Xylem H-335 - Manual (Page 23)

H-335
SDI-12 Command and Response Protocol 2-7
2.7 Send Acknowledge Command
The Send Acknowledge Command returns a simple status response which includes the address of the sensor.
Any measurement data in the sensor's buffer is not disturbed.
Command Response
"a!" "a"
Where:
a Is the sensor address ("0-9", "A-Z", "a-z", "*", "?").