Table B-3: Example aC! sequence
YD0!
Ten seconds later, after a total of 40 seconds have
passed, the data logger starts the process of
collecting data from sensor Y by issuing the first
D
command.
Y+1+2+3+4+5+6<CR><LF>
The sensor immediately responds with the sensor
address and the 6 values.
B.1.8 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 3 characters appended before the
<CR><LF>. This code is not returned in the data table but 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 available at
to learn more about how the CRC
code is developed.
B.1.9 Stopping a measurement command
A measurement command (
M!
) is stopped if it detects a break signal before the measurement is
complete. A break signal is sent by the data logger before most commands.
A concurrent measurement command (
C!
) is aborted when another valid command is sent to
the sensor before the measurement time has elapsed.
B.1.10 Send data command (aD0! … aD9!)
The Send Data command requests data from the sensor. It is issued automatically with every type
of measurement command (
aM!
,
aMC!
,
aC!
,
aCC!
). When the measurement command is a
M!
or a
MC!
, the data logger issues the
aD0!
command once a service request has been received
from the sensor or the reported time has expired. When the data logger is issuing concurrent
commands (
aC!
or
aCC!
), the Send Data command is issued after the required time has elapsed
(no service request will be sent by the sensor). In transparent mode (see
(p. 22) ), the user asserts this command to obtain data.
Depending on the type of data returned and the number of values a sensor returns, the data
logger may need to issue
aD0!
up to
aD9!
to retrieve all data. A sensor may return up to 35
SN500SS Net Radiometer
21
