我们中的大多数人从来不会对键盘花太多心思，只要它们工作正常，但如果你停下来想一想，它们是只向我们的电脑发送信号，还是在工作中有一个活跃的来回过程？今天的超级用户问答帖子回答了一位好奇的读者的问题。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

问题

超级用户阅读器Ne Mo想知道USB键盘是只发送信号还是也接收信号：

A USB keyboard does not need to receive any signals from a computer, just power, right? Or does it need to receive signals as well as send them?

USB键盘是只发送信号还是也接收信号？

答案

超级用户贡献者LawrenceC和Dmitry Grigoryev为我们提供了答案。首先，劳伦斯：

From the “Device Class Definition for Human Interface Devices (HID)” specification:

To change the keyboard LEDs, the keyboard accepts a command to do so. So it is not an “input-only” device (meaning it only outputs data to the host). That being said, there is a negotiation and enumeration process with all USB devices that require a back-and-forth conversation between the host and the device. You cannot have a “read-only” USB device. Even before USB, the computer’s keyboard controller would accept commands because it did a few things besides read the keyboard (reference):

接着是德米特里·格里戈里耶夫的回答：

Any USB device regardless of its class requires bidirectional communication to function. Every USB device (or function in terms of USB specificati***) is represented as a set of endpoints which can be thought of as buffers which accept or receive data. However, even endpoints which can only send data wait for a special packet called a token before they can reply:

The gray boxes represent the USB host while the white boxes represent USB functi*** (chart source).

Even so-called interrupt transfers are done in this way, with the USB host polling connected devices using token packets. What differs between regular (bulk) transfers and interrupt transfers is that the polling time is **all and guaranteed in the latter case. Still, all transfers are initiated by the host.

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