3.0 MODBUS Protocol Implementation
The Serial MG supports 2 transmission modes: RTU and
ASCII.
MODBUS Interfaces will support the following commands:
(FC refers to MODBUS Function Codes).
FC = 1: Read Coils
FC = 5: Write Single Coil
FC = 15: Write Multiple Coils
FC = 3: Read Holding Registers
FC = 6: Write Single Register
FC = 16: Write Multiple Registers
All other commands, exceptions, or other interface features
are not supported.
Note:
Raw addressing starts at 0. For FC-specific addressing,
the address starts at 1 for coils and 40001 for holding registers.
Denoted in the literature as 0 / 1 or 1 / 40001.
3.1
Register Size
MODBUS permits the use of a variety of register size
specifications. This device uses the standard 16-bit
unsigned integer for its register size. Additionally, register-
data representation can take the form of multi-register data
types (i.e. 32-bit integer). These will be specified
specifically for all registers that require additional
formatting.
3.2
Multi-byte Formatting
MODBUS has specified the use of ‘Big-Endian’
representation for addresses and data items. This means
that when a numerical quantity larger than a single byte is
transmitted, the most significant byte is sent first.
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