USB Cable Diagram: VBUS, Data Lines, Shield and Ground Explained

USB Cable Diagram: VBUS, Data Lines, Shield and Ground Explained — interactive diagram. Open it in the editor to customise components and wiring.

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A USB cable diagram reveals the internal conductor arrangement connecting VBUS power, differential data pairs D+ and D−, and ground from one connector to the other, forming the foundation for reliable power delivery and high-speed data transfer.

Every USB cable, regardless of connector type, carries four fundamental signal groups that define its function. VBUS on Pin 1 delivers +5 V from host to device, with current capability determined by the port type: 100 mA default for unconfigured USB 2.0, 500 mA after enumeration, 900 mA for USB 3.0, and up to 5 A for USB Power Delivery with appropriate cable rating and negotiation. The return for VBUS is the GND conductor on Pin 4, which also serves as the signal reference for differential data. Between VBUS and GND run the data conductors: D− on Pin 2 and D+ on Pin 3. These two wires form a twisted differential pair engineered to carry USB signalling at 480 Mbps for Hi-Speed USB 2.0 or 10 Mbps for Full Speed. Twisting the pair at consistent pitch cancels common-mode electromagnetic interference inductively coupled from external sources, because any noise voltage added equally to both wires cancels in the differential receiver. The braided or foil shield surrounding all four conductors provides a low-impedance path for high-frequency noise currents to return to the source rather than flowing through the signal conductors. Shield coverage must exceed 85 percent for the cable to meet FCC Class B conducted emissions limits. In standard A-to-B cables the shield connects to the metal connector shell at both ends, which bonds to chassis ground through the host and device mounting hardware. USB 3.0 cables add a second twisted pair for SuperSpeed TX (SSTX+/SSTX−) and a third for RX (SSRX+/SSRX−), plus an additional GND drain wire for the SuperSpeed shield, totalling nine conductors. USB-C cables may add additional conductors for USB4, DisplayPort alternate mode, or power delivery configuration channel CC wires. Impedance control is critical: the D+/D− pair must present 90 ohm differential impedance within ±15 percent throughout cable length to avoid reflections at the characteristic impedance discontinuity that cause eye closure at the receiver. Cable capacitance limits length to 5 m for USB 2.0 at 480 Mbps, beyond which signal attenuation degrades bit error rate below the USB specification limit.

How to wire usb cable diagram

Verify cable type before use Check manufacturer specification for data rate support and current rating. If unlabelled, use a USB cable tester to confirm all conductors are present before relying on the cable for data transfer.
Inspect connector and cable condition Visually examine both connectors for bent pins, debris, or corrosion. Inspect the cable along its length for kinks, cuts, or abraded insulation. Discard any cable showing physical damage.
Measure VBUS voltage Connect the cable to a USB port and measure Pin 1 to Pin 4 with a multimeter set to DC volts. A healthy port shows 4.75–5.25 V. Values below 4.5 V indicate excessive cable or connector resistance.
Confirm data continuity Use a cable tester or multimeter continuity mode to verify D+ and D− pass from Pin 2 to Pin 2 and Pin 3 to Pin 3. Both conductors must show continuity for data transfer to function.
Test at full speed under load Connect a known-good USB device and transfer a large file. Verify transfer speed matches the expected rate for the cable and port generation. Intermittent errors indicate marginal cable quality or connection.

Specifications

VBUS voltage	+5 V ±5% (4.75–5.25 V)
Data pair impedance	90 Ω differential ±15%
Maximum length (USB 2.0 Hi-Speed)	5 m without active repeater
Shield coverage minimum	85% optical coverage for FCC Class B

Safety warnings

Never modify USB cable internals to carry mains voltage — USB cables are rated for SELV (Safety Extra Low Voltage) circuits only, maximum 60 V DC.
Do not use damaged or abraded USB cables for charging — exposed VBUS conductors in contact with metal objects cause shorts and fire risk.
Verify cable current rating before using for high-power USB-C charging — unmarked cables may use 28 AWG VBUS conductors rated only 500 mA, not the 3–5 A needed for PD charging.

Tools needed

USB cable tester for continuity of all pins including shield continuity check
Digital multimeter for VBUS voltage and ground resistance measurements
USB protocol analyser for enumeration and data transfer speed verification
Differential oscilloscope probe for D+/D− eye diagram analysis

Common mistakes

Assuming all USB cables support data transfer — many charging-only cables omit D+ and D− conductors entirely.
Routing USB cables alongside mains power cables, coupling 50/60 Hz interference into D+/D− and causing enumeration failures.
Using a USB extension cable beyond 5 m total length without an active repeater, causing unreliable enumeration at Hi-Speed.

Troubleshooting

Device charges but is not recognised: Cause: Open D+ or D− conductor, or charging-only cable without data wires Fix: Test cable continuity on all four pins. Replace with a verified data cable. Check host USB port for bent pins that may prevent data contact while allowing power contact.
Transfer speed far below expected: Cause: Cable length exceeds specification or high cable capacitance causing signal degradation Fix: Measure cable length — replace with shorter cable or add an active repeater. Verify cable is rated for Hi-Speed USB 2.0 or SuperSpeed USB 3.0 as required.
Intermittent disconnects during transfer: Cause: Broken shield drain wire allowing noise coupling onto data pair Fix: Replace cable. If the issue is cable-specific, a USB isolator may provide temporary relief but replacing the cable is the correct long-term fix.

Frequently asked questions

Why do some USB cables only charge and not transfer data?

Charging-only USB cables contain only the VBUS and GND conductors, omitting the D+ and D− data wires to reduce cost. These cables can supply up to 500 mA for charging but present no data path. A USB protocol analyser shows no enumeration activity on such cables. Always use a verified data cable for file transfer or device programming.

What is the maximum safe current for a USB 2.0 cable?

Standard USB 2.0 cables use 28 AWG for data and 28 or 24 AWG for VBUS and GND. A 24 AWG VBUS conductor handles approximately 750 mA safely over 1–2 m. The USB 2.0 specification limits standard downstream port current to 500 mA. USB Battery Charging 1.2 allows 1.5 A, and USB Power Delivery on USB-C permits up to 5 A with appropriately rated 20 AWG cables.

How do I test a USB cable for internal wire breaks?

Use a USB cable tester that checks all four pins plus shield continuity, or use a multimeter in continuity mode between each pin at one end to the matching pin at the other. A reading above 5 ohms on VBUS or GND indicates excessive resistance. An open reading on D+ or D− explains data transfer failures despite correct power delivery.

What is USB shield drain wire and why does it matter?

The drain wire is an uninsulated conductor running alongside the shield braid or foil, providing a reliable low-resistance connection to the connector shell at each end without requiring direct contact to the shield material. It ensures the shield's RF noise currents have a defined low-impedance path. A broken drain wire raises shield impedance at high frequency, allowing radiated emissions that cause nearby device interference.

Can I extend a USB cable beyond 5 metres?

Not reliably without an active solution. The USB 2.0 specification limits cable electrical length to 5 m at Hi-Speed due to propagation delay and signal attenuation. An active USB extension repeater regenerates the signal, allowing an additional 5 m segment. Up to five hubs can be cascaded between host and device, each adding one 5 m cable segment for a theoretical maximum of 30 m total reach.