When attempting to add a State Diagram (.dia) file to a Project Navigator project, I find that the Add Source Files dialog box gives the following error:
"Only one source of this type is allowed for the same top module. This will not be added because of the conflict with the pre-existing design unit: Module|<existing state diagram name>"
In ISE 7.1i and earlier, .dia files were treated as user documents that could be opened from Project Navigator. In order to incorporate the State Diagram into the design, a user was required to generate an HDL file and then add the HDL file as the design source.
In ISE 8.1i, work was done to make a single .dia integrate as an abstract source type. When a .dia file is added in Project Navigator, it is treated as a source file (not a user document) and gets added into the hierarchy. As long as there is only one .dia file in the design, Project Navigator will pick up the corresponding HDL file for the State Diagram and pass it on to synthesis with the rest of the design. However, if there are multiple State diagrams, Project Navigator cannot yet handle this situation. The extra .dia files should be removed from the project and replaced with the generated HDL files.
This error message did not get removed from ISE 10.1 design tools. However, upon selecting OK, you will find that the State Diagram file is still added to the project and can be compiled and implemented successfully within a design containing other .dia files.