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Custom Modbus Mapping

Normally, the VIM will access a contiguous region of Modbus registers and map them directly into a single DeltaV dataset. Some devices require that registers be only read or only written, and they are interleaved together in one range. This complicates the standard mapping into DeltaV; it would require a large number of datasets. Custom Modbus Mapping can be used to satisfy the register access requirements and conserve the number of DeltaV datasets.

Configurable Modbus device definitions are required for connection definitions to Modbus devices that require specific Modbus register access, such as the Omni device. The Omni device has individual registers in its IO range that cannot be written to and others that should not be read. To allow this datasets are configured that access multiple ranges of registers, rather than the normal Modbus access method of a single range in a dataset. This method may also be used where sparse access to Modbus registers is desired rather than required.

Required Versions

For VIM1, Custom Modbus Mapping is only supported in IOD-4101 v3.9.9 with VIMNet Explorer v9.1.3.8 or later. It is not supported in earlier or later versions of IOD-4101.

For VIM2, this feature is supported in IOD-4111 v4.4.1 or later with VIMNet Explorer v9.3 or later.

File Format

A comma delimited values file (CSV) is used to map Modbus registers into DeltaV dataset reigsters.

The format of the file is specified in Custom Modbus Mapping File Format.

File Storage

The CMM files are stored in the VimNet ProgramData directory (Default: C:\ProgramData\Mynah Technologies\VimNet Explorer) and imported on opening VIMNet Explorer. Earlier version of VIMNet Explorer used the Program Files directory.

The VIMNet Properties dialog, accessible from the root IO tree item context menu, can be used to import files after VIMNet Explorer is running. This dialog shows the currently available configurations in the Configurable Modbus Devices drop-down list. The Read button allows the list to be updated after a configuration has been added to the directory.


Each Field Device can use only one CMM file at a time. The properties of a Field Device has a checkbox to enable custom mapping and a drop down list of all the available mapping files. Once a mapping is assigned to a field device, VIMNet Explorer will calculate the DeltaV dataset configuration. This mapping can be displayed by selecting the dataset in the tree. The right pane will display all the registers defined in the CMM and their location in the DeltaV datasets. The example file creates a mapping as shown in Figure 1.

Note: A maximum of 8 custom mapping devices can be configured per VIM.

If the configuration file changes after first assigning the mapping to the field device, the Refresh CMM Device option on the Field Device context menu can be used to update the configuration.

Figure 1 - Example File DeltaV Dataset Mapping