The APM 2.5 board, which was released in July 2012, is operationally identical to the APM 2.0 board, and runs the same code without modification. It does, however, include a number of improvements designed to increase its reliability and eliminate common sources of user error:
It includes reverse polarity and other power and signal protection on all main I/O pins, for enhanced robustness and reliability.
The telemetry port has been changed to a 5-pin connector that cannot be plugged in backwards, eliminating a common source of user error in APM 2.0. (An adapter cable to the standard 3DR and Xbee radios is included.)
An external magnetometer may now be used, if desired, to keep the sensor away from sources of magnetic interference. To use that, cut the trace between the micro-jumpers marked above and attach the external magnetometer to the I2C port.
Additional LEDs have been added for future use It only supports an external GPS module (the 3DR uBlox 6 is recommended). This is to ensure that the user can place the GPS in the most optimal position, with a clear view of the sky and far from sources of electromagnetic interference such as ESCs and other radios.
The dataflash chip is now built-in, rather than on a custom SD card. This is to avoid problems some users were having with APM 2.0 card insertion and contacts. There is a new connector for future expansion (to be announced later).
Aside from these changes, the boards operate the same. Please see the APM 2.0 instructions for pin definitions and other information about the APM 2.x generation boards.
Note: Because the APM 2.5 board does not have a daughterboard covering the barometric pressure sensor (the small silver rectangular sensor in the image above), it's important that the board be mounted in an enclosure to protect the sensor from airflow, which can badly throw off the altitude readings. If you choose to mount the board out in the open, make sure to cover the sensor with some cotton padding or some tissue