This Client Server article is republished in its entirety from 2003 for reference purposes.
By Reginald Wallace, Technical Support Analyst, Bentley Atlanta, Georgia 03 February 2003
Editor's note: The following article was taken from MicroStation's Help file, which contains more information about sharing DGN and DWG data.
A reference is a model that is attached to, and displayed with, an active model for printing or construction purposes. In MicroStation V8, a reference is analogous to an AutoCAD external reference (XREF) or an AutoCAD overlay. This removes the need to save or translate a DWG in DGN format in order to attach it as a reference.
When you are working with a DGN file that will be saved to a DWG file:
The DWG file format does not support all of the reference types available in MicroStation. As a result, you must use the Save As DWG/DXF Options dialog box>References tab to define how the following types of references are merged into the model:
Figure 1: The Save As DWG/DXF Options dialog box, References tab
When a MicroStation design file that is being used as a reference also has its own attachments, these become nested references. The links between these files can be maintained through many levels (depths) of nesting, so that if you open only one file, you can view the contents of many files. The individual references can be updated, and the changes will be shown in the master (or parent) file. When you attach a parent reference to your model and turn on live nesting, you can also control how many levels of nested references are attached to the model.
In AutoCAD, there are two types of references that can be made to create file associations: attachments and overlays. Both allow the display of all objects in the attached file, as well as those in the nested references. Keep in mind that although AutoCAD allows live nested references, it does not provide control of nesting depth, and thus all data in the nested attachments is implicitly displayed.
Using Live Nesting in a DGN File That Will Be Saved to DWG Format
If your DGN file will be saved to a DWG file, make sure that your reference settings are compatible with AutoCAD's reference settings. To do this, turn on live nesting for the parent reference, and set the nest depth to a high number so that all nested references are included.
To attach a reference with live nesting:
Figure 2: Attach Reference dialog box
Figure 3: Attach Reference Settings dialog box
Figure 4: Attachment Settings dialog box In the Primary toolbox, click the References icon. The Reference dialog box opens. In the dialog box's tool bar, select the Attach Reference tool. The Attach Reference dialog box opens. Select the DGN file that contains the model to attach as a reference, and click OK.The Attach References Settings dialog box opens. From the Nested Attachments options menu, choose Live Nesting.The Nested Attachments options are only available when the selected model has reference attachments. When the Live Nesting option is on, the hierarchical structure of any nested references are maintained when attaching the parent reference to a model.
In MicroStation, a nested attachment is displayed only if the child reference does not have its Ignore Attachment When Live Nesting setting turned on (in the Attachment Settings dialog box), and the parent reference has Live Nesting enabled. This is an important fact and is sometimes overlooked by users.
In the Nest Depth field, enter a high value (for example, 10).Set the depth setting to a high number to include even the most deeply nested references when the file is saved to DWG. Although the nested attachments will display in the view window, you will see only the parent reference in the References dialog box's list box. This setting can be selected when attaching the reference file.
An AutoCAD overlay is a type of reference. Unlike an AutoCAD attached reference, if it becomes a nested reference, it is not displayed, nor are any of the files attached to it. Thus, an overlay might be used when designers working on a portion of the project need to see information that will not necessarily be displayed at the parent file level. To create a MicroStation reference attachment that behaves like an AutoCAD overlay, turn on the Ignore Attachment When Live Nesting setting in the Attachment Settings dialog box.
Figure 5: Save As DWG/DXF Options dialog box, General tab When you save a DGN file to a DWG file, you can save reference attachments to DWG files as overlays. To do this, open the Save As DWG/DXF Options dialog box and turn on Create Overlays for Reference Attachments (No Live Nesting). When this setting is on, all references are created as overlays, whether or not the Ignore Attachment When Live Nesting setting is on. This setting effectively disables live nesting in the DWG output file. It is useful for files where live nesting is turned off for all attachments.
A model in a DWG file can be attached as a reference to a design or sheet model in the open file (DGN or DWG). To attach the model in a DWG file as a reference, follow the procedure for attaching references.
You can save your DWG attachments in MicroStation V8 down to the v7 format. The reference attachments are automatically saved over to DGN. In MicroStation V8, it is not necessary to convert the attachments. You will either have to open each reference and do a Save As v7, or batch convert them all at once.
For additional information on Reference file functionality in MicroStation V8, check out the Technotes/FAQ's at http://communities.bentley.com/Wiki/view.aspx/Reference_File_Nesting_With_DGN_And_DWG_Files_TechNote