Thermo Scientific AquaSensors™ DataStick™ Modbus ...

The Modbus adapter responds with an exception if both registers of a float data type are not read or
written in the same request. This will reduce the likelihood of data corruption within the Modbus
Master.
There exist three banks of registers that deserve further discussion. They are banks 42XXX, 43XXX,
and 44XXX. Register bank 42XXX contains Process Value, Calibration, and Configuration variables of
type float. Register bank 43XXX contains the same Process Value, Calibration, and Configuration
variables, but of type integer. Register bank 44XXX contains the corresponding Scale Factors for the
integer variables in bank 43XXX. The order of the variables in banks 43XXX and 44XXX is identical
so that a Modbus master can access the Scale Factor for an integer Process Value simply by adding
1,000 to the integer Process Value register number.
5.1 Variable Classes
The class of each variable is shown in the register map in Section 6 of this manual. Please note that
there is not a one-to-one correspondence between variables and registers. This is because variables of
type float each occupy two registers.
To make Modbus master programming easier, variables of each class are grouped together in the register
map in banks. Each bank is 1,000 registers in size, but the number of variables defined in each bank is
restricted to fewer than 100 so they can all be read/written by a single Modbus read/write request.
5.1.1 Class Process Value
This class of variable represents the current value of the process that is being measured by the DataStick.
For example, a pH sensor installed at a wastewater treatment plant might measure the pH of the treated
water leaving the plant. The Sensor Value may be monitored continuously by the plant operator and
may be recorded to demonstrate compliance with applicable EPA regulations.
This class of variable originates in the DataStick and exists in two types; float and integer. The former
is for Modbus masters that support floating-point math, and the latter is for Modbus masters that do not.
5.1.2 Class Scale Factor
Variables of this class are of type integer.
5.1.2.1 Integer representation of Floating Point numbers
Not all PLC’s support floating-point math. For this reason all values of type float are also available as
type integer.
Integer data types include a corresponding Scale Factor that defines the relationship between the integer
type of the value and the float type.
The Modbus Adapter calculates the integer type of the value from the float type by multiplying the float
type by the scale factor. For example, for a Sensor Type of pH, the Scale Factor for the Sensor Value is
100. If the float Sensor Value is 7.04, then the integer Sensor Value is 7.04 x 100 = 704.
Thermo Scientific AquaSensors DataStick Modbus Communications User Guide 11