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@kannan It's like the message says. DXF can only have a single top cell but your layout seems to have multiple of them. Delete all top cells you don't need and save to DXF for click and the cell and use "Save Selected Cell As" from the context menu.
This seems weird, is not the layout cell being
viewed always "a" cell, singular? Or is it something
like "view as new top cell" does not in fact make
a cell a "true" topcell, for this purpose? That only
the true GDS topcell can be translated?
Perhaps you would "select all" of whatever you
want to export to DXF format, and "make cell"
and save that cell out as a GDS file, and work
from that database which you know is a single
Ok, maybe I need to be more specific.
In a layout, you can have many cells. Most cells will be placed inside other cells. There are cells however which are not placed in other cells. These are "top" cells. The form the root of the cell tree.
A layout in GDS can have multiple top cells. Each top cell may form another design. For example, standard cell libraries may be made up of several top cells - one representing an inverter, the next one an AND gate and so forth.
But some formats such as DXF do not support this mode. They will only accept layouts which have a single top cell.
KLayout has a "current cell" - this is the cell you're looking at. This may be a top cell but it may also be a child cell. Either way, this is just a view, not the nature of the layout. The layout is made from all cells you see in the cell tree.
You can save only single cells to files using "Save selected cell as ..." from the cell tree's context menu. This will create a layout file with only this particular cell and the child cells used therein. In this case, the layout written will have a single top cell and hence you can write single cells (plus child cells) to DXF this way, even if the layout has multiple top cells.