The decision also reversed a trial court ruling that Infineon hadn't infringed Rambus patents, sending the patent infringement issue back to the lower court.

"A cloud has been lifted from over our heads," said Steve Tobak, a Rambus senior vice president. "We followed the rules."

Those rules were established by the standards body Jedec during deliberations on a synchronous DRAM standard. In 2001, a U.S. district court in Richmond, Va., found that Rambus had committed fraud by attending Jedec meetings while it was filing patents for SDRAM technology. Rambus then challenged memory makers for violating its patents.

In reversing the lower court, the appeals panel also vacated a ruling known as a "judgment as a matter" law that defined how patent claims are formulated. Hence, the case was sent back to the district court to hear Rambus' claim of patent infringement by Infineon.

The three-judge appeals court, which deliberated for over six months, "clarified some complex issues," Tobak said. "This is a ruling that has legs."

A key issue in the case was Rambus' duty to disclose its patent applications to Jedec members. Legal analysts said the appeals court found that Rambus' duty, as a participant in the Jedec standards process, "arises only when work formally begins on the proposed standard," said David Balto, a Washington IP attorney and former Federal Trade Commission (FTC) official. The duty to disclose did not cover a Jedec participant's future plans, he added.

"Having framed the duty of disclosure in the above terms, the court concluded that Rambus did not breach its duty as to the SDRAM standard because none of the claims in its patents and pending patent applications reads on that standard," Balto added.

Rambus still faces antitrust charges before the FTC, which filed a complaint against the Los Altos, Calif., company last June. Company officials predicted the appeals court ruling would have a "positive impact" on that case, in which Rambus is accused of deceiving Jedec and harming competition.

A trial before an FTC administrative law judge is scheduled for March. Balto said the key issues in the anti-trust case are competitive harm and the need for standards groups like Jedec to clarify their rules on disclosure.

Rambus is also pursuing patent infringement cases against Micron Technology Inc. of Boise, Idaho, and Hynix Semiconductor Inc. of South Korea.

Rambus' stock soared on news of the favorable ruling and was up over 50 percent by the end of trading on Wednesday.