I was trying without success to use the Sparkfun adaptor with an XBee to get wireless operation for my Rover. After getting the elusive schematic for the APM2.5 and the schematic for the Sparkfun adaptor it became obvious that there are 1k resistors in series with the input and output of the APM port. There are 1k pull up resistors on the sparkfun adaptor. With this arrangement it is impossible for the Serial Port on the APM to pull down the bus to less than 2.5 volts. To achieve a 0 state TTL level, the bus has to get down to somewhere around 0.5 volts ( see http://www.allaboutcircuits.com/textbook/digital/chpt-3/logic-signal-voltage-levels/
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The solution may be to change the pull up resistor on the input to the Sparkfun adaptor to 10k or more, but I chose to use a different adaptor.