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MicroStation tags and AutoCAD attributes are similar. However there are differences to consider when you are working in a DGN file that will be converted to DWG, or when you are working directly in a DWG file.
In MicroStation, tags are defined as "non-graphical attributes that may be attached to elements."
In AutoCAD, an attribute is defined as "a text string associated with an attribute that identifies a particular attribute."
In simpler terms, both are usually text-type elements used to relate specific information about a particular element such as a valve type, or about an area of a drawing such as a room number.
In MicroStation, the first step in defining a tag is to create a tag set-a group of associated tags. A tag set defines the tag name, value, data type, and display options. Tag sets do not exist in AutoCAD, so when a DGN file containing tags is saved to DWG, the tag sets are dropped. Conversely, when a DWG file containing attributes is opened in MicroStation or saved to DGN, a tag set is created for each attribute in each shared cell definition.
To create a tag set in MicroStation, go to the Element pull-down menu and select Tags > Define. The Tag Set dialog box will open. Click the Add button under the Sets window in this dialog box. The Tag Set Name dialog box will open. Enter the name of the tag set to be added, and click OK. The new tag set name will now appear in the Sets window.
To create a tag, select the tag set it will be associated with in the Sets window of the Tag Sets dialog box, then click the Add button below the Tags window. The Define Tag dialog box will open. In the Tag Name window, enter the name of the tag.
The Prompt window is optional. Its default is Value, but can be customized to show the prompt for the tag value that will appear in the Attach Tags dialog box when the tag is about to be attached to an element.
MicroStation allows you to use character, integer, or real (real numbers) tag type. Character type is text data with a maximum limit of 1024 characters. Integer type is a four-byte numerical value. Real type is an eight-byte floating point value. AutoCAD supports only character-type tags. Real- or integer-type tags in a DGN file will automatically convert to character-type tags when the file is saved to DWG.
If the Variable toggle is on, the tag value is considered variable. If it is on, the Attach Tags dialog box will open when you use the Attach Tags tool. The tag value can then be entered in this dialog box. If this toggle is off, the default value for the tag is used when the tag is attached.
If the Default toggle is on, the tag attaches automatically with the default value. If the Default value is not set, the value for character type tags will be NULL and 0 for the numeric types.
If the Confirm toggle is on, a dialog box will open when Attach Tags tool is used. The tag value can then be entered into this dialog box.
If a default tag value is to be used, it should be entered in the Default Tag Value window.
The Display Tag toggle is on by default. This will display the tag when it is placed. If it is off, the tag will not display. Turn the display off when associating the tag with non-visual elements or for fast display.
Once the tag set and tags are defined, the tag can now be placed in the file. To place tags, use the Attach Tags tool or the ATTACH TAGS key-in. When the tool or key-in is used, the Attach Tags dialog box will open. From this dialog box, select the tag set containing the tag(s) to be placed.
If the Association toggle in this dialog box is on, the tag must be attached to an element in the file. If it is off, the tag will be placed without an association to any element in the file. When placed, the tag name will be displayed, not the tag value. This method is used most when creating cells with tags.
The cell is created, and the tag is placed without association. When the cell is placed in a drawing file, the tag value is displayed instead of the tag name.
This is similar to the way attributes work in AutoCAD, where attributes must be part of a block (cell). The block is created, and the attributes are placed in the block. Just as placing a non-associative tag in a cell, the tag name is displayed instead of the value. It is only when the block is placed in another file that the tag value is displayed.
Because an AutoCAD block, like a MicroStation cell, is nothing more than a drawing file, a common mistake made with tags in DWG files is to attach the file intended to be a block as a reference file. When this happens, the tag name, not the value, is displayed. These files, or blocks, must be placed as a block in order to see the tag value.