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Modbus
Modbus is an industrial protocol, used to read data from multiple (industrial) sensors on a single RS485
network. Modbus enabled sensors typically measure non-navigation-related physical quantities like
temperature, pressure, level or flow.
When Modbus is enabled, the MiniPlex-3 becomes a Modbus master, utilizing the Modbus-RTU protocol
via port NMEA Out2. Received sensor data is converted into NMEA 0183 $YXXDR sentences. The
MiniPlex-3 will group as many sensor values as possible into one XDR sentence as the maximum
sentence length or 82 characters permits. The exact number of sensor values depends on the chosen
precision and length of Transducer IDs entered for each slave entry.
Up to 30 slave definitions can be entered in the table on the Modbus tab. Each entry defines one slave
address and one register number, along with other parameters regarding the register and data type and
the required parameters for the XDR sentence.
Figure 63
Modbus Table Columns
The Modbus table contains the following columns:
Index: The leftmost column is the entry number. Clicking on a number enables the Delete
button to delete the entry.
Slv: The Modbus slave address of the sensor to be read. Valid addresses are 1 to 247.
Reg: The Modbus register number of the sensor to be read. The MiniPlex-3 uses 0-based
protocol addresses, ranging from 0 to 65535.
T: Modbus Register type:
H: Holding register (function code 03)
I: Input register (function code 04)
D: Data type:
I: Signed integer (16-bit/1 register)
U: Unsigned integer (16-bit/1 register)
L: Signed long integer (32-bit/2 registers)
F: Floating point (32-bit/2 registers)
R: Reverse register order in case of a multi-register read operation (Data types “L” and
“F”).
P: Precision or number of decimals for value in Modbus register (0 to 7).
T: Transducer Type character in the generated XDR sentence.
Transducer ID: Transducer ID in the generated XDR sentence, maximum 15 characters.
U: Units character in the generated XDR sentence.
Status: Modbus slave status.
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