B.1.6 Start measurement commands (aM!)
A measurement is initiated with the
M!
command. The response to each command has the form
atttn<CR><LF>
, where
a
= sensor address
ttt
= time, in seconds, until measurement data is available; when the data is ready, the sensor
notifies the data logger, and the data logger begins issuing
D
commands.
n
= the number of values returned when one or more subsequent
D
commands are issued; for
the
aM!
command,
n
is an integer from 0 to 9.
When the
aM!
command is issued, the data logger pauses its operation and waits until either it
receives data from the sensor or the time (
ttt
) expires. Depending on the scan interval of the data
logger program and the response time of the sensor, this may cause skipped scans to occur. To
avoid this, ensure that the scan interval is greater than the longest measurement time (
ttt
).
Table B-2: Example aM! sequence
0M!
The data logger makes a request to sensor 0 to start a measurement.
00352<CR><LF>
Sensor 0 immediately indicates that it will return two values within the
next 35 seconds.
0<CR><LF>
Within 35 seconds, sensor 0 indicates that it has completed the
measurement by sending a service request to the data logger.
0D0!
The data logger immediately issues the first
D
command to collect
data from the sensor.
0+.859+3.54<CR><LF>
The sensor immediately responds with the sensor address and the
two values.
B.1.7 Start measurement commands with cyclic
redundancy check (aMC! and aCC!)
Error checking is done by using measurement commands with cyclic redundancy checks (
aMC!
or
aCC!
). This is most commonly implemented when long cable lengths or electronic noise may
impact measurement transmission to the data logger. When these commands are used, the data
returned in response to
D!
or
R!
commands must have a cyclic redundancy check (CRC) code
appended to it. The CRC code is a 16-bit value encoded within three characters appended before
the <CR><LF>. This code is not returned in the data table but is instead checked by the data
logger as it comes. The code returned is based on the SDI-12 protocol. See the SDI-12
communication specification for version 1.4 version 1.4 (January 2019) available at
to learn more about how the CRC code is developed.
MetSENS-Series Compact Weather Sensors
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