Today I tried a simple test with the Redpark iOS serial cable for iPhone, trying to connect from a simple iOS test app through the cable to a 3DR telemetry radio in order to send mavlink commands to a receiving radio mounted on a drone.
Unfortunately it looks like Redpark cable only puts out a 3.3V logic level signal. This doesn’t work with the 3DR radio, which apparently needs 5V signals. Sure enough, plugging the radio into the cable results in some flaky behavior.
My next step will be to try a logic-level shifter between the 3dr radio and the Redpark cable. I may also introduce a 5V power supply to power the radio separately from the iPhone supply.