<PAGE>
                                                                   EXHIBIT 10.20

 
                     ELEVENTH AMENDMENT TO LEASE AGREEMENT
                     -------------------------------------
                                        

          THIS ELEVENTH AMENDMENT TO LEASE AGREEMENT (hereinafter referred to as
the "Eleventh Amendment") is made as of the 16th day of July, 1998, by and
between WEEKS REALTY, L.P. (hereinafter referred to as "Landlord") and CHECKMATE
ELECTRONICS, INC. (hereinafter referred to as "Tenant").

                             W I T N E S S E T H :
                             -------------------- 
                                        
          WHEREAS, ASC North Fulton Associates Joint Venture and Tenant entered
into that certain Lease Agreement dated July 17, 1990, as amended by that
certain First Amendment to Lease dated December 20, 1990, as amended by that
certain Second Amendment to Lease dated October 15, 1992, as amended by that
certain Third Amendment to Lease April 30, 1993, as amended by that certain
Fourth Amendment to Lease July 15, 1993, as amended by that certain Fifth
Amendment to Lease dated August 16, 1994, as amended by that certain Sixth
Amendment to Lease dated February 20, 1995, as amended by that certain Seventh
Amendment to Lease dated January 18, 1996, as amended by that certain Eighth
Amendment to Lease dated April 1, 1996, as amended by that certain Ninth
Amendment to Lease Agreement dated August 18, 1997, and as further amended by
that certain Tenth Amendment to Lease Agreement dated May 20, 1998 (hereinafter
collectively referred to as the "Agreement") for the lease of 70,938 square feet
of office/ warehouse space at 999, 1003, 1009, and 1011 Mansell Road, Roswell,
Georgia which is more particularly described in the Agreement and certain
easements, rights and privileges appurtenant thereto (hereinafter referred to as
the "Leased Premises"); and

          WHEREAS, Weeks Realty, L.P. succeeded to the interest of the landlord
under the Agreement and is the Landlord with respect to the Leased Premises; and

          WHEREAS, the Agreement will expire by its terms on September 30, 1999
and Tenant desires to enter into this Eleventh Amendment in order to extend the
term of the Agreement;

          NOW, THEREFORE, for and in consideration of Ten Dollars ($10.00) paid
by Landlord and Tenant to one another, and for other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged by
Landlord and Tenant, Landlord and Tenant amend the Agreement as follows:

          1.  The Agreement is hereby extended for an additional two (2) year
term effective October 1, 1999 and continuing until midnight on September 30,
2001 on all of the same terms, covenants and conditions as the original
Agreement with the same base year except that the base rental for the new term
shall be as set forth below:

     1003 Mansell Road, Suite A-1 (4,200 square feet):
     -------------------------------------------------
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     October 1, 1999 - September 30, 2000   $2,865.98/month    $34,391.70/year
     October 1, 2000 - September 30, 2001   $2,951.95/month    $35,423.45/year
 
     1003 Mansell Road, Suite A-A (4,430 square feet):
     -------------------------------------------------
     October 1, 1999 - September 30, 2000   $3,022.92/month    $36,275.06/year
     October 1, 2000 - September 30, 2001   $3,113.61/month    $37,363.31/year
 
     1009 Mansell Road, Suite D (11,200 square feet):
     ------------------------------------------------
     October 1, 1999 - September 30, 2000   $7,642.60/month    $91,711.20/year
     October 1, 2000 - September 30, 2001   $7,871.88/month    $94,462.54/year
 
     1009 Mansell Road, Suite F (2,350 square feet):
     -----------------------------------------------
     October 1, 1999 - September 30, 2000   $1,603.58/month    $19,242.98/year
     October 1, 2000 - September 30, 2001   $1,651.69/month    $19,820.27/year
 
     1009 Mansell Road, Suite G, J, K (11,466 square feet):
     ------------------------------------------------------
     October 1, 1999 - September 30, 2000   $7,824.11/month    $93,889.34/year
     October 1, 2000 - September 30, 2001   $8,058.84/month    $96,706.02/year
 
     1011 Mansell Road, Suite C (15,522 square feet):
     ------------------------------------------------
     October 1, 1999 - September 30, 2000   $10,591.82/month   $127,101.90/ year
     October 1, 2000 - September 30, 2001   $10,909.58/month   $130,914.96/ year
 
     999 Mansell Road (9,600 square feet):
     -------------------------------------
     October 1, 1999 - September 30, 2000   $4,636.15/month    $55,633.84/year
     October 1, 2000 - September 30, 2001   $4,775.24/month    $57,302.86/year
 
     1003 Mansell Road, Suite C (7,370 square feet):
     -----------------------------------------------
     October 1, 1999 - September 30, 2000   $3,453.95/month    $41,447.41/year
     October 1, 2000 - September 30, 2001   $3,557.57/month    $42,690.83/year
 
     1003 Mansell Road, Suite D (4,800 square feet):
     -----------------------------------------------
     October 1, 1999 - September 30, 2000   $3,213.60/month    $38,563.20/year
     October 1, 2000 - September 30, 2001   $3,310.01/month    $39,720.10/year
 
Total Leased Premises (70,938 square feet):
-------------------------------------------
     October 1, 1999 - September 30,2000    $44,854.71/month   $538,256.63/year
     October 1, 2000 - September 30,2001    $46,200.37/month   $554,404.34/year

which payments shall be due and payable on or before the first day of each
calendar month during the applicable term, together with any other additional
rental as set forth hereunder or as set forth in the Agreement.

          3.  As consideration for Tenant's performance of all obligations to be
performed by Tenant under the Agreement, Landlord shall contribute the sum of
Seventy 

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Thousand and no/100 Dollars ($70,000.00) (the "Allowance") towards the
cost of tenant improvements to the Leased Premises. The Allowance shall be used
for alterations, improvements, fixtures and equipment which become part of or
are attached or affixed to the Leased Premises, including walls, wall coverings
and floor coverings, but excluding trade fixtures, furniture and furnishings or
other personal property. In the event the cost of tenant improvements exceeds
the cost of tenant improvement Allowance, the excess shall be paid by Tenant
within thirty (30) days of Tenant's receipt of Landlord's notice.

          4.  Except as expressly modified by this Eleventh Amendment, all
provisions, terms and conditions of the Agreement shall remain in full force and
effect.

          5.  In the event a provision of this Eleventh Amendment conflicts with
a provision of the Agreement, the Eleventh Amendment shall supersede and
control.

          6.  All terms and phrases used herein shall have the same meaning as 
assigned to them in the Agreement.

          7.  This Eleventh Amendment shall not be of any legal effect or
consequence unless signed by Landlord and Tenant, and once signed by Landlord
and Tenant it shall be binding upon and inure to the benefit of Landlord,
Tenant, and their respective legal representatives, successors and assigns.

          8.  This Eleventh Amendment has been executed and shall be construed 
under the laws of the State of Georgia.



                  [SIGNATURES CONTAINED ON THE FOLLOWING PAGE]

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   IN WITNESS WHEREOF, the undersigned have caused this Eleventh Amendment to be
executed under seal and delivered as of the day and year first above.
 

                                         LANDLORD:
Signed, sealed and delivered             WEEKS REALTY, L.P.,
in the presence of:                      a Georgia limited partnership
 
 
/s/ Kelly A. Kinnery                     By:  Weeks GP Holdings, Inc.,
--------------------                          a Georgia corporation,         
Witness                                       its sole general partner 
                                            
/s/ Patrick L. Adams
--------------------
Notary Public                                 By:    /s/ Forrest W. Robinson
                                                     -----------------------
                                              Name:  Forrest W. Robinson
                                                     -----------------------
                                              Its:   President/C.O.O.
                                                     -----------------------



                                         TENANT:
Signed, sealed and delivered             CHECKMATE ELECTRONICS, INC.,
in the presence of:


/s/ Tanya K. Meiss                       By:    /s/ John J. Neubert   
------------------                              -------------------        
Witness                                  Name:  John J. Neubert     
                                                -------------------     
                                         Its:   Exec. V.P. and CFO
                                                -------------------      
/s/ Valerie J. David                                                
--------------------                                                
Notary Public                            ATTEST: 
                                                                    
                                         By:    /s/ Margaret L. Burkett    
                                                -----------------------     
                                         Name:  Margaret L. Burkett    
                                                -----------------------
                                         Its:   Corporate Secretary
                                                ----------------------- 
                                                                    
                                         (Corporate Seal)

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