Welcome to EnviroDIY, a community for do-it-yourself environmental science and monitoring. EnviroDIY is part of 
Just an FYI, as I understand the SDI-12 spec, the receiving instrument should receive 3.5V for a ‘0’ or space. The mayfly port is only capable of supplying 3.3V – so it may work for most SDI-12 instruments, and is working for one SDI-12 instrument I’m using
Typically with off-spec issues, things can work on some instruments and then not on others. So hopefully the problem is something simple, but just sharing my analysis.
