9 ms·
The DisplayPort Pin 20 Problem
- cesarb 8y agoFor 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)?
- russdill 8y agoI've only seen high end scopes and signal generators used for such a purpose.
- AlphaWeaver 8y agoUser 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 8y agoComment: https://news.ycombinator.com/item?id=17163948 https://news.ycombinator.com/item?id=17163948 TL;DR: US$30
- awat 8y agoInteresting, early on with a 4K monitor on a Mac I was having trouble with kernel panics I wonder if this was the cause.
- crazy5sheep 8y agoIt'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 8y agoI 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. :(
- bdonlan 8y agoSeems like this problem could have easily been solved by putting a diode on the device side.
- gsich 8y agoThis is the correct solution.
- codedokode 8y agoThere is a voltage drop on a diode, around 0.2 - 0.7 volts, cannot this become a problem?
- Faaak 8y agoThen the supply before the diode supplies 3.3V + the voltage drop; for example 3.6V.
- gizmo686 8y agoSo now you need a 3.6V power regulator, in additition to the 3.3V one you likely already have for all the components built for 3.3V.
- 0110011 8y agoOr another diode to put in-line with the DP circuitry. It's probably not ideal from a low-power perspective but if that's what you gotta do to make robust and compliant hardware, you do it.
- Faaak 8y agoA Schottky will drop .3V; That's within the range (±10%) specified in the specs
- kevincox 8y agoYes. But now you have given up all of your tolerance so you better have a perfect (or over-voltage) power supply. This means that you now have less choice in power supply.
- myself248 8y agoSo, 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 8y agoThat would be my expectation then, since an extension cable should behave (as much as possible) as the original port you'd be using.
- jannes 8y agoI'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 8y agoI'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 8y agoPerhaps 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 8y agoPutting the computer to sleep resets it for me. Only happens on anything that isn’t DP (HDMI and DVI->DP) Annoying for sure.
- joosters 8y agoSame here! My computer is a Mac mini and also using a mini DP to DP cable. There’s a 20% chance that the monitor will get stuck in power-saving mode when turning it on. When this happens, rebooting my Mac is the only way I’ve found to make it come back. I’ve tried many cables :(
- ocdtrekkie 8y agoThis explains so much. I have a feeling this is why the Surface Dock generally won't work with dongles, for instance.
- Azerb 8y agoYes you’ll want to use active dongles with the Surface Dock, those will run off the 3.3v supply.
- Bud 8y agoWhat's the easiest way to tell whether pin 20 is wired, if you don't have a very very expensive cable tester?
- vesinisa 8y agoA 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_(connector).PNG https://en.wikipedia.org/wiki/File:Mini_DisplayPort_(connect...
- digi_owl 8y agoGiven the size of most multimeter probes i have seen, i can't shake the feel that it could get very fiddly very fast.
- seanp2k2 8y agoGet / make a breakout board http://www.elabguy.com/displayport.php http://www.elabguy.com/displayport.php
- lightedman 8y agoI keep a set of probe cables with the ends clipped off, and just a couple of wires left unclipped, twisted and tinned together for strength/rigidity. Works great for tiny trace testing.
- digi_owl 8y agoSeems 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 8y agoApplies 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 8y ago"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 8y agoI 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.
- Too 8y agoLiberal in what you receive should only apply for human user interfaces. If you apply the rule on machine to machine interfaces it eventually leads to all applications having to be bug-for-bug compatible with the most popular implementation. (Internet explorer 6 usually being the prime example of this)
- teilo 8y agoWow. 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 8y agoThis tester does this and more: https://www.amazon.com/Noyafa-NF-633-DisplayPort-Testing-Continuity/dp/B01MZ4JPZH https://www.amazon.com/Noyafa-NF-633-DisplayPort-Testing-Con... 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"
- codedokode 8y agoThey should have used different pins for source and sink power.
- deleted 8y ago[deleted]
- mort96 8y agoA 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 8y agoThey 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 8y ago> 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.
- empyrical 8y agoA 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 8y agoMore likely a de-sense issue. Poor grounding/shielding on either the connector or the adaptor can impact the performance of the antennas.
- fuzzy2 8y agoBut 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 8y agofor 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.
- IshKebab 8y agoIt is clearly a short circuit. The amount of current that flows depends on the output impedance which I imagine isn't specified.
- deckardyc 8y ago2012 thread on this exact problem. (2nd post is particularly interesting.) http://www.wsgf.org/forums/viewtopic.php?f=66&t=24405&sid=b7d5b709837c32d16154933c4578b3a9&start=10 http://www.wsgf.org/forums/viewtopic.php?f=66&t=24405&sid=b7... 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.
- qwerty456127 8y ago> 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 8y agoChange the cable, not the device.
- slykar 8y agoI'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 8y agoSo 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.