<PAGE>   1


                                                                    EXHIBIT 2.1


                                  COMMON STOCK
                         SECURITIES PURCHASE AGREEMENT

         This agreement (this "Agreement") is made effective as of July l,
1998, between Advent Global GECC III Limited Partnership, a Delaware limited
partnership (the "Purchaser"), and the Seller set forth on the signature page
attached hereto (the "Seller").

         WHEREAS, the Purchaser desires to purchase and the Seller desires to
sell that number of shares of Common Stock (the "Common Stock"), par value $.01
per share, of PaySys International, Inc., a Florida corporation (the
"Company"), listed next to the Seller on Schedule A (the "Shares"), in exchange
for cash, on the terms and subject to the conditions stated herein; and

         WHEREAS, the Purchaser and the Company are parties to that certain
Securities Purchase Agreement made effective as of June 17, 1998 (the "Series
A-l Agreement"), wherein the Purchaser is acquiring Series A-l Convertible
Participating Preferred Stock par value $.01 per share, of the Company, in
exchange for the Shares.

         NOW, THEREFORE, in consideration of the mutual promises and covenants
set forth herein, the parties hereto agree as follows:


                                   SECTION l

                    AUTHORIZATION AND SALE OF THE SECURITIES

         1. 1 Sale of the Securities. Subject to the terms and conditions
hereof, at the Closing (as defined below), the Seller shall sell to the
Purchaser, and the Purchaser shall purchase from the Seller that number of
Shares specified opposite the Seller's name on the Schedule A in exchange for
cash, as set forth, as to the Seller, as set forth on Schedule A.


                                   SECTION 2

                             CLOSING DATE: DELIVERY

         2.1 Closing. The closing of the purchase and sale of the Shares (the
"Closing") shall take place at the location and time set forth below on July 1,
1998, or at such other place and time upon which the Seller and the Purchaser
shall agree. The date of the Closing is referred to as the "Closing Date".

         2.2 Location of Closing. The Closing shall take place at the offices
of Finn Dixon & Herling LLP at One Landmark Square, Stamford, Connecticut, at
10:00 a.m. on the Closing Date.

         2.3 Delivery. At the Closing, the Seller shall deliver to the
Purchaser the appropriate 



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certificate(s) representing the Shares purchased by the Purchaser at the
Closing, duly executed with appropriate stock powers, signatures guaranteed,
which shall be delivered against payment of the consideration therefor in the
amount specified an Schedule A, by certified check or wire transfer of same day
funds (or any combination thereof).


                                   SECTION 3

                  REPRESENTATIONS AND WARRANTIES OF THE SELLER

         The Seller hereby represents and warrants to the Purchaser with
respect to the sale of the Shares as follows:

         3.1 Title to Shares. The Seller is the record and beneficial owner of
the Shares set forth opposite the Seller's name on Schedule A, free and clear
of any Security Interest, and has full power and authority to convey such
Shares, free and clear of any Security Interest, and, upon delivery of and
payment for such Shares as herein provided, the Purchaser will acquire good,
marketable and valid title thereto, free and clear of any Security Interest.

         3.2 Authority Relative to this Agreement. The Seller has the full
legal right and power and all authority and approval required to enter into,
execute and deliver this Agreement and to perform fully the Seller's
obligations hereunder and thereunder. This Agreement has been duly executed and
delivered by the Seller and constitutes the legal, valid and binding obligation
of the Seller enforceable against the Seller in accordance with its terms.

         3.3 Absence of Conflicts. The execution and delivery of this Agreement
and the consummation of the transactions contemplated hereby and the
performance by the Seller of this Agreement in accordance with its terms and
conditions will not: (i) require any notice to, filing or registration with, or
permit, license, variance, waiver, exemption, franchise, order, consent,
authorization or approval of, any other person; (ii) violate, conflict with or
result in a breach of any provision of or constitute a default (or an event
which, with notice or lapse of time or both, would constitute a breach or
default) under, or result in the termination of, or accelerate the performance
required by, or result in the creation of any Security Interest on the Shares
held by the Seller or upon the assets, properties or businesses of the Seller
under any of the terms, conditions or provisions of any contract or other
agreement (written or oral) to which the Seller is a party or by or to which
the Seller or the Shares held by the Seller are bound or subject; or (iii)
violate any Law or any judgment ruling, order, writ, injunction or award of any
Governmental Authority which is applicable to the Seller or to the Shares held
by the Seller.

         3.4 Brokers or Finders. Neither the Purchaser nor the Seller has
incurred or will incur, directly or indirectly, as a result of any action take
by the Seller, any liability for brokerage or finders' fees or agents'
commissions or any similar charges in connection with this Agreement or any
transaction contemplated hereby.



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                                   SECTION 4

                REPRESENTATIONS AND WARRANTIES OF THE PURCHASER

         The Purchaser hereby represents and warrants to the Seller with
respect to the purchase of the Shares as follows:

         4. 1 Experience. The Purchaser has substantial experience in
evaluating and investing in private placement transactions of securities in
companies similar to the Company so that the Purchaser is capable of evaluating
the merits and risks of the Purchaser's investment in the Company and has the
capacity to protect the Purchaser's own interests. The Purchaser represents and
warrants to the Seller that it is aware that the purchase of Shares involves
substantial risk and that its financial condition and investments are such that
it is in a financial position to hold the Shares for an indefinite period of
time and to bear the economic risk of and withstand a complete loss of such
investment.

         4.2 Investment. The Purchaser is acquiring the Shares for investment
for the Purchaser's own account, not as a nominee or agent, and, except as
contemplated by the Series A-l Agreement, not with the view to, or for resale
in connection with, any distribution thereof. The Purchaser understands that
the Shares have not been, and will not be, registered under the Securities Act
of 1933, as amended (the "Securities Act"), or the securities laws of any state
by reason of exemptions from the registration provisions of the Securities Act
and such laws which depend upon, among other things, the bona fide nature of
the investment intent and the accuracy of the Purchaser's representations as
expressed herein.

         4.3 Rule 144. The Purchaser acknowledges that the Shares must be held
indefinitely unless subsequently registered under the Securities Act or an
exemption from such registration is available. The Purchaser is aware of the
provisions of Rule l44 promulgated under the Securities Act which permit the
limited resale of shares purchased in a private placement subject to the
satisfaction of certain conditions, including, among other things, (i) the
existence of a public market for the Shares, (ii) the availability of certain
current public information about the Company, (iii) the resale occurring not
less than one year after a party (who is not an "affiliate") has purchased and
fully paid for the shares to be sold, (iv) the sale being effected through a
"broker's transaction" or in transactions directly with a "market maker" (as
provided by Rule l44(f)) and (v) the number of shares being sold during any
three-month period not exceeding specified limitations.

         4.4 No Public Market. The Purchaser understands that no public market
now exists for the Shares and that there is no assurance that a public market
will ever exist for the Shares.

         4.5 Access to Data. The Purchaser has had an opportunity to discuss
the Company's business, management, and financial affairs with the Company's
management and the opportunity to review the Company's facilities and business
plan. The Purchaser has also had an opportunity to ask questions of officers of
the Company, which questions were answered to its satisfaction. The Purchaser
acknowledges that it has had an opportunity to conduct its own independent due
diligence investigation of the Company.



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         4.6 Authorization. This Agreement, when executed and delivered by the
Purchaser, will constitute the valid and legally binding obligation of the
Purchaser, enforceable in accordance with its respective terms, subject to (i)
laws of general application relating to bankruptcy, insolvency, and the relief
of debtors, and (ii) rules of law governing specific performance, injunctive
relief, or other equitable remedies. The Purchaser has full partnership, power
and authority to enter into and to perform its obligations under this Agreement
in accordance with its respective terms. The Purchaser represents that it has
not been organized, reorganized or recapitalized specifically for the purpose
of investing in the Company.

         4.7 Brokers or Finders. Neither the Purchaser nor the Seller has
incurred, or will incur, directly or indirectly, as a result of any action
taken by the Purchaser, any liability for brokerage or finders' fees or agents'
commissions or any similar charges in connection with this Agreement or any
transaction contemplated hereby.

         4.8 Accredited Investor. The Purchaser (i) has adequate means of
providing for his current needs, (ii) has no need for liquidity in his
investment in the Shares, and (iii) is able to bear the economic risk of losing
its entire investment in the Shares. The Purchaser has its principal office in
the state set forth in Section 7.5 hereto. The Purchaser is an accredited
investor, as such term is defined in Rule 501(a) of Regulation D promulgated
under the Securities Act.

                                   SECTION 5

                 CONDITIONS TO THE OBLIGATIONS OF THE PURCHASER

         Thc obligations of the Purchaser to purchase the Shares at the Closing
is, at the option of the Purchaser, subject to the fulfillment on or prior to
the Closing Date of the following conditions:

         5.1 Representations and Warranties. The representations and warranties
made by the Seller in Section 3 of this Agreement shall have been true and
correct when made, and shall be true and correct in all material respects as of
the Closing Date.

         5.2 Consents. The Seller shall have obtained, or shall obtain within
the time periods required by applicable law, all necessary blue sky law permits
and qualifications, or secured exemptions therefrom, required by any state for
the sale of the Shares at the Closing and shall have obtained all other
consents, permits and waivers necessary to consummate and the transactions
contemplated by this Agreement, and any consents required under any agreements,
licenses, leases or commitments of the Seller.

         5.3 Proceedings and Documents. All corporate and other proceedings in
connection with the transactions contemplated hereby and all documents and
instruments incident to such transactions shall be reasonably satisfactory in
substance and form to counsel for the Purchaser and they shall have received
all such counterpart originals or certified or other copies of such documents
as they may reasonably request.


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         5.4 Opinions of Counsel. The Purchaser shall have received from
counsel for the Seller (who shall by reasonably acceptable to the Purchaser) an
opinion, dated the Closing Date, satisfactory in form and substance to the
Purchaser and their special counsel, and which shall be substantially in the
form of Exhibit 1 attached hereto.

         5.5 No Litigation. No action, suit or other proceeding shall be
pending or threatened before any court, tribunal, or governmental authority
seeking or threatening to restrain or prohibit the consummation of the
transactions contemplated hereby, or seeking to obtain substantial damages in
respect thereof or which would otherwise materially and adversely affect the
Company, its subsidiaries, or their respective business, assets, prospects or
financial condition.

         5.6 Compliance with Laws. The offer and purchase and sale of the
Shares to the Purchaser shall be legally permitted by all laws and regulations
to which the Sellers or the Purchaser are subject.

         5.7 Simultaneous Transactions. The sale and purchase at all Shares
listed on Schedule A is conditioned upon the simultaneous (i) closing of the
purchase by Purchaser of additional shares of Common Stock from selling
stockholders in an amount that, when added to the Shares purchased hereunder,
shall total not less than 699,301 shares of Common Stock and (ii) Second
Closing (as such term is defined in the Series A-1 Agreement).

                                   SECTION 6

                 CONDITIONS TO THE OBLIGATIONS OF THE SELLER

         The Seller's obligation to sell the Shares at the Closing is, at the
option of the Seller, subject to the fulfillment on or prior to the Closing
Date of the following conditions:

         6.1 Representations and Warranties. The representations and warranties
made by the Purchaser in Section 4 of this Agreement shall have been true and
correct when made, and shall be true and correct in all material respects as of
the Closing Date.

         6.2 Blue Sky Law. The Seller shall have obtained all necessary blue
sky law permits and qualifications, or secured exemptions therefrom, required
by any state for the offer and sale of the Shares at the Closing.

         6.3 No Litigation. No action, suit or other proceeding shall be
pending or threatened before any court, tribunal or governmental authority
seeking or threatening to restrain or prohibit the consummation of the
transactions contemplated hereby, or seeking to obtain substantial damages in
respect thereof or which would otherwise materially and adversely affect the
Company, its business, assets, prospects or financial condition.



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                                   SECTION 7

                               GENERAL PROVISIONS

          7.1 Governing Law. All or part of this Agreement and the legal
 relations between the parties hereto has been negotiated in the State of
 Georgia and will be enforced under the laws of the State of Georgia without
 regard to its conflicts of laws provisions.

          7.2 Successors and Assigns: Third Party Beneficiaries. Except as
 otherwise expressly limited herein, the provisions hereof shall inure to the
 benefit of, and be binding upon, the successors (including successor trustees,
 in the case of a trustee), assigns, heirs, executors. and administrators of
 the parties hereto. Nothing in this Agreement, expressed or implied, is
 intended to confer upon any party other than the parties hereto and their
 respective successors and assigns any rights, remedies, obligations, or
 liabilities under or by reason of this Agreement.

          7.3 Entire Agreement: Amendment and Waiver. This Agreement
 constitutes the full and entire understanding and agreement between the
 parties with regard to the subject matter hereof and thereof and supersede all
 prior agreements among the parties. All prior negotiations and agreements
 shall be merged into this Agreement. Any term of this Agreement may be
 amended, and the observance of any term hereof may be waived (either generally
 or in a particular instance) only with the written consent of the Purchaser
 and the Seller. Any amendment or waiver effected in accordance with this
 Section 7.3 shall bc binding upon each of the parties hereto.

          7.4 Survival. The representations, warranties, covenants, and
 agreements made herein shall survive any investigation made by the Purchaser
 and the closing of the transactions contemplated hereby for two years from the
 Closing.

          7.5 Notices, etc. All notices and other communications required or
 permitted hereunder shall be in writing and shall be (i) mailed by registered
 or certified mail, postage prepaid, (ii) delivered by reliable overnight
 courier service, or (iii) otherwise delivered by hand, by messenger or by fax,
 addressed (A) if to the Purchaser, c/o Advent International Corporation, 10l
 Federal Street, Boston, MA 02110, Attention: Marcia Hooper, telecopier no.
 (617) 951-0566, or at such other address as the Purchaser shall have furnished
 to the Sellers in writing, with a copy to Finn Dixon & Herling LLP, One
 Landmark Square, Stamford, Connecticut 06901, Attention: Michael J. Herling,
 telecopier no. (203) 348-5777, or (B) if to the Seller, to such Seller's
 address set forth on Schedule A or at such other address as such Seller shall
 have furnished to the Purchaser in writing.

          7.6 Delays or Omissions. No delay or omission to exercise any right,
 power, or remedy accruing to either party upon any breach or default under
 this Agreement, shall be deemed a waiver of any other breach or default
 theretofore or thereafter occurring. Any waiver, permit, consent, or approval
 of any kind or character on the part of either party of any breach or default
 under this Agreement, or any waiver on the part of any party of any provisions
 or conditions of this Agreement, must be in writing and shall be effective
 only to the extent specifically set forth in such writing. All remedies,
 either under this Agreement or by law or otherwise afforded to 



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any of the parties, shall be cumulative and not alternative.

         7.7 References. Unless the context otherwise requires, any reference
to a "Section" refers to a section of this Agreement. Any reference to "this
Section" refers to the whole number section in which such reference is
contained.

         7.8 Severability. If any provision of this Agreement is held to be
unenforceable under applicable law, then such provision shall be excluded from
this Agreement and the balance of this Agreement shall be interpreted as if
such provision were so excluded and shall be enforceable in accordance with its
terms. The court in its discretion may substitute for the excluded provision an
enforceable provision which in economic substance reasonably approximates the
excluded provision.

         7.9 Dispute Resolution. Any claim or controversy arising out of or
relating to this Agreement shall be settled by non-binding arbitration in
accordance with the CPR Institute for Dispute Resolution's Rules for
Non-Administered Arbitration of Business Disputes by three arbitrators, of whom
each party shall appoint one. Any Arbitrator not appointed by a party shall be
selected from the CPR Institute for Dispute Resolution Panels of Distinguished
Neutrals. The arbitration shall be governed by re United States Arbitration
Act, 9 U.S.C. 1-16. The place of arbitration shall be Atlanta, Georgia, or such
other places as the parties shall mutually agree. The Arbitrators are not
empowered to award damages in excess of actual damages or to award punitive
damages. All applicable statutes of limitation shall be tolled while the
procedures specified in this Section 7.9 are pending so long as both parries
are diligently pursuing the arbitration. If one party is not diligently
pursuing the arbitration, then the party diligently pursuing the arbitration
may notify the other that such tolling shall cease. The parties will take such
action, if any, required to effectuate such tolling.


         7.10 Pronouns. All pronouns and any variation thereof refer to the
masculine, feminine or neuter, singular or plural, as the identity of the
person or persons may require.

         7.11 Counterparts. This Agreement may be executed in any number of
counterparts, each of which shall be deemed an original and enforceable against
the parties actually executing such counterpart, and all of which, when taken
together, shall constitute one instrument.

         7.12 Remedies. The parties to this Agreement acknowledge and agree
that a breach of any of the covenants of the Seller or the Purchaser set forth
in this Agreement may not be compensable by payment of money damages and,
therefore, that the covenants of the foregoing parties set forth in this
Agreement may be enforced in equity by a decree requiring specific performance.

         7.13 Certain Definitions. As used in this Agreement, the following
terms shall have the following meanings unless the context otherwise requires.

         (i) "Governmental Authority" means any federal, state, municipal or
other governmental or quasi-governmental department, commission, board, bureau,
agency or instrumentality, or 



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any court of the United States of America or any political subdivision thereof,
or of any other country.

         (ii) "Law" means any foreign, federal, state or local constitution,
         statute, regulation, rule, ordinance, code, plan, injunction, order,
         decree, ruling, charge or treaty.

         (iii) "Security Interest" means any adverse claim, mortgage, pledge,
         lien, encumbrance, opinion, restriction on transfer, easement, right
         of way, matter of survey, charge, or other security interest.

Executed effective as of the date first set forth above.


<TABLE>
<CAPTION>

THE PURCHASER:                      ADVENT GLOBAL GECC III LIMITED PARTNERSHIP

<S>                                 <C>
                                    By: Advent Global Management Limited Partnership, General
                                        Partner

                                    By: Advent International  Limited Partnership,  General Partner 
                                    By: Advent International Corporation, General Partner

                                    By:
                                       -------------------------------------------------------
                                       Marcia Hooper
                                       Vice President

THE SELLER:                         INTELLIGENT SYSTEMS CORPORATION
                                    By:
                                       -------------------------------------------------------
                                    Name:  Bonnie L. Herron
                                    Title:  Vice President
</TABLE>



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                                                                     SCHEDULE A

<TABLE>
<CAPTION>

----------------------------------------------------------------------------------------
   Seller's Name and Address           No. of Shares                  Cash Consideration
----------------------------------------------------------------------------------------
<S>                                    <C>                            <C>
----------------------------------------------------------------------------------------
Intelligent Systems Corporation           437,063                        $2,500,000.00
----------------------------------------------------------------------------------------
</TABLE>



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