The IMPEDANCE_RULE property, attached to a net, specifies an impedance restriction between any two pins on a net or between any pin and Tpoint connection on a net. This property is used by DRC checking and routing.
If a connection has an impedance constraint specified by both an IMPEDANCE_RULE property and an electrical constraint set, the IMPEDANCE_RULE value is used. Use Constraint Manager for pin-pairs.
The format of the IMPEDANCE_RULE property in the $A_PROPERTIES section of a netlist is
Specifies the checking of impedance of everyconnect line (cline) segment in the net. | ||
Required. Specifies the impedance constraint value in ohmsexpressed as a decimal number. | ||
Specifies the tolerance for an impedance value specified either as avalue in ohms or a percentage. You must follow a percentage with a % (percent sign). If you do not specify a value, 2% is assumed. | ||
Specifies the name of the net to which the impedance applies. | ||
原文：http://www.cnblogs.com/temo/p/7727098.html