What facilities does EAGLE CAD have for helping to do layout with matched length groups and differential pairs? Can you apply such a constraint in the auto-router? As a follow-on to this, what (other) free Electrical CAD tools support this type of feature?
Edit If you don't believe any free CAD packages support this design feature, what are some low-cost options that do support it?
Answer
Eagle actually has both of these features as of version 6. From their website:
Benefit from differential pair routing and automatic meanders
Special signals can be routed with exactly the same length. Differential pair routing is used for pairs of differential (equal and opposite) signals across the board to create a balanced transmission system. Creating meanders helps users automatically give a signal a certain length in the PCB. Meanders in traces are used to increase delay times in high speed digital circuits.
http://www.cadsoftusa.com/eagle-pcb-design-software/new-in-v6/?language=en (halfway down)
See for yourself.
The Meander button:
Length matching of a differential pair (from manual):
I don't actually know how to implement these techniques, but I can point you to the documentation. Check the manual (warning 300+ page PDF) on page 175 to 178 - http://www.cadsoft.de/wp-content/uploads/2011/05/V6_manual_en.pdf
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