Thanks, Shannon, for your response, clarifications and tremendous work with the Mayfly. Addressing your questions:
I’m powering the CS451 with the same 12v battery that powers the Mayfly from the 12-5v USB converter. The ground (-) of the Mayfly & battery are common. I could not find anywhere in the Campbell Sci specs the voltage level for the signal. But I did find in the SDI-12 Serial-Digital Interface Standard (SDI-12 Support Group, Utah) that the standard for the signal is essentially 0-5v. So, I plugged the signal line in Digital Pin #7 and it’s been working fine. If I had an oscilloscope I could confirm the voltages.