1
 
                                                                     EXHIBIT 5.1
 
                                                                  April 22, 1996
 
National Data Corporation
National Data Plaza
Atlanta, Georgia 30329-2010
 
         Re:  Registration Statement on Form S-4 Covering a
              Maximum of 3,210,532 Shares of Common Stock
 
Ladies and Gentlemen:
 
     This opinion is being rendered in connection with the proposed merger of
C.I.S. Technologies, Inc. ("CIS") with a subsidiary of National Data Corporation
(the "Company"), in which the Company will issue up to 3,210,532 shares of its
$.125 par value per share common stock (the "Shares"), upon the terms and
conditions set forth in its Registration Statement on Form S-4 (the
"Registration Statement"), as filed by the Company with the Securities and
Exchange Commission under the Securities Act of 1993, as amended, on April 22,
1996.
 
     As counsel for the Company, we have examined such corporate records and
documents as we have deemed relevant and necessary as the basis for this
opinion, and we are familiar with the actions taken by the Company in connection
with the authorization, registration, issuance, and sale of the Shares.
 
     Based upon the foregoing, it is our opinion that the Shares will, upon
their issuance in accordance with the terms and conditions set forth in the
Agreement and Plan of Merger by and among CIS, NDC Merger Corp. and the Company,
dated as of April 15, 1996, be duly authorized and validly issued, fully paid
and non-assessable under the Delaware General Corporation Law as in effect on
this date.
 
     We hereby consent to the use of this opinion as an exhibit to the
Registration Statement and the reference to our firm in the section entitled
"Legal Matters."
 
                                          Very truly yours,
 
                                          ALSTON & BIRD
 
                                          By: /s/  Joel J. Hughey
                                             -----------------------