Campbell CS455 Manual descarregar pdf (Página 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 logger

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.

1 2 ... 12 13 14 15 16 17 18 19 20 21 22 ... 33 34

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