Campbell OBS500, Operator'S Manual

Page: 62 / 64

Appendix D. SDI-12 and RS-232 Measurement Commands for OS Version 1
D-2
TABLE D-1. SDI-12 and RS-232 Measurement Commands
Commands Process Values Returned
aM6!
aC6!
Measure
Send Data
obs (tu)
ss (tu )
ratio (tu)
temperature (degc)
raw obs (volts)
raw ss (volts)
open current (ma)
close current (ma)
wet dry (0=dry 1=wet)
aM7!
aC7!
Close Wiper
Send Data
Close wiper position count
Close max current count
Close timeout count
Close current (ma)
aM8!
aC8!
Send Wiper Data Open close total count
Open wiper position count
Open max current count
Open timeout count
Close wiper position count
Close max current count
Close timeout count
aC9! Open Wiper
Measure 100 Times
Close
Send Data
obs median
obs mean
obs standard deviation
obs min
obs max
ss median
ss mean
ss standard deviation
ss min
ss max
With the SDI-12 concurrent measurements ( aCx! ), the datalogger
does not request the data until the next interval hits. For example,
if you have a 30-minute interval, you will not see the data for 30
minutes. There is not an equivalent M command to the aC9!
command since the M command is limited to nine returned values.
As the measurement data is transferred between the probe and datalogger
digitally, there are no offset errors incurred with increasing cable length as seen
with analog sensors. However, with increasing cable length, there is still a
point when the digital communications will break down, resulting in either no
response or excessive SDI-12 retries and incorrect data due to noise problems.
Using SDI-12 commands which add a CRC check (e.g., aMC! ), can
significantly improve incorrect data issues.
NOTE