
The DisplayPort Pin 20 Problem - PascLeRasc
http://monitorinsider.com/displayport/dp_pin20_controversy.html
======
myself248
So, cables shouldn't have a wire on pin 20.

This means that you could have a dongle (say, a DP-to-VGA converter) that
works when plugged into the source, but doesn't work when plugged into a cable
and a gender-changer to effectively form an extension cable.

I'd argue, therefore, that extension cables (purpose-built with M-to-F
connectors) should wire pin 20, and only M-M cables should omit it.

~~~
simcop2387
That would be my expectation then, since an extension cable should behave (as
much as possible) as the original port you'd be using.

------
jannes
I've been having problems connecting an external monitor to my MacBook for a
long time.

I'm using a MiniDP to DP cable and sometimes the external monitor just
wouldn't get any signal after sleeping the laptop with the cable plugged in.
Unplugging the cable or power cycling the monitor doesn't help at all. I have
to completely restart the laptop to get a monitor signal again. I wonder if
that's due to the pin 20 issue.

The funny thing is that I actually bought 4 different cables already and all
of them have the same problem.

~~~
Sholmesy
I've been having a similar issue, across multiple different DP cables, (DP ->
DP).

My screen will just shut off, and refuse to turn back on for minutes at a
time. It will also randomly flicker once every 10-15 minutes (for like 3-4s,
then back to normal). I've tried two different DP cables, and two different DP
ports, but its the same issue, and the HDMI ports I have don't support 144hz
1440p =\\.

Such a frustrating interface. Would love if someone could weigh in with
advice.

~~~
stanley
Perhaps unrelated, but I have a similar problem with my hackintosh and a
monitor connected via HDMI. The other monitor (same model) connected to the
same GPU via DisplayPort has no such issues. Restarting the machine seems to
be the only way to get the HDMI monitor to start working again.

~~~
pram
Putting the computer to sleep resets it for me. Only happens on anything that
isn’t DP (HDMI and DVI->DP)

Annoying for sure.

------
cesarb
For Ethernet, we have inexpensive cable testers which show whether a cable is
wired correctly (and expensive cable testers which also check the electrical
characteristics). Is there anything like that for DisplayPort or USB-C (or
both, since DisplayPort can also use USB-C cables)?

~~~
AlphaWeaver
User falcon620 posted a comment further down in this thread with a link to a
device to test DP cables that was much cheaper than I expected it to be!

~~~
exikyut
Comment:
[https://news.ycombinator.com/item?id=17163948](https://news.ycombinator.com/item?id=17163948)

TL;DR: US$30

------
deckardyc
2012 thread on this exact problem. (2nd post is particularly interesting.)

[http://www.wsgf.org/forums/viewtopic.php?f=66&t=24405&sid=b7...](http://www.wsgf.org/forums/viewtopic.php?f=66&t=24405&sid=b7d5b709837c32d16154933c4578b3a9&start=10)

My new video card + 3 different (cheap) DP cables: PC endlessly power cycled
every second.

Bought proper DP cables from Accell. Problem gone.

[https://amzn.to/2KWPSDZ](https://amzn.to/2KWPSDZ)

Never cheaping out on DP cables again.

------
bdonlan
Seems like this problem could have easily been solved by putting a diode on
the device side.

~~~
simcop2387
You can likely get around the diode drop that this would impart by clever use
of a P-channel mosfet.

[https://hackaday.com/2011/12/06/reverse-voltage-
protection-w...](https://hackaday.com/2011/12/06/reverse-voltage-protection-
with-a-p-fet/)

This ends up using the body-diode that's present in all mosfets (it exists
because you can't manufacture one without it) to get the initial protection,
that then lets you turn on the mosfet fully, getting a much smaller voltage
drop. You'd still need to be a bit cleverer to handle both sides but it
shouldn't be too hard.

~~~
hatsunearu
That only solves the problem if you're protecting against backward batteries,
not the "two sources" problem

~~~
simcop2387
Two sources will look surprisingly the same, this still will require both
sides to have the protection but once they do it's exactly the same. The one
with the lower voltage will end up looking like it's got a small negative
voltage being put on it, shutting off the mosfet. You'd still want over
current protection, just like any power source, for devices that don't have
protection or a broken cable that's shorting, etc.

~~~
hatsunearu
No. Try simulating it. It won't work. I mean, if you did exactly what that
circuit shows, you're not gonna go anywhere, so you need some modifications
(like changing the zener diode to a schottky, etc).

The reason why that thing works is the fact that the body diode puts the input
voltage onto the source voltage (sans Vf) which establishes a nice and high
Vgs (negative, but you get the point).

Let's assume you have a similar circuit on both sides. The bus voltage is
3.3V, because if it's not, then you don't have a bus. Somehow you need the
gate voltage on one to be nearly equal to the bus voltage on one side, but on
the other, near to ground. How are you going to do that? That circuit (and any
trivial variation thereof) is not going to cut it.

Like, any sort of diode that connects the bus voltage to the gate on either
side isn't going to work asymmetrically that pulls one low and one high, so
it's not going to work. You need active monitoring of some sort.

I've been seeing a lot of hilariously bad comments regarding electronics that
seem to think any of this shit is easy--it's not.

~~~
foodevl
It's actually far easier than you're making it out to be, because you seem to
be assuming an ideal voltage source. Most linear regulators can only source
current, not sink it. So it is perfectly fine to simply put a LDO at both ends
and tie them together. The bus voltage will be the higher of the two. No extra
diodes needed, certainly no active monitoring, and no extra current will
follow even if the regulators are putting out slightly different voltages.
Maybe add a small series resistance to limit initial inrush current as
capacitors balance, but that's it.

~~~
simcop2387
That's a really interesting and novel way to address it. It does leave a
potential issue though, it could lead to double the current being available to
a device in the middle which could mean it could overdraw for it's own specs
and melt or start a fire. Unlikely though.

I'd also say that with an LDO, the LDO is providing the active monitoring
since it'll need to monitor the output anyway in order to regulate properly.
You'd definitely need a resistor following it, not as much to limit inrush or
anything, but to prevent the two regulators from causing oscillations (one
turns off because the voltage looks good, which causes the other to cycle
too).

------
digi_owl
Seems like if you leave it up to the cable manufacturers to implement parts of
a standard (beyond X wires at Y gauge minimum) they are bound to fuck
something up.

Just look at the amount of screw-ups over the resistors used in USB C cable
variants, in particular the one used to indicate that the cable is an A to C
converter.

~~~
MichaelGG
Applies to software too. Standards need to be strongly defined with no leeway.
Parsing should be tight and leave no room for creativity. (Text protocols like
HTTP I'm looking at you!) Anything that deviates should be rejected by
reference implementations instead of trying to be "robust" by accepting junk.

~~~
rsync
"Standards need to be strongly defined with no leeway."

Actually, I think there should be leeway - but only in one direction.

Specifically, I am thinking of the suggestion: "be conservative in what you
send and liberal in what you receive".

~~~
sigotirandolas
I think that this is what the parent was arguing about: Being liberal about
what you receive often leads to every different implementation handing the
undefined cases in their own particular way, which leads to incompatibilities
or one of the implementation becoming the de-facto standard.

------
teilo
Wow. I think I am going to build a test device for this. It would be very
simple: Two female DP ports with pins 20 wired together through a battery,
resistor, and LED. For good measure, it should be four ports so we can test
mini-DP as well. If the bulb lights up when you plug both cable ends in, then
the cable is bad.

~~~
falcon620
This tester does this and more:

[https://www.amazon.com/Noyafa-NF-633-DisplayPort-Testing-
Con...](https://www.amazon.com/Noyafa-NF-633-DisplayPort-Testing-
Continuity/dp/B01MZ4JPZH)

[http://www.noyafa.com/show/364.html](http://www.noyafa.com/show/364.html)

"There's a separate led to test if display port power (pin 20) is connected"

------
Bud
What's the easiest way to tell whether pin 20 is wired, if you don't have a
very very expensive cable tester?

~~~
vesinisa
A multimeter in continuity mode?

Pinout for DP:
[https://en.wikipedia.org/wiki/File:DisplayPort_Connector.svg](https://en.wikipedia.org/wiki/File:DisplayPort_Connector.svg)

Mini-DP (female, reverse in cable):
[https://en.wikipedia.org/wiki/File:Mini_DisplayPort_(connect...](https://en.wikipedia.org/wiki/File:Mini_DisplayPort_\(connector\).PNG)

~~~
digi_owl
Given the size of most multimeter probes i have seen, i can't shake the feel
that it could get very fiddly very fast.

~~~
seanp2k2
Get / make a breakout board
[http://www.elabguy.com/displayport.php](http://www.elabguy.com/displayport.php)

------
awat
Interesting, early on with a 4K monitor on a Mac I was having trouble with
kernel panics I wonder if this was the cause.

~~~
crazy5sheep
It's a known issue that latest version of Mac OS has problem to connect to
external 4k monitors. I did not experience kernel panics, but lagging on
display, high memory and cpu usage on the windowserver daemon.

~~~
Corrado
I just changed jobs and they gave me a late model MBP connected to an external
4k display. The lag is palpable, on almost any action. ALT-Tab between windows
takes many hundreds of milliseconds. Typing in the terminal is so slow I often
times have to go back and correct my commands because I over typed them.

I thought it was just a combination of low-end MBP (cheapest 13" available)
and crappy 4K monitor (Sceptre U274K). Are you saying that there is an actual
problem with macOS High Sierra that is causing my lag? I've already changed DP
cables and that didn't help at all. :(

------
slykar
I've been bitten by this with almost every cable. I couldn't find a shop that
would be able to tell me whether they sell cables that don't do this.

I simply destroyed the pin and isolated it with some paper...

------
amluto
So USB-C isn’t the only standard that requires cables to do something non-
obvious to avoid subtle breakage.

Damn it, spec designers, stop screwing this up. In DP’s case, it sounds like
it would have been straightforward to require ports to put a diode on pin 20.
(Heck, any reasonable implemention should be protected against short circuits
and overcurrent. Protecting against backfeeding seems like it should add
negligible expense on top.

------
ocdtrekkie
This explains so much. I have a feeling this is why the Surface Dock generally
won't work with dongles, for instance.

~~~
Azerb
Yes you’ll want to use active dongles with the Surface Dock, those will run
off the 3.3v supply.

------
qwerty456127
> If you happen to have such a cable, your best advice is to throw it away and
> buy a new one that doesn't have this issue

Why throw it away if I can happen to get a DP-powered device in future? And
why has the FAQ article been removed?

~~~
hrrsn
Change the cable, not the device.

------
fuzzy2
But that's not a short circuit, like the article mentions, is it? The voltage
difference means _some_ current will flow, but that's generally harmless.

~~~
foxhill
for even a small voltage differential, with a resistance of near-zero, the
current could be very large. basically until one of the regulators gives way,
which is likely way passed the point at which damage can occur.

------
empyrical
A while ago I has a MiniDP->DVI adapter on my macbook air that would cause the
wifi card to lose all signal. I wonder if this is why?

~~~
Azerb
More likely a de-sense issue. Poor grounding/shielding on either the connector
or the adaptor can impact the performance of the antennas.

------
codedokode
They should have used different pins for source and sink power.

~~~
mort96
A display port cable is ghe same in both ends.

Maybe they should've designed display port to have different connecrors for
source and sink, but I must imagine they had compelling reasons to not do
that.

~~~
gizmo686
They could crossover in the cable. So both sides provide power on pin 20, and
leave pin 21 notconnected, then have the cable wire pin 20 to pin 21.

Then, when a device needs to draw power, they connect pin 21 as the source and
leave 20 notconnected. If both devices supply power, then both power circuits
are open.

This still isn't ideal, as extension cables would need to be non-crossover and
you would have a problem if you ever tried using an extension cable in place
of a normal cable.

~~~
DSMan195276
> This still isn't ideal, as extension cables would need to be non-crossover
> and you would have a problem if you ever tried using an extension cable in
> place of a normal cable.

I don't think that's really a big problem:

1\. If your device has a female port (most devices), supply power on pin 20,
source power on pin 21

2\. If your device has a male port ('dongle' type devices), supply power on
pin 21, source power on pin 20.

3\. M-F (extension) cables are non-crossover

4\. M-M and F-F cables/adapters are crossover

With that configuration I don't think there's any way to plug something in
incorrectly or accidentally use the wrong cable. You can't use an extension
cable in place of a normal cable without using a M-F adapter, which would do
the crossover for you. It gets a bit fuzzier when involving converters to
other port types, but I think it's actually less complicated then the current
model in that case. As long as you just supply or source power on the right
pins for the DP port type you have (male or female), then there is nothing
else to worry about when creating a converter.

At the very least, I can't imagine it would be any worse then the current
situation. With the current setup extension cables should have pin-20
connected (Though it sounds like they might not), but M-M/F-F cables should
not, which IMO is much messier then the above.

~~~
gizmo686
That seems like it should work, although it is a different (and better)
proposal than what I had.

Essentially, your proposal is to have the invariant that males supply power on
pin 20 and source on pin 21, while females supply on 21 and source on 20. It
is easy to see that this invariant is maintained by all of your proposed
cables. And, since the only way to plug ports together is male-female you are
guaranteed to always be matching supply with source.

Since this invariant is maintained at every terminal, there is, as you
identify, no added difficulty in creating converters, as they would only
target one gender of the port anyway.

Out of curiosity, is there any port that takes this approach (either for power
or data pins)?

~~~
cesarb
"Traditional" USB does that: USB-A ports source power, USB-B ports sink power.
(Also, USB-A ports are always towards the host, which controls the
bidirectional serial link.) The standard cable has a USB-A plug on one end,
and a USB-B plug on the other end. There can be no USB-A to USB-A (or USB-B to
USB-B) cable, since they're forbidden by the standard, so you can't connect
two sources (or two sinks) together.

And of course it didn't work: there are many standard-breaking USB cables with
identical plugs on both ends, and standard-breaking devices which require
them.

~~~
DSMan195276
That's not quite what I described. With my design either port can source or
sink power, they just do it on opposite pins. And it wouldn't be USB-A vs
USB-B, but male vs female

So for USB, you would need a 5-pin port, where two are devoted to power. And
both USB-A and USB-B would have the same wiring, but male ports source and
supply on opposite pins as the female ones. Then M-M or F-F cables crossover
the power lines.

