Enron Floating Point Support; Configuring The Floating Point Data Transfer - ProSoft Technology CompactLogix MVI69E-MBS User Manual

MVI69E-MBS ♦ CompactLogix™ Platform
Modbus Serial Enhanced Communication Module

8.3.1 Enron Floating Point Support

Many manufacturers have implemented special support in their drivers for what is
commonly called the Enron version of the Modbus protocol. In this implementation,
addresses greater than 7000 are presumed to contain floating-point values. The
significance to this is that the count descriptor for a data transfer now denotes the
number of floating-point values to transfer, instead of the number of words.

8.3.2 Configuring the Floating Point Data Transfer

A common question is how to handle floating-point data when using the module as a
Modbus master. This really depends on the slave device and how it addresses this
application.
Just because your application is reading or writing floating-point data, does not mean
that you must configure the Float Flag, Float Start, and Float Offset parameters within
the module.
These parameters are only used to support what is typically referred to as Enron or
Daniel Modbus, where one register address must have 32 bits, or one floating point
value. Below is an example:
Example #1
Modbus Address
47101
47102
47103
47104
With the module configured as a master, you only need to enable these parameters to
support a write to this type of addressing (Modbus FC 6 or 16).
If the slave device uses addressing as shown in Example #2, then you do not need to do
anything with the Float Flag or Float Start parameters, as this addressing scheme uses
two Modbus addresses to represent each floating-point value:
Example #2
Modbus Address
47101
47103
47105
47107
Because each 32 bit REAL value is represented by two Modbus addresses (example
47101 and 47102 represent TEMP Pump #1), then you do not need to set the Float
Flag, or Float Start for the module for Modbus FC 6 or 16 commands being written to the
slave.
ProSoft Technology, Inc.
Data Type
32 bit REAL
32 bit REAL
32 bit REAL
32 bit REAL
Data Type
32 bit REAL
32 bit REAL
32 bit REAL
32 bit REAL
Parameter
TEMP Pump #1
Pressure Pump #1
TEMP Pump #2
Pressure Pump #2
Parameter
TEMP Pump #1
Pressure Pump #1
TEMP Pump #2
Pressure Pump #2
Reference
User Manual
Page 127 of 159