Vaisala WS425 - Manual (Page 53)

Chapter 4 ________________________________________________________________ Operation
VAISALA _______________________________________________________________________ 53
SDI-12 Communications Protocol
SDI-12 data recorders and sensors communicate by an exchange of
ASCII characters on the data line. The data recorder sends a break to
wake up the sensors on the data line. A break is continuous spacing on
the data line for at least 12 milliseconds. The data recorder then sends a
command. The sensor, in turn, returns the appropriate response. Each
command is for a specific sensor. The first character of each command is
a unique sensor address that specifies with which sensor the recorder
wants to communicate. Other sensors on the SDI-12 bus ignore the
command and return to low-power standby mode. When a data recorder
tells a sensor to start its measurement procedure, the recorder does not
communicate with any other sensor until the data collection from the first
sensor is complete.
A typical recorder/sensor measurement sequence proceeds in the
following order:
1. The data recorder wakes all sensors on the SDI-12 bus with a
break.
2. The recorder transmits a command to a specific, addressed sensor,
instructing it to make a measurement.
3. The addressed sensor responds within 15.0 milliseconds returning
the maximum time until the measurement data will be ready and
the number of data values it will return.
4. If the measurement is immediately available, the recorder transmits
a command to the sensor instructing it to return the
measurement(s). If the measurement is not ready, the data recorder
waits for the sensor to send a request to the recorder, which
indicates that the data is ready. The recorder then transmits a
command to get the data.
5. The sensor responds, returning one or more measurements.
Baud Rate and Byte Frame Format
The baud rate for SDI-12 is 1200 b. Table 14 below shows the byte frame
format for SDI-12.
Table 14 Byte Frame Format for SDI-12
1 start bit
7 data bits, least significant bit transmitted first
1 parity bit, even parity
1 stop bit