Thermo Scientific AquaSensors™ DataStick™ Modbus ...

5 Register Numbers and Byte/Word Order
According to the Modicon Compact 984 Ladder Logic Manual, 043503387, the I/O Reference
Numbering System defines reference number 4XXXX as “an output or holding register. A 4X register
may be used to store numerical data (binary or decimal) in state RAM or to send the data from the CPU
to an output unit in the control system.” For this reason, the register map for the Modbus Adapter is
defined to begin at 40,001. The Modbus Adapter is the CPU in the above statement, and the customer’s
Modbus master is the output unit in the control system.
As defined, some registers are read/write; some are read-only, and some are write-only. If a write-only
register is read by the Modbus master, the register’s value will be 0.
The defined registers are grouped into banks so that all registers in a bank can be read with a single
Modbus read request. If the register number received in a query is outside the range of the defined
banks, the Modbus Adapter will respond with an Exception Code of 02 indicating an illegal data
address. The legality of some register numbers depends on the type of Sensor Head installed in the
DataStick. For example, the Cell Constant register is legal only if a Contacting Conductivity or
Electrodeless Conductivity Sensor Head is installed in the DataStick. DataStick commands and
responses are defined in Section 7 of this document. If the register number received in a query is
invalid, the Modbus Adapter will respond with a value of 0. It is the responsibility of the PLC
programmer to know which register numbers are valid based on the Sensor Type.
For consistency, all values of data type float are aligned at even-numbered registers.
Values of type float are represented in 32-bit IEEE 754 floating-point format. This requires two
registers per value. By default, the lower register number contains the upper 16 bits and the higher
register number contains the lower 16 bits as shown in Table 6. This is known as “Big-Endian” word
order. Floats can be made Little-Endian in their word order by changing the value of the PLC Data
Type accordingly as shown in Table 7. The byte order can also be affected by PLC Data Type. For
compatibility with Thermo Scientific AquaSensors products such as AquaComm and the AV38, set PLC
Data Type to Big Endian (2).
Table 6: Word order of values of type float
42001
42002
Upper 16 bits
Lower 16 bits
Sensor Value (float)
FPConvert can be used to determine hexadecimal representations of floating-point values (and vice
versa). This utility can be obtained from http://www.61131.com.
Table 7: Defines the valid values for PLC Data Type.
Value Type
0 Little Endian – ControlLogix
1 (default) Little Endian Word Swap – SLC
2 Big Endian
3 Big Endian Word Swap
Thermo Scientific AquaSensors DataStick Modbus Communications User Guide 10