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I've been investigating why my 2560×1440 DisplayPort monitor and my mini-DisplayPort MacBook Air don't play along, and, it turns out, this is likely due to a wire in the cable that connects the two together, which is supposed to be absent.

http://support.apple.com/kb/TS3533

Certain cables advertised as being VESA 1.1a compliant may contain a construction issue that may result in the DisplayPort power pins at either end of the cable shorting together. VESA 1.1a compliant cables must isolate the DisplayPort power pins at either end of the cable.

This makes no sense to me! Why is it that both the monitor and the graphics card have this pin connected, but a standard-compliant cable is supposed to provide isolation of the pin?

A "construction issue" causing the "pins at either end of the cable shorting together"? Isn't that how wired cables are supposed to work?

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  • \$\begingroup\$ Not sure of the answer but I noticed that version is one of the older ones available for free (with registration) here vesa.org/vesa-standards/free-standards. It says it contains an adaptor checklist. \$\endgroup\$
    – PeterJ
    Commented Dec 24, 2013 at 5:15

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You are shorting two different Voltage Sources together. Both will compete to provide the power on the line. Not a good idea.

From http://www.displayport.org/faq/

Q: Why isn’t there a wire on pin 20 (POWER) on the standard external DisplayPort cables?

A: The reason power isn’t included in standard cables is because both source and sink devices are designed to provide power. Captive, attached cables often include the power wire. If it is desired, for example, that a particular source device utilize the power available from the mating sink device, then that Source device could include an attached or dedicated cable that carries the DisplayPort power signal. Same could be applied to a sink device.

Since Displayports allow for daisy chaining and other features, sometimes its good for a sink device (like the monitor) to provide power. Sometimes it is not. A direct connection between a Source (like your Computer) and the Sink device is not a situation where both power pins should be connected/used.

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Why is it that both the monitor and the graphics card have this pin connected, but a standard-compliant cable is supposed to provide isolation of the pin?

This allows for greater flexibility in connecting displayport compatible devices. For instance, you might want to use a displayport extender that uses standard CAT5/6 wiring, commonly found in office buildings, to communicate with a display far from the source. You need an adapter. Wouldn't it be convenient if the adapter didn't need to have AC power on one or both ends of the connection?

Displayport provides a small amount of power at both the source and target (sink) so that such an adapter can be designed which doesn't require external power.

This power pin is present on both sides of the connection, but if you're using no adapters, and you don't need power inbetween the two devices, you don't connect the pins.

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  • \$\begingroup\$ Is it not possible for the active hardware to be smart enough to detect whether the cable is as such defective, and avoid the interference issues? For example, it appears that this issue is only caused on some combination of devices; how come the rest of the devices are not affected? \$\endgroup\$
    – cnst
    Commented Apr 29, 2014 at 20:32
  • \$\begingroup\$ @cnst Anything is possible, so sure, they could have added some sort of auto detection or auto negotiation, but that would add significant electrical complexity and possible software support on each end. Alternately, they can leave out one wire on a cable. There's a tradeoff, and it sounds like they chose the simpler, cheaper option. \$\endgroup\$
    – Adam Davis
    Commented Apr 30, 2014 at 1:12
  • \$\begingroup\$ I would argue that in the end it's actually not the cheaper option. One way or the other, it appears that most cables on the market are non-compliant (especially aftermarket ones for the laptops with mini-DP, which are never included with most DP monitors), and it does appear to be causing various hard-to-troubleshoot issues with various monitors. As such, I believe the total cost of this for the industry (tech support, wasted troubleshooting time, RMA of "defective" monitors etc) is quite substantial and likely overall exceeds the extra manufacturing costs or "savings". \$\endgroup\$
    – cnst
    Commented Jun 8, 2015 at 17:15
  • \$\begingroup\$ @cnst Perhaps. You'd have to talk to the displayport consortium to change the specification. \$\endgroup\$
    – Adam Davis
    Commented Jun 8, 2015 at 17:19

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