大多數時候，我們很容易就知道計算機上的各種埠和旁邊的列印符號是用來做什麼的，但偶爾會出現一些新的或不同的符號。有鑑於此，今天的超級使用者問答帖子回答了一位好奇讀者的問題。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

問題

超級使用者讀者BloodPhilia想知道USB-C埠旁邊的D形圖示是什麼意思：

There is a **all D-shaped icon next to my USB-C port that looks like two “Ds” or a “P and D” (a **aller “P” placed inside a larger “D”). I have tried looking for it online, but I cannot seem to find it anywhere. What does it stand for and what does its functionality entail?

At first, I thought it stood for power delivery, but I could not find any matching symbols related to the “standard” on Google. Also, my laptop has a separate power input for charging.

USB-C埠旁邊的D形圖示是什麼意思？

答案

超級使用者貢獻者Journeyman Geek和chx為我們提供了答案。首先，熟練工極客：

is a DisplayPort icon, which indicates that the port supports an Alternate Mode, and a simple passive USB-C to DP cable (or a monitor that does a USB-C connection in DP mode) would work. Not all USB-C connectors support DisplayPort, so it is a very helpful marking to see “printed” on a device.

然後是chx的答案：

The other answer is fully correct, but let me elaborate on why it is necessary. The USB-C connector is the biggest mess I have seen in computing connectors. I have been a columnist/editor for a computer magazine in the nineties, so I have seen a lot of connectors, trust me.

USB-C has four high speed lanes that can carry a wide variety of signals, which is really nice, but the problem is there are no requirements to show or indicate to the user what the capabilities are using colors or ic***. Traditionally, the mess was somewhat the other way around, with the same signal, but a lot of different connectors (SCSI, both parallel and serial, is notorious for this).

You needed adapters then as well, but at least you had a really good idea what it could be just by looking at a connector (outside of the mess with EGA and CGA starting in 1984 before VGA killed both of them off in 1987, something that affected far fewer people). The hope is that this time (eventually), you will not need adapters and everything will just work.

Here are a few things it might or might not be capable of, but only one at a time:

• Charging the device itself with 20V / 3A.
• Charging a connected device with 20V / 3A.
• Providing a DisplayPort 1.4 signal (you will need a passive adapter). The icon mentioned is the DisplayPort logo, so it is used correctly to help clarify a little bit of the chaos that USB-C is (but you still cannot tell whether the port is 1.3 or 1.4 capable though).
• Providing an HDMI 1.4b signal (you will need a passive adapter). Perhaps the HDMI logo will be used, but then again, it may not.
• Providing a Thunderbolt 3.0 signal, which is PCI Express, DisplayPort 1.2, USB 3.1 (Generation 2), and USB Power delivery multiplexed into a signal served over the same connector using more expensive active cables. Typically, a Lightning Bolt port is used, but guess what? That is not a requirement.
• Providing USB 3.1 (Generation 2, a.k.a. 10 Gbit/s speed USB). Some motherboards will give you USB-C connectors carrying USB 3.1 (Generation 1), formerly known as USB 3.0 signals, at 5 Gbit/s just to have more variety, because clearly, there is not enough of that.
• There is MHL too.

All in all, be very glad that you have at least some indication of what your port is capable of. You do not always have that luxury:

It is literally anyone’s guess what are these ports are capable of. They really should have provided some way to clarify this mess, but they did not. If a good variety of colors and ic*** versus the space c***traints are not viable, then provide a standard way for software to display a capability list for users. A diagnostics tool that you could plug into a USB-C port and have it give you a list of what it is capable of would also be very helpful (i.e. this port can provide signals A, B, C, and accepts inputs D, E, F).

Because the “just work” dream is clearly off, if you plug a USB-C cable into a monitor, you do not really have any idea whether it will work or not. The monitor might require a Thunderbolt signal, a DisplayPort signal may be enough, or just USB may be enough because it is using DisplayLink technology, in which case, the host device needs a proprietary driver.

Further, even if the right Alternate Modes are present, sometimes they just do not work well. A Google engineer has tested a lot of USB-C cables on Amazon and most were out of spec. Demanding 10 Gbit/s from such a thin cable combined with the complexity of the whole thing makes it totally unsurprising that theoretically compatible devices have problems.

For the past 10-15 years, everyone has grown accustomed to the fact that you just plugged in a USB device and it worked (perhaps after installing a driver). USB-C, however, is a brave new world.

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