This new feature is currently released in Beta mode. Technical information available on the IOIO wiki. The way this works is that IOIO will attempt to communicate with the Android device with the OpenAccessory protocol. When this is not supported, it will seamlessly fall back to ADB. This enables you to connect the same IOIO board to both new and old devices. Your applications can be very easily be ported to the new mode, requiring only a few non-intrusive modifications to your application's metadata.
What is this all about? What is the relation between this new technology, IOIO and the other boards out there? I will try to provide some answers and clear some of the confusion that was caused as result of the proximity of all these announcements.OpenAccessory is a new Android feature, which enables connecting external peripherals to an Android device over a USB connection. This feature exposes a standardized interface on the USB bus, as well as a Java API that enables an Android application to communicate with the accessory on the other end. This feature is supported on Android 2.3.4 and higher. The OpenAccessory protocol allows the Android device to act as either a host or a device on the USB bus (the host mode is only supported on Android 3.x and higher and only on certain devices).
OpenAccessory is a low-level protocol: it features a single full-duplex communication channel between the Android device and the accessory, over which arbitrary bytes can be sent back and forth - much like a UART connection. It leaves to the accessory designer to design the higher-level protocol, i.e. what messages to send and what their meaning is to the Android application and to the accessory.
Read more about OpenAccessory here. The Accessory Developer's Kit (ADK) is a reference implementation of an OpenAccessory-enabled board, developed by Google and announced together with OpenAccessory. This board is essentially an Arduino Mega with an on-board USB host shield. It comes with an Arduino-side C++ library, which implements the protocol. Following Google, several vendors have released compatible boards.The term "ADK" is often used synonymously with "OpenAccessory", i.e. one might say "this new board supports ADK" when they actually mean it supports the OpenAccessory protocol.In my personal opinion, the ADK has been released mostly for promoting the OpenAccessory protocol and providing a quick-start and demo board, rather than intended to be a consumer product.The Android Debug Bridge (ADB) is a debug protocol which existed on every Android device since the early days of Android. Technically, this protocol allows a host connected over USB to open various kinds of communication channels to the Android device. On the Android-side of these communication channels are different services, such as debug, file-system access, Linux shell access. Another note-worthy service (on which IOIO has been based) allows forwarding of the data sent over the channel to a TCP socket. This allows an Android application to listen on a certain port and accept connections coming from the outside world, and do so without the need to modify the OS.ADB's main advantages over OpenAccessory are:Available on any Android device.Provides useful features, such as file-system access (IOIO uses this for firmware upgrades).More mature, does not suffer from some problems currently existing in OpenAccessory.Simpler to work with on the Android application side - just listen on a TCP socket.OpenAccessory's main advantages over ADB are:Better throughput and latency.Does not require the user to enable USB debugging.More secure (IOIO takes its own measures to guarantee that the power of ADB cannot be exploited by a malicious firmware).Allows applications to be notified upon connection of the accessory. The user can choose which application to launch when the accessory connects. This might be doable with ADB too, but IOIO doesn't do that.See Inopia's excellent in-depth comparison here.
Supports All Android Versions - since IOIO works with both OpenAccessory and ADB it can communicate with a very large variety of existing Android devices, leveraging OpenAccessory when it exists and leveraging the additional features of ADB when they exist. Other boards, which do not support ADB, are limited to all but the newest Android devices out there.Functionality - IOIO is almost exactly identical to the Arduino Mega in terms of pin counts and functions. The only difference I could spot is in the number of PWM channels (IOIO-9, Mega-16) and TWI channels (IOIO-3, Mega-1).Cost - at $50, IOIO currently seems to be the cheapest available commercial board out there. A close alternative is a DIY version offered here, costing $55 and requires some work.High-Level Software - the other boards out there expect you to write both an Android application and embedded-C code for the board, designing your own communication protocol. IOIO does all that for you, leaving you to write only the Android-side code, while using a high-level Java API for controlling the board's functions.Support Forum and Wiki - IOIO has an active discussion group and an extensive documentation wiki, which continues to grow quickly. The IOIO project is committed to the hobbyist community, and to the hobbyist community only!Size - IOIO is probably the smallest board out there - almost as small as you could get with 48 I/O pins, numerous supply pins and a USB connector. It is much smaller than the ADK board.Bootloader - IOIO's firmware includes a secure bootloader, which enables firmware upgrades to be performed through the Android device.Power Supply - IOIO has an on-board 5V switch-mode regulator capable of delivering up to 1.5A. This allows for simultaneous charging of the Android and powering two standard servos without problem. Some of the other boards will require an external 5V supply to support this use-case. In addition, IOIO has an on-board trimmer which allows limiting the Android's charging current. This is very useful for battery-operated setups, when you don't want the Android device to drain your battery.Open-Source - Unlike some of the other alternatives - the IOIO's hardware, firmware and software are completely open-source with a FreeBSD license (very permissive). This approach has been chosen because I believe this is what works best for the hobbyist community, and allows people to customize the product for their needs, contribute to it, understand it best, compete on its pricing.In conclusion, despite my obvious bias, I believe IOIO is very competitive with other OpenAccessory-enabled platforms. To be fair, here is another view.