Do not use a generic passive USB-A-to-USB-A cable to connect two computers or charge one USB host from another. The USB 2.0 specification lists a cable assembly with a Standard-A plug at both ends as prohibited. If specialized equipment uses an A-to-A service or proprietary connection, use only the exact cable and procedure documented by that equipment manufacturer.
A Standard USB-A Plug Is Intended for the Host Side
USB uses defined host/device roles and approved cable assemblies. A passive Standard-A-plug-to-Standard-A-plug cable can connect two host-side power/data ports in a way the standard does not permit. Connector fit is therefore not evidence that the connection is safe or functional.
For a related connector and cable-specification path, see Male vs Female Cable Connectors: Identification Guide.
Use the Correct Host-to-Device Cable Path
| Task | Typical cable path | Check |
|---|---|---|
| Computer to printer/audio interface | USB-A host → USB-B device | Exact Type-B shape and data generation |
| Computer to legacy phone/accessory | USB-A host → Micro-B or Mini-B device | Device receptacle, data and charging requirements |
| USB-A charger/computer to USB-C device | Compliant USB-A-to-USB-C cable | Charging/data capability and device support |
| Special service equipment | Manufacturer-declared proprietary assembly only | Exact model, procedure and cable part number |
The USB Type-B guide explains Standard-B, Mini-B and Micro-B device-side variants.
Computer-to-Computer Transfer Needs a Purpose-Built Bridge
To transfer files directly between two computers over USB, use a purpose-built USB transfer/bridge cable with the required electronics and supported software, or use a network/storage workflow. Do not substitute a passive A-to-A cable. A bridge product must state its operating-system support and transfer method.
Treat Proprietary A-to-A Connections as Model-Specific
Some test fixtures, enclosures or legacy products may physically use Type-A at both ends. That does not make a generic cable suitable. Check the equipment manual and approved part number; the wiring, role and power behavior may be proprietary. If the documentation is unavailable, do not experiment by connecting two powered ports.
The same caution applies to ports that look like USB but are used only for factory service, firmware loading or a vendor-specific accessory. Match the exact equipment model and documented function; never infer an electrical role from the connector shell alone.
Identify an Unknown Cable before Connecting It
- Photograph both connector ends and read any markings.
- Record the exact equipment models associated with the cable.
- Check the manufacturer manual for the approved assembly.
- Do not use a continuity-only check to infer USB compliance or safe power behavior.
- Use the USB connector identification guide if either end is uncertain.