KROHNE Summit 8800 Vol 3 - Manual (Page 136)

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SUMMIT 8800 COMMUNICATION 08
8 .4 .3 Parameters
Modbus is not a well defined standard. It started as a very simple protocol, but has been modi-
fied to fit different applications. Such changes have never been included in the standard, result-
ing in a wide variety of conflicting versions. Therefore, to cover all applications, many param-
eters have been included in the Summit.
Figure 185 Modbus slave registers
Each line represents one register with the following parameters:
Address Modbus address, any number between 0 to 65535 is valid. There is no standard for
addressing registers. Most often blocks of the same type are used, e.g. 10001-19999 for
read only bits, but check the capabilities or needs of the master device before defining
your addresses.
Variable Name of the variable from the ID tree
Type Defines the content of the variable used: available in different types: e.g. character,
short integer, long integer (32 bits integers), floating point (32 bit single precision IEEE
floating point), double (64 bit: double precision IEEE floating point), time and short string
(8 characters).
The original standard only defines bits (coils) and integers (inputs). Presently long and
floats are very common, while the flow computer stores most values as doubles to have
sufficient accuracy. Therefore, not all types may be supported by the master, e.g. older
PLC’s often do not support doubles. So change the type to fit the Modbus master. The
SUMMIT 8800 will automatically scale it to the type selected.
Highest accuracy of numbers is achieved using double precision IEEE numbers to 64
bit resolution. All numbers can be cast as types other than their default type, this will
be indicated by brackets for the type, e.g., (float) - Indicating the number is not using its
default type setting.
NOTE:
Highest accuracy of numbers is achieved using double precision IEEE numbers to 64 bit
resolution.
Bytes ID size in bytes. Information purposes only and cannot be changed.
Order Byte order. Default is ascending 12345678. Different computer architectures use
their memory in different ways to store data. Example, when a transmitting computer
takes a 32-bit computer word composed of four 8 bit bytes and places it on a serial
communication line, the receiving computer needs to know how the bytes need to be
stored: does it start with the least significant byte first (Little-endian), or the most
significant byte first (Big-endian). Motorola and Intel microprocessors are generally
incompatible (cannot share the same memory) because of this byte order problem. In
most cases, it is impossible to predict how a given system will handle this problem.
For this reason the SUMMIT 8800 made this configuration possible, with the same
processor it will be able to display the same order, e.g., 1234, or any other combinations
needed, e.g., an order of 2143.
The byte order will normally be the same for all registers.