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                                                                 EXHIBIT 10.11.6

                      FIRST AMENDMENT TO SECURITY AGREEMENT

        THIS FIRST AMENDMENT TO SECURITY AGREEMENT (this "Amendment") dated as
of December 21, 1999, is made by STB Systems, Inc., a Texas corporation
("Debtor") and Bank One, Texas, N.A., individually and as Agent for the Lenders
under the Credit Agreement described below ("Secured Party").

                                   BACKGROUND

        1.   Debtor, Secured Party and the lenders named therein entered into
that certain Credit Agreement dated as of November 21, 1997, (as heretofore
amended, supplemented, or restated, the "Original Credit Agreement"), for the
purpose and consideration therein expressed, pursuant to which Debtor executed
(i) in favor of Bank One, Texas, N.A. ("Bank One") that certain promissory note
dated as of November 23, 1999, payable to the order of Bank One in the aggregate
principal amount of $12,500,000 (the "Original Bank One Note") and (ii) in favor
of Comerica Bank-Texas ("Comerica") that certain promissory note dated as of
November 23, 1999, payable to the order of Comerica in the aggregate principal
amount of $12,500,000; in each case which renewed and extended the promissory
notes described therein (the "Original Comerica Note"; the Original Bank One
Note and the Original Comerica Note are herein collectively referred to as the
"Original Notes").

        2.   Pursuant to the Original Credit Agreement, Debtor executed in favor
of Secured Party that certain Security Agreement dated as of November 21, 1997
(the "Original Security Agreement");

        3.   Debtor, Secured Party and the lenders named therein are or are to
become parties to an Amended and Restated Credit Agreement of even date herewith
(as amended, supplemented, or restated, the "Credit Agreement"), amending and
restating the Original Credit Agreement for the purpose and consideration
therein expressed.

        4.   Pursuant to the Credit Agreement, Debtor executed (i) in favor of
Bank One that certain promissory note of even date herewith, payable to the
order of Bank One in the aggregate principal amount of $12,500,000, renewing and
extending the Original Bank One Note (the "New Bank One Note") and (ii) in favor
of Comerica that certain promissory note of even date herewith, payable to the
order of Comerica in the aggregate principal amount of $12,500,000, renewing and
extending the Original Comerica Note (the "New Comerica Note"; the New Bank One
Note and the New Comerica Note, as from time to time amended, and all promissory
notes given in substitution, renewal or extension therefore or thereof, in whole
or in part, being herein collectively called the "Notes"); and

        4.   Pursuant to the Credit Agreement, Lenders have agreed to extend
credit to Debtor.

        5.   In order to induce Lenders to extend such credit pursuant to the
Credit Agreement, Debtor has agreed to enter into this Amendment for the
purposes as stated herein.


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        NOW, THEREFORE, in consideration of the premises and for other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, Debtor and Secured Party hereby agree as follows:


                                    AGREEMENT

        1.   DEFINED TERMS. Capitalized terms used but not defined herein shall
have the meanings given them in the Credit Agreement.

        2.   OTHER DEFINED TERMS. Unless the context otherwise requires, the
following terms when used in this Amendment shall have the meanings assigned to
them in this Section 2.

             "Amendment" means this First Amendment to Security Agreement.

             "Security Agreement" means the Original Security Agreement as
        amended hereby.

        3.   AMENDMENTS TO ORIGINAL SECURITY AGREEMENT.

             A. General Definitions. The definition of "Credit Agreement" in
        Section 1.1 of the Original Security Agreement is hereby amended in its
        entirety to read as follows:

             "CREDIT AGREEMENT" means that certain Amended and Restated Credit
        Agreement dated as of December 21, 1999, among Debtor, Secured Party and
        certain lenders (the "LENDERS"), as from time to time amended,
        supplemented or restated."

             B. Terms. Secured Party and Debtor hereby agree that (i) all
        references to the "Notes" in the Original Security Agreement shall be
        deemed to refer to the Notes as defined herein and (ii) all references
        to the "Loan Documents" in the Original Security Agreement shall be
        deemed to refer to the Loan Documents as defined in the Credit
        Agreement.

             C. Address. Debtor's address on the signature page of the Original
        Security Agreement is hereby amended in its entirety to read as follows:

                             "3400 Waterview
                             Richardson, TX 75080"

        3.   RATIFICATION OF ORIGINAL SECURITY AGREEMENT. Debtor acknowledges
that its obligations and covenants under the Original Security Agreement are not
impaired by the execution and delivery of the Notes or the Credit Agreement and
shall remain in full force and effect. The Original Security Agreement as hereby
amended is hereby ratified and confirmed in all respects. Any reference to the
Security Agreement in any Loan Document shall be deemed to refer to this
Amendment also. The execution, delivery and effectiveness of this



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Amendment shall not, except as expressly provided herein, operate as a wavier of
any right, power or remedy of Secured Party or Lenders under the Original
Security Agreement or any other Loan Document nor constitute a waiver of any
provision of the Original Security Agreement or any other Loan Document. This
Amendment is a Loan Document, and all provisions in the Credit Agreement
pertaining to Loan Documents apply hereto.

        4.   FURTHER ASSURANCES. Debtor will, at its expense and at any time and
from time to time, promptly execute and deliver all further instruments and
documents and take all further action that may be necessary or desirable or that
Secured Party may request in order (i) to perfect, continue the perfection of,
and protect the security interest created or purported to be created by the
Security Agreement and the first priority of such security interest; (ii) to
enable Secured Party to exercise and enforce its rights and remedies thereunder
in respect of the Collateral; or (iii) to otherwise effect the purposes of the
Security Agreement, including but not limited to: (A) executing and filing such
financing or continuation statements, or amendments thereto, as may be necessary
or desirable or that Secured Party may request in order to perfect, continue the
perfection of, and preserve the security interest created or purported to be
created thereby; (B) delivering to Secured Party (upon request, to the extent
not otherwise required hereunder to be delivered without request) all originals
of chattel paper, documents or instruments which are from time to time included
in the Collateral; and (C) furnishing to Secured Party from time to time
statements and schedules further identifying and describing the Collateral and
such other reports in connection with the Collateral as Secured Party may
reasonably request, all in reasonable detail.

        5.   GOVERNING LAW. THIS AMENDMENT SHALL BE GOVERNED BY AND CONSTRUED
AND ENFORCED IN ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS AND THE LAWS OF
THE UNITED STATES OF AMERICA; EXCEPT TO THE EXTENT THAT THE PERFECTION AND THE
EFFECT OF PERFECTION OR NON-PERFECTION OF THE SECURITY INTEREST CREATED HEREBY
ARE GOVERNED BY THE LAWS OF A JURISDICTION OTHER THAN THE STATE OF TEXAS.

        6.   COUNTERPARTS. This Amendment may be separately executed in
counterparts and by the different parties hereto in separate counterparts, each
of which when so executed shall be deemed to constitute one and the same
Amendment. This Amendment may be validly executed by facsimile or other
electronic transmission.



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        IN WITNESS WHEREOF, the undersigned have executed this Amendment as of
the date first above written.

                                       DEBTOR:

                                       STB SYSTEMS, INC.

                                       By: /s/  BRYAN F. KEYES
                                          ------------------------------------
                                       Name:
                                       Title: Vice President

                                       SECURED PARTY:
                                       BANK ONE, TEXAS, N.A., as Agent

                                       By:  /s/ RICK ROGERS
                                          ------------------------------------
                                       Name:
                                       Title: Managing Director