my current project. And I think I spotted a small mistake, it looks like you have mistakenly placed the pull up and the pull down 100k resistors both on AUX_P of the sink instead of having the pull up on AUX_N of the sink.

Thought I should let you know in case a rev 2.1 ever makes its way out.

PS: Thanks for putting this on github, its been really helpful in helping me make my own displayport kvm.

[jorticus]

Good spotting, that's definitely a mistake, though interestingly it did not cause any problems for me (my DP monitor works fine with two different source devices). I believe HPD is the signal that really matters for display detection.

This is how it should be:

And from the DP spec:

The DisplayPort Source Device must weakly pull down the AUX+ line and weakly pull up the AUX- line 
with 100kΩ (+/-5%) resistors between the AC-coupling capacitor and the Source Connector to assist 
detection of DisplayPort Source and Powered DisplayPort Source by the Sink Device. 

All Sink Devices must have AC-coupling capacitors, whether they implement DisplayPort Source Detection 
or not. The Sink Devices must very weakly pull up AUX+ line and very weakly pull down AUX- line with 
1MΩ (+/-5%) resistors between the Sink Connector and the AC-coupling capacitors.

Interestingly the TI datasheet doesn't mention needing 1MΩ resistors for the DFP ports, but the spec implies they are required.

Thanks! I might have enough of a reason now to spin a rev3.

[Yantrio]

I had found a couple of other examples online in places that were using 1MΩ on the sink too, but I couldn't find anything to back that decision as to why.

Thanks for the snippet from the displayport spec, that's handy to note and I'll go read into it further.

Thanks for getting back so quickly! and I'll keep an eye on the repo for your V3 in the future if it happens.

Also, if you're happy with BGA the newer HD3SS214 looks interesting to use (if you wanted DP 1.4).

[psyspy1]

@jorticus Hello! I am working on a hobby project that will integrate your design. Pardon my ignorance, I am confused by the source/sink and upstream/downstream terminologies used in the DP spec and TI datasheet. I want to switch between a pc and laptop to the monitor. I am assuming PC and laptop are the sources and monitor is the sink. Is it the other way around?

[jorticus]

@psyspy1 yeah this always confuses me...

Sink/Source refer to the direction of data: Source = Computer (ie, sourcing image data) Sink = Monitor/Display (ie, sinking image data)

While Upstream/Downstream are from the perspective of the cable, so imagine a chain such as

[Display (DFP)] ---cable--- [(UFP) KVM Switch (DFP)] ---cable--- [(UFP) Computer]

Then from the perspective of the KVM Switch: Upstream Port (UFP) is connected to the Display / Sink Downstream Port (DFP) is connected to the Computer / Source (I think... not 100% sure about this)

Note that the chip itself is bidirectional and doesn't care about which side has the Source & Sink (ie, you could use the chip to switch between two monitors(Sinks) to a single computer(Source))

Short answer - yes, PC/Laptop are Sources, Monitor is a Sink. And if you follow my schematic, it'll do exactly what you want.

[psyspy1]

@jorticus Thank you!

I am confused by this statement from the spec:

The DisplayPort Source Device must weakly pull down the AUX+ line and weakly pull up the AUX- line 
with 100kΩ (+/-5%) resistors between the AC-coupling capacitor and the Source Connector to assist 
detection of DisplayPort Source and Powered DisplayPort Source by the Sink Device.

Shouldn't the two sources on the left in your diagram pull down the AUX+ line and pull up the AUX- line? And the sink connector on the right pull up AUX+ and pull down AUX- with 1MΩ resistors?

[psyspy1]

I think I understand it now.

In the 2:1 setup(two sources -> 1 sink), all 3 connectors in the diagram are sources. That's why you pull down the AUX+ line and pull up the AUX- line on the source output connector.

If it were the other case (1:2, two sinks -> 1 source), then the right sink connector will pull up AUX+ line and pull down AUX- line with 1MΩ resistors.

Thanks for your explanation.