8. Operation
8.1 Sensor measurements
The SN500SS responds to the SDI-12 commands shown in
(p. 12). When using an
M!
,
M1!
,
M2!
,
M3!
, or
M4!
command, the data logger waits for the time specified by the sensor,
sends the
D!
command, pauses its operation, and waits until either it receives the data from the
sensor or the sensor timeout expires. If the data logger receives no response, it will send the com-
mand a total of three times, with three retries for each attempt, or until a response is received.
Because of the delays this command requires, it is only recommended in measurement scans of
10 seconds or more.
A
C!
,
C1!
,
C2!
,
C3!
, or
C4!
command follows the same pattern as an
M!
,
M1!
,
M2!
,
M3!
, or
M4!
command with the exception that it does not require the data logger to pause its operation
until the values are ready. Rather, the data logger picks up the data with the
D!
command on the
next pass through the program. Another measurement request is then sent so that data is ready
on the next scan.
NOTE:
This section briefly describes using the SDI-12 commands. Additional SDI-12 information is
available in
(p. 19), or at
.
The
MC!
and
CC!
commands are the same as the previous commands, but where the C at the
end of the command forces a validation for the data received from the sensor using a checksum.
If the checksum is invalid, the data logger will re-request the data up to three times. The check-
sum validation increases the measurement time by about 40 milliseconds if there are no errors.
Retries will increase the measurement time in proportion to the number of retries. The checksum
option is necessary only for long cable lengths or when electronic noise may impact meas-
urement transmission to the data logger.
SN500SS Net Radiometer
11
