Vaisala WXT510 - Manual (Page 142)

User’s Guide ______________________________________________________________________
140 _________________________________________________________________ M210470EN-D
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.
SDI-12 Timing
Figure on page shows a timing diagram for a SDI-12 command and its
response. The tolerance for all SDI-12 timing is ±0.40 milliseconds.
The only exception to this is the time between the stop bit of one
character and the start bit of the next character. The maximum time for
this is 1.66 milliseconds, with no tolerance.
- A data recorder transmits a break by setting the data line to spacing
for at least 12 milliseconds.
- The sensor will not recognize a break condition for a continuous
spacing time of less than 6.5 milliseconds. The sensor will always
recognize a break when the line is continuously spacing for more
than 12 milliseconds.