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You cannot interface with generic USB devices from a stock OS iPad unless you are a manufacturer in Apple's MFi (http://developer.apple.com/programs/mfi/) program. There appears to be a ton of paperwork involved to just apply. The external accessories API is of no help on stock OS devices, unless you are manufacturing your own custom MFi-compliant device.

Alternatives

• If your USB device can pretend to be a MIDI instrument or controller, you might be able to interface with it over USB by sending/receiving MIDI messages from the iPad.
• Another option is to use a small inexpensive board, such as a Raspberry Pi Zero W, and use it to programmatically transfer USB data via WiFi to an iOS device.

You cannot directly interface with generic USB devices from a stock OS iPad unless you are a manufacturer in Apple's MFi (http://developer.apple.com/programs/mfi/) program. There appears to be a ton of paperwork involved to just apply. The external accessories API is of no help on stock OS devices, unless you are manufacturing your own custom MFi-compliant device.

Alternatives

• If your USB device can pretend to be a MIDI instrument or controller, you might be able to interface with it over USB by sending/receiving MIDI messages from the iPad.
• Another option is to use a small inexpensive board, such as a Raspberry Pi Zero W, and use it to programmatically transfer USB data via WiFi to an iOS device.

You cannot interface with generic USB devices from a stock OS iPad unless you are a manufacturer in Apple's MFi (http://developer.apple.com/programs/mfi/) program. There appears to be a ton of paperwork involved to just apply. The external accessories API is of no help on stock OS devices, unless you are manufacturing your own custom MFi-compliant device.

Added (2011):

If your USB device can pretend to be a MIDI instrument or controller, you might be able to interface with it over USB by sending/receiving MIDI messages from the iPad.

Added (2018):

Another option is to use a small inexpensive board, such as a Raspberry Pi Zero W, and use it to programmatically transfer USB data via WiFi to an iOS device.

You cannot interface with generic USB devices from a stock OS iPad unless you are a manufacturer in Apple's MFi (http://developer.apple.com/programs/mfi/) program. There appears to be a ton of paperwork involved to just apply. The external accessories API is of no help on stock OS devices, unless you are manufacturing your own custom MFi-compliant device.

Alternatives

• If your USB device can pretend to be a MIDI instrument or controller, you might be able to interface with it over USB by sending/receiving MIDI messages from the iPad.
• Another option is to use a small inexpensive board, such as a Raspberry Pi Zero W, and use it to programmatically transfer USB data via WiFi to an iOS device.

You cannot interface with generic USB devices from a stock OS iPad unless you are a manufacturer in Apple's MFi (http://developer.apple.com/programs/mfi/) program. There appears to be a ton of paperwork involved to just apply. The external accessories API is of no help on stock OS devices, unless you are manufacturing your own custom MFi-compliant device.

Added:

If your USB device can pretend to be a MIDI instrument or controller, you might be able to interface with it over USB by sending/receiving MIDI messages from the iPad.

You cannot interface with generic USB devices from a stock OS iPad unless you are a manufacturer in Apple's MFi (http://developer.apple.com/programs/mfi/) program. There appears to be a ton of paperwork involved to just apply. The external accessories API is of no help on stock OS devices, unless you are manufacturing your own custom MFi-compliant device.

Added (2011):

If your USB device can pretend to be a MIDI instrument or controller, you might be able to interface with it over USB by sending/receiving MIDI messages from the iPad.

Added (2018):

Another option is to use a small inexpensive board, such as a Raspberry Pi Zero W, and use it to programmatically transfer USB data via WiFi to an iOS device.