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E.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! is issued, the data logger pauses its operation and waits until either it receives the
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. In this case
make sure your scan interval is longer than the longest measurement time (ttt).
Table E-2: Example aM! sequence
0M!
00352<CR><LF>
0<CR><LF>
0D0!
0+.859+3.54<CR><LF>
E.1.7 Start concurrent measurement commands (aC!)
A concurrent measurement (aC!) command follows the same pattern as the aM! command with
the exception that it does not require the data logger to pause its operation, and other SDI-12
sensors may take measurements at the same time. The sensor will not issue a service request to
notify the data logger that the measurement is complete. The data logger will issue the aD0!
command during the next scan after the measurement time reported by the sensor has expired.
To use this command, the scan interval should be 10 seconds or less. The response to each
command has the form atttn<CR><LF>, where
a = the sensor address
ttt = time, in seconds, until the measurement data is available
The data logger makes a request to sensor 0 to start a measurement.
Sensor 0 immediately indicates that it will return two values within the
next 35 seconds.
Within 35 seconds, sensor 0 indicates that it has completed the
measurement by sending a service request to the data logger.
The data logger immediately issues the first D command to collect
data from the sensor.
The sensor immediately responds with the sensor address and the two
values.
SR50A-Series Sonic Ranging Sensors
53
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