The RATSNEST command assesses all of the airwire connections in order to achieve the shortest possible paths, for instance, after components have been moved. After reading a netlist via the SCRIPT command, it is also useful to use the RATSNEST command to optimize the length of airwires.
The RATSNEST command also calculates all Polygons belonging to a
signal. This is necessary in order to avoid the calculation of
airwires for pads already connected with polygons. All of the calculated
areas are then being displayed in the "real mode". Normally, this
decreases the screen redraw speed. You can switch back to the faster
"outline mode" with the RIPUP command.
The automatic calculation of the polygons can be turned off with SET POLYGON_RATSNEST OFF;.
RATSNEST ignores airwires representing signals which have their own layer in a multilayer board (e.g. layer $GND for signal GND), apart from signals connecting smd pads to a supply layer with a via-hole.
Note that RATSNEST doesn't mark the board drawing as modified, since the calculated polygon data (if any) is not stored in the board, and the recalculated airwires don't really constitute a modification of the drawing.
Zero length airwires
If two or more wires of the same signal on different routing layers end at the same point without being connected through a pad or a via, a zero length airwire is generated, which will be displayed as an X-shaped cross in the Unrouted layer. The same applies to smds that belong to the same signal and are placed on opposite sides of the board.
Such zero length airwires can be picked up with the ROUTE command just like ordinary airwires. They may also be handled by placing a VIA at that point.
Making sure everything has been routed
If there is nothing left to be routed, the RATSNEST command will respond with the message
Ratsnest: Nothing to do!Otherwise, if there are still airwires that have not been routed, the message
Ratsnest: xx airwires.will be displayed, where xx gives the number of unrouted airwires.
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