Android supports a variety of USB peripherals and Android USB accessories
(hardware that implements the Android accessory protocol) through two modes: USB
accessory and USB host. In USB accessory mode, the external USB hardware acts as
the USB host. Examples of accessories might include:

- robotics controllers
- docking stations
- diagnostic and musical equipment
- kiosks
- card readers

and many more. This gives Android-powered devices that don't have host
capabilities the ability to interact with USB hardware. Android
USB accessories must be
designed to work with Android-powered devices and must adhere to the [Android
accessory communication protocol](http://accessories.android.com/demokit). In
USB host mode, the Android-powered device acts as the host. Examples of devices
include digital cameras, keyboards, mice, and game controllers. USB devices that
are designed for a wide range of applications and environments can still
interact with Android applications that can correctly communicate with the
device.

Figure 1 shows the differences between the two modes. When the Android-powered
device is in host mode, it acts as the USB host and powers the bus. When the
Android-powered device is in USB accessory mode, the connected USB hardware (an
Android USB accessory in this case) acts as the host and powers the bus.

![Diagram showing the difference between host and accessory modes](https://developer.android.com/static/images/usb-host-accessory.png)

**Figure 1.** USB Host and Accessory Modes

USB accessory and host modes are directly supported in Android 3.1 (API level
12) or newer platforms. USB accessory mode is also backported to Android 2.3.4
(API level 10) as an add-on library to support a broader range of devices.
Device manufacturers can choose whether or not to include the add-on library on
the device\\'s system image.

**Note:** Support for USB host and accessory modes are ultimately dependant on
the device\\'s hardware, regardless of platform level. You can filter for devices
that support USB host and accessory through a
[`<uses-feature>`](https://developer.android.com/guide/topics/manifest/uses-feature-element) element.
See the USB [accessory](https://developer.android.com/develop/connectivity/usb/accessory) and
[host](https://developer.android.com/develop/connectivity/usb/host) documentation for more details.

## Debug considerations

When debugging applications that use USB accessory or host features, you most
likely have USB hardware connected to your Android-powered device. This
prevents you from having an `adb` connection to the Android-powered device
using USB. You can still access `adb` over a network connection. To enable `adb`
over a network connection:

1. Connect the Android-powered device using USB to your computer.
2. From your SDK `platform-tools/` directory, enter `adb tcpip 5555` at the command prompt.
3. Enter `adb connect <device-ip-address>:5555` You should now be connected to the Android-powered device and can issue the usual `adb` commands like `adb logcat`.
4. To set your device to listen on USB, enter `adb usb`.