EXHIBIT 5.1


                                                March 11, 2005

Transaction Systems Architects, Inc.
224 South 108th Avenue
Omaha, Nebraska 68154

                     Re: Registration Statement on Form S-8

Ladies and Gentlemen:

         We are acting as counsel to Transaction Systems Architects, Inc., a
Delaware corporation (the "Company"), in connection with the registration of
3,000,000 shares (the "Shares") of the Company's Common Stock, par value $0.005
per share, pursuant to a Registration Statement on Form S-8 to be filed by the
Company on or about March 11, 2005 (the "Registration Statement").

         We have examined such documents, records, and matters of law as we have
deemed necessary for purposes of this opinion. Based on such examination and on
the assumptions set forth below, we are of the opinion that the Shares that may
be issued and sold pursuant to the Company's 2005 Equity and Performance
Incentive Plan (the "Plan") and authorized forms of stock option agreements
thereunder will be, when issued and sold in accordance with the Plan and such
agreements, duly authorized, validly issued, fully paid and nonassessable.

         In rendering the foregoing opinion, we have (i) relied as to certain
factual matters upon certificates of officers of the Company and public
officials, and we have not independently checked or verified the accuracy or
completeness of the statements contained therein, (ii) assumed that the
signatures on all documents examined by us are genuine and that where any such
signature purports to have been made in a corporate, governmental, fiduciary, or
other capacity, the person who affixed such signature to such documents had
authority to do so, and (iii) assumed the authenticity of all documents
submitted to us as originals and the conformity to original or certified copies
of all copies submitted to us as certified or reproduction copies. In addition,
our examination of matters of law has been limited to the General Corporation
Law of the State of Delaware and the federal laws of the United States of
America, in each case as in effect on the date hereof.

         We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement.

                                                Very truly yours,


                                                /s/ Jones Day