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Instead of timing, it’s possible there’s a voltage problem. Do you have the sdi-12 data pin going directly to the Mayfly? Then the power and ground to another battery? The sdi-12 specification actually requires that the voltage of the “spacing” be over 3.5V. The Mayfly is only powered at 3.3V, so it’s not actually ever getting the voltage high enough to meet sdi-12 specs. The decagon and zebra tech sensors I’ve worked with have seemed to be fine with that but the Campbell might not be. Can you try putting a level shifter on the data pin to boost the voltage? I think a standard Uno runs at 5V so you could also try the basic communication on an Uno if you have one handy to see if it works.
