Campbell Scientific CS450/CS455 Pressure Transducer - Manual (Page 17)

CS450/CS455 Pressure Transducer
3.2 SDI-12 Commands
The CS450/CS455 uses an SDI-12-compatible hardware interface and supports
a subset of the SDI-12 commands. The most commonly used command is the
“aM!” command, issued by the datalogger. Here, a represents the sensor
address. The communication sequence begins with the datalogger waking the
sensor and issuing the aM! command. The transducer responds to the
datalogger indicating that two measurements will be ready within two (2)
seconds. Subsequent communications handle data reporting from the sensor to
the datalogger.
The SDI-12 protocol has the ability to support various measurement
commands. The CS450/CS455 supports the commands that are listed in the
following table.
TABLE 3-5. SDI-12 Commands
SDI-12 Command Command Function Values Returned
aM! Configured settings Pressure/Level,
Temperature
aM1! PSIG, °C Pressure, Temperature
aM2! PSIG, °F Pressure, Temperature
aM3! kPa, °C Pressure, Temperature
aM4! kPa, °F Pressure, Temperature
aM5! Sensor’s Serial Number Serial Number
aM6! Ohms, ohms, °C ΔR, Rb, Temperature
aM7! Configured settings
(provides data in less than
0.8 seconds)
Pressure/Level
aM8! Configured settings
(provides average of data
based on user selected
samples)
Pressure/Level,
Temperature
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 like aMC!, which adds a CRC check, can
significantly improve incorrect data issues.)
3.2.1 SDI-12 Addresses
Multiple sensors can be connected to a single digital I/O channel (control port)
on an SDI-12-compatible datalogger; each sensor must have a unique SDI-12
address. Possible addresses are 0 to 9, A to Z, and a to z.
11