

                              Erickson & Sederstrom
                           A Professional Corporation

                               Regency Westpointe
                     10330 Regency Parkway Drive, Suite 100
                           Omaha, Nebraska 68114-3761
                                November 20, 1998



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

Re:  Transaction Systems Architects, Inc. (the "Company")

Gentlemen:

     We refer to the registration,  on a Registration Statement on Form S-8 (the
"Registration  Statement")  under the  Securities  Act of 1933,  as amended,  of
$30,000,000 of deferred  compensation  obligations  (the  "Obligations")  of the
Company under the Company's  Deferred  Compensation  Plan (the "Plan").  We have
reviewed the Plan, the Deferred Compensation Plan Trust, the charter and by-laws
of the Company,  corporate proceedings of the Board of Directors relating to the
Plan, and such other  documents,  corporate  records and questions of laws as we
have deemed necessary to the rendering of the opinions expressed below.

     Based upon the foregoing, we are of the opinion that the Obligations,  when
issued in accordance  with the provisions of the Plan, will be valid and binding
obligations of the Company,  enforceable in accordance with their terms,  except
as may be  limited  by the  effect of  bankruptcy,  insolvency,  reorganization,
moratorium,  and similar  laws  affecting  the rights and  remedies of creditors
generally,  by the effect of general  principles of equity (whether in an action
at law or a  proceeding  in  equity),  or by the  effect of the laws of usury or
other laws or equitable  principles  relating to or limiting  the interest  rate
payable on indebtedness.

     We express on  opinion  as to the laws of any  jurisdiction  other than the
laws of the State of Nebraska.

     We hereby  consent  to the  filing  of this  opinion  as a  exhibit  to the
Registration Statement.

                                             Very truly yours,

                                             Samuel E. Clark

