Most of us never give much thought to our keyboards as long as they work well, but if you stop to think about it for a moment, do they only send signals to our computers or is there an active back-and-forth process at work? Today’s SuperUser Q&A post has the answer to a curious reader’s question.
Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.
SuperUser reader Ne Mo wants to know if USB keyboards only send signals or if they receive them too:
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?
Do USB keyboards only send signals or do they receive them too?
SuperUser contributors LawrenceC and Dmitry Grigoryev have the answer for us. First up, LawrenceC:
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):
Followed by the answer from Dmitry Grigoryev:
Any USB device regardless of its class requires bidirectional communication to function. Every USB device (or function in terms of USB specifications) 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 functions (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 small and guaranteed in the latter case. Still, all transfers are initiated by the host.
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