Well, I have regressed even further. Now I can’t even get the modem to turn on. I confirm ~4.2V between pins 10 and 1 on the Xbee, so it is definitely getting power (and from the LiPo to boot) but it is no longer responding to the first initialization in e.g. logging_to_EnviroDIY.ino. Unforunately I don’t have any development board other than the MayFly to test on. Am I missing something major here?