In-Situ Aqua TROLL 400 Modbus and SDI-12 Reference Guide - Manual (Page 79)

8.37.5 Sensor Data Register Map Offsets
These registers specify the register number of the first register in each sensor’s Sensor Data
Header Block. These registers assume the 4x (holding register) reference ID and therefore will
not contain the reference ID as part of the value (i.e. 40001 will be stored as 1).
If an attempt is made to access a sensor status register that is not mapped to a sensor (one that
exceeds the maximum number of sensors supported by the probe), the probe returns an
exception response with error code 0x02 (illegal data address).
If a sensor connection is open (the sensor is not plugged in or is not part of the current
configuration) the probe returns 0. This permits a valid block read of the mapped sensor status
registers regardless of the current configuration of the probe and sensors.
If an attempt is made to access a sensor connection register that is not mapped to a sensor
(one that exceeds the maximum number of sensors supported by the probe), the probe will
return an exception response with error code 0x02 (illegal data address).
8.37.6 Sensor Data Cache Timeout
Sensors shall measure all of their parameters whenever a parameter value register is read. The
parameters are recorded in a data cache and a cache timeout is started. If any subsequent
parameter value from the sensor is read within the specified cache timeout, the device shall
return the value recorded in the cache instead of making a new measurement. The default value
of the cache timeout is device specific. If the cache timeout value is set to 0 milliseconds, each
Modbus message to read one or more parameters will cause a new sensor reading to be taken.
Logs running on a probe will also populate the data cache when a measurement is taken.
The end-of-session timeout supersedes the cache timeout. If an end of session timeout occurs,
the cache for all sensors shall be cleared.
8.38 Current Loop Configuration
Attempting to access current loop configuration registers 9501 through 9507 will result in an
exception response with exception code 0x02 (illegal data address).
8.39 Last Logged Record Registers
These registers are not supported by the device since data logging is not supported. Attempting
to access these registers will result in an exception response with exception code 0x02 (illegal
data address).
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