Keyboards, mice, printers, flash drives and phones all connect to a computer through the same kind of port. That port is the universal serial bus, and its name tells you how it sends data.
USB (universal serial bus) is an industry standard interface for connecting devices to a computer. It uses serial transmission: data is sent one bit at a time. When a device is plugged in, the computer automatically detects it and loads the correct driver. The cable can also supply power to the device. The main drawbacks are a limited cable length and slower speeds on older USB versions.
Fig. 1: many different devices connect through the same USB standard
Section 01
What is USB?
Each word in the name matters. In the lab, the USB note shows a byte travelling along the cable one bit after another.
Universal: one standard used by almost every manufacturer and device, so devices work with almost any computer.
Serial: data is transmitted one bit at a time, so it does not suffer from skew.
Bus: a set of wires that carries data (and here, power) between the computer and the device.
Examiner trap
USB is serial, not parallel. If a question asks how USB transmits data, "one bit at a time" is the key phrase.
Section 02
Plugging in.
This is the sequence the examiners want when they ask how USB is used to transmit data.
1. The device is connected to a USB port, and the computer automatically detects it.
2. The device is recognised and the correct driver is loaded (or the user is asked to install one).
3. Data is then transmitted serially, one bit at a time, between the device and the computer.
4. The same cable can supply power to the device, for example to charge a phone.
Worked example A new mouse
Describe what happens when a new USB mouse is plugged into a laptop.
The laptop detects that a device has been connected.
It recognises the device as a mouse and loads the correct driver automatically.
Movement data is sent to the laptop serially, one bit at a time, and the mouse is powered through the USB cable.
Answer: Detected, driver loaded, data sent serially, powered by the cable
Stage 1 · Learn
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How does USB transmit data?
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What happens when a USB device is first plugged in?
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What does the word universal in USB tell you?
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Besides data, what else can a USB cable carry?
Section 03
Benefits and drawbacks.
Learn a few of each. Questions often ask for two benefits and one drawback.
Benefits
Drawbacks
Industry standard: almost all devices and computers support it
Maximum cable length is limited (a few metres, about 5 m for standard cables)
Devices are automatically detected and drivers loaded
Older versions of USB have a slower transmission speed
Connectors only fit one way (or are reversible), so they are hard to plug in wrongly
Different connector types (such as USB-A, micro USB, USB-C) can need adaptors
Can supply power to, or charge, the device
The power supplied is limited, so large devices need their own supply
Newer versions are backward compatible with older ones
Your turn
Give one reason a USB printer cable is unsuitable for connecting a printer 30 metres away.
USB cables have a limited maximum length (a few metres), so the signal would not travel 30 m reliably.
Examiner trap
"It is fast" is too vague to earn a benefit mark. Give a specific benefit such as automatic detection, power supply or being an industry standard.
Stage 2 · Exam
Exam-style questions
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Which is a drawback of USB?
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Which is a benefit of using USB to connect a keyboard?
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Why does USB not suffer from skew?
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A student connects a new USB 3 flash drive to an old USB 2 port. What happens?
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USB, mastered.
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