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Modbus Interface
12.5.3
Modbus register ranges
The Model 3711 Modbus interface supports Function Codes 1, 2, 3, 4, 5, 6, 15, and 16. These
functions must reference registers that have been mapped in the Modbus Registers window. The
function may reference registers that are within a mapped range, or that include the Starting Register
and/or the Ending Register.
Registers can be mapped independently for each function. Each register references two or more
points, identified by TLP notation. Up to 15 registers can be mapped for each function. Mapping for
function 3 is split into two lines to allow mapping of up to 30 registers.
Two types of mapping are provided:
•
Point indexing – The Starting Register is mapped to the specified point. Subsequent registers,
through the Ending Register, are mapped to the same point type and parameter and increment
the point logical number.
•
Parameter indexing– The Starting Register is mapped to the specified point. Subsequent
registers, through the Ending Register, are mapped to the same point type and point logical
number, and increment the parameter number.
Example
When mapping registers, note the following:
•
Be sure that the Modbus register range is large enough to accommodate the referenced data,
taking datatype size into consideration.
•
For functions 1, 2, 5, and 15, the mapped point should be a single-byte parameter, and
preferably a status parameter (only bit 0 is used), because these functions pack the data into a
binary format for transmission. Each address span must be unique within the function for
proper operation. If the address spans are not unique, the first valid address is used.
•
For functions 3, 4, and 16, you can create a continuous data table by making the registers
continuous (i.e., the Starting Register of one line is one greater than the Ending Register of the
previous line). This allows access to all referenced data with one request.
144
Modbus starting register: 1000
Modbus ending register: 1002
Device parameters: AIN A1 SCANPR (3 1, 2)
Mapped registers, point indexing:
•
AIN A1 SCANPR (3 1, 2)
•
AIN A2 SCAN PR (3 2, 2)
•
AIN A3 SCAN PR (3 3, 2)
Mapped registers, parameter indexing
•
AIN A1 SCANPR (3 1, 2)
•
AIN A1 FILTER (3 1, 3)
•
AIN A1 MINRAW (3 1, 4)
®
Micro Motion
Model 3711 Gas Flow Computers
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