IBM Modbus Plus - Manual (Page 14)

Modbus Plus X-Link Driver User’s Guide
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4.1 Data Types and Lengths
The Modbus Plus Driver supports three X-Link data formats; Bits and
Integers, as well as Floats.
The Modbus Plus protocol does not support floating point. The driver will
truncate any floating point numbers before transmission, and expand the
integer numbers to floating point after retrieval when floating points are
requested.
The type of Modbus Plus command that the driver generates depends on the
address type specified.
The length of the data portion in a command is constrained by the Modbus
Plus protocol. Mappings that are configured with the X-Link reading or
writing the data on the Modbus Plus network should be restricted to a
maximum size of:
Data Type X-Link Access Maximum Size (bytes)
COIL Read 250
COIL Write 100
Registers Read 250
Registers Write 200
Global Data Read/Write 64