The layout file describes a complete layout including symbols, vias,
elements and layers with lines, rectangles and text. This is the most
complex file of all. As Pcb
has evolved, the file format has
changed several times to accommodate new features. Pcb
has
always been able to read all older versions of the .pcb
file.
This allows the migration of older designs to newer versions of the
program. Obviously older versions of Pcb
will not be able
to properly read layout files stored in newer versions of the file
format.
In practice it is very common for footprint libraries to contain
elements which have been defined in various versions of the Pcb
file format. When faced with trying to understand an element file or
layout file which includes syntax not defined here, the best approach
is to examine the file src/parse_y.y which is the definitive
definition of the file format.
The PCB layout file contains the following contents, in this order (individual items are defined in File Syntax:
PCB
Grid
Cursor
Flags
Groups
Styles
Symbols
Vias, Rats, Layers, and Elements
Netlists