Micro-A to Micro-B on USB 3.0: Where the OTG Host Link Is Used
Most cables connect a host to a device, but both ends of a Micro-A to Micro-B assembly look like device connectors. The difference is the ID contact inside the shell, which tells the hardware which side acts as host. Understanding that single pin explains why this niche cable is still written into industrial and embedded specifications.
Quick Answer / Summary
A USB 3.0 Micro-A to Micro-B cable links a portable OTG host to a peripheral. The Micro-A end includes an ID contact tied low, which signals the port to act as host through OTG negotiation. Micro-B ends are typically used on devices that act as peripherals. Micro-A is wider than Micro-B and carries the same ten-contact SuperSpeed layout, so the link can run at 5 Gbps while the host role is assigned by the ID line.
Micro-A and Micro-B: same family, different roles
Both shells belong to the USB 3.0 Micro connector family and share the same contact count and SuperSpeed positions. The Micro-A plug is wider because it adds the ID contact, and the ID line is what OTG uses to decide host or device role.
A Micro-B plug omits or leaves the ID unconnected, so it defaults to device behavior. That is why a cable with Micro-A on one end and Micro-B on the other defines a host-to-device relationship the moment it is inserted, with no software configuration.
What the ID contact does
When the ID pin is grounded, the port switches to host and takes responsibility for supplying bus power and enumerating the attached device. When ID is open, the port behaves as a peripheral. On Micro-A and Micro-B hardware, this is the mechanism behind OTG host switching: the cable, rather than a menu, selects the role.
Where these cables are actually used
Micro-A to Micro-B appears in embedded and industrial equipment where one small device must host another. Handheld instruments, portable data loggers and dedicated control units often expose a Micro-A port so they can read from a Micro-B peripheral such as a card reader or a mobile drive.
The connector is also common on early 3.0 mobile hardware, where the same physical port handled both host and peripheral roles depending on the cable fitted. Designers chose it because a single receptacle could serve both functions with the correct assembly.
Bus power when the portable device is host
In an OTG host role, the portable device supplies VBUS to the peripheral. That device is often battery powered, so the current budget is tighter than a desktop port. A standard port provides one unit load of 100 mA before configuration and five unit loads (500 mA) after, and a charging port can reach 900 mA. A peripheral that draws beyond the host budget will brown out, which is why low power peripherals are the norm in these builds.
Why the assembly is niche but specified
The cable solves a specific problem: connect two small devices without a computer in between. General purpose cables cannot do this, because their connector types imply a fixed role. Where an equipment manual names a Micro-A to Micro-B assembly, substituting a different shell changes the OTG behavior and the link will not establish.
Availability is limited because volumes are low, so buyers often specify the assembly directly with a supplier rather than hunting retail stock. Contact layout, keying and the ID connection are the details worth confirming before a build.
Details to confirm on a purchase order
Confirm that the Micro-A end has the ID contact connected as the host role requires, that both shells carry the full ten-contact SuperSpeed layout, and that the length and jacket suit the enclosure. Small differences in molding or keying can make a cable that looks correct fail to seat in a recessed port.
Comparison Table
| Feature | Micro-A | Micro-B |
|---|---|---|
| Shell width | Wider, adds ID contact | Narrower, no host ID |
| Default role | Host when ID grounded | Peripheral |
| Contact count | Ten with SuperSpeed row | Ten with SuperSpeed row |
| Typical use | Portable OTG host | Device or peripheral |
| Bus power | Source in host role | Sink on device side |
Table: Micro-A versus Micro-B on USB 3.0
Practical Application Scenarios
- A handheld data logger reads a card reader with no computer present, and the Micro-A to Micro-B cable assigns the host role the moment it is inserted.
- A peripheral browns out during a transfer because it draws more than the battery powered host can supply on VBUS.
- A replacement cable with a different shell fails to establish the link, because the host role depends on the ID contact in the Micro-A end.
- An equipment manual names a Micro-A to Micro-B assembly, and confirming the ten-contact layout avoids a build that will not seat.
Related Products and Further Reading
- Micro and Type-C cable catalog – The catalog lists Micro-A, Micro-B and Type-C assemblies with contact and keying options.
- contact us about a custom assembly – Contact us with your OTG role requirements to confirm the ID wiring before production.
- certifications overview – The certifications overview lists the compliance documentation available for embedded and industrial builds.
- 3.0 Micro-B cable guide – The 3.0 Micro-B cable guide covers the device side of the same Micro connector family.
Frequently Asked Questions
What is a Micro-A to Micro-B cable used for?
It connects a portable OTG host to a Micro-B peripheral, letting one small device control another without a computer in between.
How is Micro-A different from Micro-B?
Micro-A is wider and includes an ID contact that assigns the host role when grounded. Micro-B omits the host ID and behaves as a peripheral.
Do these cables support 5 Gbps?
Yes. Both shells carry the same ten-contact layout with the SuperSpeed pairs, so the link can run at 5 Gbps when hardware supports it.
Why is this cable hard to find in stores?
Volumes are low because the use case is narrow, so buyers usually order the assembly directly from a supplier instead of retail.