Reply To: SDI-12 Change Address for Meter Hydros 21 CTD Sensor

It sounds like your PC is just not seeing the Mayfly. You should make sure the Mayfly is turned on at the main power switch, and that the small power-select slide switch is set to the USB/Lipo setting (not Ext). Sometimes the switches can be a little glitchy, so you can try sliding them back and forth to make sure they’re getting good contact. And also ensure that you’re using a proper microUSB cable, like the one that came with the starter kit, if you bought one. Some users have reported issues with cheaper cables due to bad connections, broken wires, or bent pins on the cable. And verify that you’ve chosen the proper COM port in the Arduino IDE settings. Sometimes your computer will assign a new COM port to the Mayfly, but the IDE doesn’t always update the COM setting to the newest port, so it’s looking for the Mayfly on the wrong port.