<PAGE>   1


                                                                   EXHIBIT 10.21


                                 FIRST AMENDMENT
                                       TO
                   FIRST AMENDED AND RESTATED CREDIT AGREEMENT


         THIS AMENDMENT is entered into as of November 12, 1999, among ULTRAK,
INC., a Delaware corporation ("Borrower"), certain lenders, and BANK ONE, TEXAS,
N.A., as Administrative Agent ("Administrative Agent").

         Borrower, Administrative Agent and Lenders are party to that certain
First Amended and Restated Credit Agreement (as renewed, extended, amended and
restated, the "Credit Agreement") dated as of August 12, 1999, providing for,
among other things, a revolving credit facility and a term loan facility. The
parties to this Amendment have agreed to amend the Credit Agreement as set forth
herein.

         1.       Defined Terms; References. Unless otherwise stated in this
Amendment (a) terms defined in the Credit Agreement have the same meanings when
used in this Amendment and (b) references to "Sections," "Schedules" and
"Exhibits" are to sections, schedules and exhibits to the Credit Agreement.

         2.       Amendments.

         a.       Section 1.1 is amended by deleting therefrom the following
                  defined terms: (i) Adjusted Fixed Assets, and (ii) Book Value.

         b.       The following defined terms in Section 1.1 are amended in
                  their entirety as follows:

                           "Borrowing Base" means, at any time, the sum, without
                  duplication, of (a) 80% of Eligible Accounts plus (b) 50% of
                  Eligible Inventory, plus (c) 50% of the Market Value of
                  Borrower's Permitted Investment in Detection Systems, Inc.,
                  plus (d) 80% of the Market Value of Borrower's Permitted
                  Public Company Holdings, plus (e) 100% of the then-current
                  account balance in the Money Market Account.

                           "Permitted Acquisition" means any Acquisition for
                  which the prior written consent of Required Lenders has been
                  obtained (such consent to be in the sole and absolute
                  discretion of each of those Lenders).

         c.       Section 3.2(c) is amended by adding a new clause (iv) as
                  follows:

                           (iv) Immediately upon the sale of any of Borrower's
                  investment in Detection Systems, Inc., Borrower shall make a
                  mandatory prepayment on the Obligation to Administrative Agent
                  (with any related Funding Loss) in an amount equal to 100% of
                  the Net Proceeds of such sale or other disposition.
                  Notwithstanding Section 3.11, the amount prepaid to
                  Administrative Agent may be applied against either the
                  Principal Debt under the Revolving Facility or the Term Loan
                  or any other portion of the Obligation, in the sole and
                  absolute discretion of Administrative Agent and Required
                  Lenders.

         d.       Section 3.2(f) is amended by adding a new clause (iv) as
                  follows:

                           (iv) Immediately upon the sale of any of Borrower's
                  investment in Detection Systems, Inc., Borrower shall make a
                  mandatory prepayment on the Obligation to Administrative Agent
                  (with any related Funding Loss) in an amount equal to 100% of
                  the


<PAGE>   2


                  Net Proceeds of such sale or other disposition.
                  Notwithstanding Section 3.11, the amount prepaid to
                  Administrative Agent may be applied against either the
                  Principal Debt under the Revolving Facility or the Term Loan
                  or any other portion of the Obligation, in the sole and
                  absolute discretion of Administrative Agent and Required
                  Lenders. No such mandatory prepayment that is applied against
                  the Term Loan may be reborrowed.

         e.       Section 8.4 is amended in its entirety as follows:

                           8.4 Inspections. Upon reasonable request and at least
                  5-days' advance notice (but during the pendency of an Event of
                  Default, no advance notice is required), each Company shall
                  allow Administrative Agent or any Lender (or their respective
                  Representatives) to inspect any of that Company's properties,
                  to review reports, files and other records and to make and
                  take away copies, to conduct tests or investigations and to
                  discuss any of its affairs, conditions and finances with its
                  other creditors, directors, officers, employees, outside
                  accountants or representatives from time to time, during
                  reasonable business hours (but during the pendency of an Event
                  of Default, at any time). Without limiting the foregoing, the
                  Companies shall allow Administrative Agent and Lenders to
                  perform field examinations to test such systems and controls
                  of the Companies as they deem appropriate, such field
                  examinations at any time prior to an Event of Default to occur
                  no more frequently than once per calendar quarter (provided
                  that such limitation shall not apply after the occurrence and
                  during the continuation of an Event of Default). Borrower
                  shall reimburse Administrative Agent and each Lender for the
                  reasonable expenses of each such field examination (the
                  expected expense relating to each such field exam is $10,000).

         f.       Section 9.7(g) is amended in its entirety as follows:

                           (g) Borrower's investment in Detection Systems, Inc.
                  as of November 12, 1999, provided that any part of such
                  investment that is sold or otherwise disposed of may not be
                  subsequently reacquired;

         g.       Section 9.8 is amended in its entirety as follows:

                           9.8 Distributions; Other Payments. No Company may
                  declare, make or pay any Distribution except (i) Distributions
                  paid in the form of additional equity that is not required to
                  be redeemed or is redeemable at the option of the holder if
                  certain events or conditions occur or exist or otherwise, (ii)
                  Distributions to Borrower from any other Company, (iii)
                  Distributions from a Foreign Subsidiary to Ultrak Holdings,
                  (iv) Distributions from Ultrak Operating to Ultrak GP and
                  Ultrak LP, (v) Distributions of up to $117,210 per year
                  representing preferred stock dividends from Borrower to Mr.
                  George K. Broady made in respect of Borrower's Series A
                  Preferred Stock, and (vi) from November 12, 1999, through the
                  entire remaining term that this agreement is in effect, up to
                  an aggregate of $100,000 in stock repurchases by the Companies
                  (for the avoidance of doubt, the restriction set forth in this
                  clause (vi) shall apply to the Companies taken as a whole and
                  is not meant to apply to each Company individually).

         h.       Section 9.9 is amended in its entirety as follows:

                           9.9 Disposition of Assets. No Company may sell,
                  assign, lease, transfer or otherwise dispose of any of its
                  assets (including, without limitation, equity interests in any


                                       2
<PAGE>   3


                  other Company) other than a Permitted Asset Sale.
                  Notwithstanding the foregoing, Borrower may sell its Permitted
                  Investments described in SECTION 9.7(f) without being required
                  to prepay the Obligation.

         i.       Section 10.4 is amended in its entirety as follows:

                           10.4 Capital Expenditures. For any fiscal year of
                  Borrower, capital expenditures for the acquisition,
                  construction, improvement or replacement of land, buildings,
                  equipment or other fixed or capital assets or leaseholds
                  (excluding expenditures properly chargeable to repairs or
                  maintenance) for the Companies shall not exceed (a) $6,700,000
                  for Borrower's fiscal year ending December 31, 1999, and (b)
                  $4,000,000 for any fiscal year thereafter.

         j.       Schedule H is amended in its entirety, and all references in
                  the Credit Documents shall be deemed to refer to, the Schedule
                  H attached hereto as Annex A.

         k.       Schedule J is amended in its entirety, and all references in
                  the Credit Documents shall be deemed to refer to, the Schedule
                  J attached hereto as Annex B.

         3. Conditions Precedent. Notwithstanding any contrary provisions
contained in the Credit Documents, this Amendment is not effective unless and
until:

         a.       the representations and warranties in this Amendment are true
                  and correct;

         b.       Administrative Agent receives counterparts of this Amendment
                  executed by each party named below; and

         c.       Administrative Agent receives a copy of the resolutions
                  adopted by the Board of Directors of Borrower authorizing the
                  transactions contemplated by this Amendment (such resolutions
                  being in form and substance acceptable to Administrative
                  Agent), certified as true and correct by the Secretary or an
                  Assistant Secretary of Borrower.

         4. Ratifications. This Amendment modifies and supersedes all
inconsistent terms and provisions of the Credit Documents, and except as
expressly modified and superseded by this Amendment, the Credit Documents are
ratified and confirmed and continue in full force and effect. The parties hereto
agree that the Credit Documents, as amended by this Amendment, continue to be
legal, valid, binding and enforceable in accordance with their respective terms.
Without limiting the generality of the foregoing, Borrower hereby ratifies and
confirms that all Liens heretofore granted to Administrative Agent on behalf of
Lenders were intended to, do, and continue to secure the full payment and
performance of the Obligation. Borrower agrees to perform such acts and duly
authorize, execute, acknowledge, deliver, file and record such additional
assignments, security agreements, modifications or amendments to any of the
foregoing, and such other agreements, documents, and instruments as
Administrative Agent may reasonably request in order to perfect and protect
those Liens and preserve and protect the rights of Administrative Agent and
Lenders in respect of all present and future Collateral.

         5. Representations and Warranties. Borrower hereby represents and
warrants to Administrative Agent and Lenders that (a) this Amendment and any
other Credit Documents to be delivered under this Amendment have been duly
executed and delivered by or on behalf of Borrower and each other Company party
to them, (b) no action of, or filing with, any Governmental Authority is
required to authorize, or is otherwise required in connection with, the
execution, delivery, and performance by Borrower or any other


                                       3
<PAGE>   4


Company of this Amendment or any other Credit Document to be delivered under
this Amendment, (c) this Amendment and any other Credit Documents to be
delivered under this Amendment are valid and binding upon Borrower and the other
Companies and are enforceable against Borrower and such other Companies in
accordance with their respective terms, except as limited by any applicable
Debtor Relief Laws, (d) the execution, delivery and performance by Borrower and
the other Companies of this Amendment and any other Credit Documents to be
delivered under this Amendment do not require the consent of any other Person
and do not and will not constitute a violation of any Governmental Requirements,
agreements or understandings to which Borrower or any other Company is a party
or by which Borrower is bound, (e) the representations and warranties contained
in the Credit Agreement, as amended by this Amendment, and any other Credit
Documents are true and correct in all material respects as of the date of this
Amendment, and (f) as of the date of this Amendment, no Event of Default or
Potential Default exists.

         6. Waiver. Effective only upon receipt by Administrative Agent of this
Amendment executed by each Person named below, this Amendment shall become
notice to Borrower that Administrative Agent and Lenders hereby temporarily
waive any Event of Default or Potential Default arising under the Credit
Agreement with respect to the Companies' non-compliance with Sections 10.1 and
10.2 for the period ending September 30, 1999. The waivers set forth in the
immediately-preceding sentence shall expire on January 31, 2000, and be of no
further force and effect, and the Events of Default thus temporarily waived
fully reinstated, unless Required Lenders agree in writing before January 31,
2000 to a permanent waiver of such Events of Default. Except as stated above (i)
this waiver is not a consent or waiver in respect of any existing or future
Events of Default or Potential Defaults or a waiver of Lender's rights to insist
upon Borrower's compliance with its obligations under each Credit Document and
(ii) each Credit Document is unchanged and continues in full force and effect.

         7. Inspections. The parties hereto contemplate that Administrative
Agent will perform a field examination of the Companies within 45 days of the
date of this Amendment in accordance with its rights under Section 8.4 of the
Credit Agreement. The parties hereto further contemplate that Wells Fargo Bank
(Texas), National Association will perform its own field examination of the
Companies approximately 60 days following the conclusion of the Bank One, Texas,
N.A. examination described above. In both cases, Borrower shall reimburse
Administrative Agent and Wells Fargo Bank (Texas), National Association,
respectively, for their reasonable expenses in connection with such field
examinations (such expenses are not expected to exceed $10,000 per examination).
The Companies also hereby consent to Administrative Agent and any Lender who
performs any field examination or other inspection of the Companies to discuss
the results of such field examination or inspection with Administrative Agent
and the other Lenders and to provide copies of any documents, reports or
analyses prepared or obtained in connection with such field examination or
inspection to Administrative Agent and the other Lenders. Administrative Agent
and each Lender hereby agree that any such documents, reports or analyses that
might from time to time be provided to it by any other party to this Amendment
(other than a Company) shall be delivered to it as a courtesy of the party
delivering it, that Administrative Agent and each such Lender has the
opportunity to inform itself from other sources as to any relevant information
regarding the Companies in order to make its own independent credit decisions,
and releases the party delivering any such documents, reports or analyses from
any liability whatsoever relating to errors or omissions contained therein. It
is contemplated that Administrative Agent and Wells Fargo Bank (Texas), National
Association will promptly provide to each other copies of any final reports
prepared in connection with the field examinations referred to above. In no way
will the terms of this paragraph limit or restrict the ability of Administrative
Agent and the other Lenders from exercising their inspection rights in
accordance with Section 8.4 of the Credit Agreement.

         8. References. All references in the Credit Documents to the "Credit
Agreement" refer to the Credit Agreement as amended by this Amendment. This
Amendment is a "Credit Document" referred to in


                                       4
<PAGE>   5


the Credit Agreement and the provisions relating to Credit Documents in the
Credit Agreement are incorporated by reference, the same as if set forth
verbatim in this Amendment.

         9. Counterparts. This Amendment may be executed in any number of
counterparts with the same effect as if all signatories had signed the same
document.

         10. Parties Bound. This Amendment binds and inures to the benefit of
Borrower, Lenders and Administrative Agent and, subject to Section 14.12 of the
Credit Agreement, their respective successors and assigns.

         11. Entirety. THIS AMENDMENT, THE CREDIT AGREEMENT AS AMENDED BY THIS
AMENDMENT, AND THE OTHER CREDIT DOCUMENTS REPRESENT THE FINAL AGREEMENT BETWEEN
THE PARTIES FOR THE TRANSACTIONS HEREIN AND THEREIN, AND MAY NOT BE CONTRADICTED
BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENT BETWEEN THE
PARTIES. THERE ARE NO UNWRITTEN ORAL AGREEMENT BETWEEN THE PARTIES.

                      REMAINDER OF PAGE INTENTIONALLY BLANK
                             SIGNATURE PAGES FOLLOW


                                       5
<PAGE>   6


         EXECUTED as of the date first stated above.


Address for Notice                    ULTRAK, INC., as Borrower

Ultrak, Inc.
1301 Waters Ridge Drive
Lewisville, Texas 75057
Attn: Mr. Tim D. Torno,
      Vice President - Finance        By:
Fax No.: (972) 353-6679                  ---------------------------------------
                                          Tim D. Torno, Vice President - Finance

Address for Notice                    BANK ONE, TEXAS, N.A.,
                                      as Administrative Agent and a Lender
Bank One, Texas, N.A.
1717 Main Street, 3rd Floor
Dallas, Texas  75201
Attn: Alan L. Miller,                 By:
      Vice President                     ---------------------------------------
Fax No.: (214) 290-2305                   Alan L. Miller, Vice President


Address for Notice                    WELLS FARGO BANK (TEXAS),
                                      NATIONAL ASSOCIATION,
Wells Fargo Bank (Texas),             as a Lender
National Association
1445 Ross @ Field, 3rd Floor
Dallas, Texas 75202
Attn: Kyle Hranicky,                  By:
      Vice President                     ---------------------------------------
Fax No.: (214) 969-0370                  Kyle Hranicky, Vice President



<PAGE>   7


         To induce Administrative Agent and Lenders to enter into this
Amendment, each guarantor named below (a) consents and agrees to this
Amendment's execution and delivery, (b) ratifies and confirms that all
guaranties, assurances, and Liens granted, conveyed, or assigned to
Administrative Agent on behalf of Lenders under the Credit Documents are not
released, diminished, impaired, reduced, or otherwise adversely affected by this
or any prior amendment, and continue to guarantee, assure, and secure the full
payment and performance of all present and future Obligation, (c) agrees to
perform such acts and duly authorize, execute, acknowledge, deliver, file and
record such additional guaranties, assignments, security agreements, deeds of
trust, mortgages, and other agreements, documents, instruments, and certificates
as Administrative Agent may reasonably deem necessary or appropriate in order to
create, perfect, preserve, and protect those guaranties, assurances, and Liens,
and (d) waives notice of acceptance of this consent and agreement, which consent
and agreement binds each of the undersigned and their respective successors and
permitted assigns and inures to Administrative Agent and Lenders and their
respective successors and permitted assigns.


                                   ULTRAK GP, INC.



                                   By:
                                      ------------------------------------------
                                      Tim D. Torno, Vice President - Finance


                                   ULTRAK, LP, INC.



                                   By:
                                      ------------------------------------------
                                      Tim D. Torno, Vice President - Finance


                                   ULTRAK OPERATING, L.P.

                                   By: Ultrak GP, Inc. its General Partner



                                   By:
                                      ------------------------------------------
                                       Tim D. Torno, Vice President - Finance


                                   DIAMOND ELECTRONICS, INC.



                                   By:
                                      ------------------------------------------
                                       Tim D. Torno, Vice President - Finance


                                   MONITOR DYNAMICS, INC.


<PAGE>   8


                                   By:
                                      ------------------------------------------
                                      Tim D. Torno, Vice President - Finance


                                   ABM DATA SYSTEMS, INC.



                                   By:
                                      ------------------------------------------
                                      Tim D. Torno, Vice President - Finance


<PAGE>   9


                                     Annex A
                                       to
                                 First Amendment
                                       to
                   First Amended and Restated Credit Agreement

                                    EXHIBIT H

                              BORROWING BASE REPORT


       FOR THE MONTH ENDED                 ,           (the "SUBJECT MONTH")
                          ----------------- ----------


ADMINISTRATIVE AGENT:    Bank One, Texas, N.A.              DATE:       ,
                                                                 ------------

BORROWER:                Ultrak, Inc.


================================================================================

         This report is delivered under the First Amended and Restated Credit
Agreement (as renewed, extended and amended, the "CREDIT AGREEMENT") dated as of
August 12, 1999, between Borrower, certain Lenders and Administrative Agent.
Terms defined in the Credit Agreement have the same meanings when used, unless
otherwise defined, in this report.

<TABLE>
<CAPTION>
====================================================================   ===========================================

                            LINE                                               AT END OF SUBJECT QUARTER
--------------------------------------------------------------------   -------------------------------------------
<S>                                                                    <C>                      <C>
1.        Total Accounts - Line 1 minus Line 2(d)                      $
--------------------------------------------------------------------   -----------------------   -------------------
2.        Accounts excluded from the Borrowing Base due to the         $
          exclusions set forth in the definition of "Eligible
          Accounts"
--------------------------------------------------------------------   -----------------------   -------------------
         (a)      Excluded under clause (a)                            $
--------------------------------------------------------------------   -----------------------   -------------------
         (b)      Excluded under clause (b)                            $
--------------------------------------------------------------------   -----------------------   -------------------
         (c)      Excluded under clause (c)                            $
--------------------------------------------------------------------   -----------------------   -------------------
         (d)      Total ineligible Accounts - sum of Lines 2(a),       $
                  2(b) and 2(c)
--------------------------------------------------------------------   -----------------------   -------------------
3.        Total Eligible Accounts - Line 1 minus Line 2(d)             $
--------------------------------------------------------------------   -----------------------   -------------------
4.        80% of Line 3                                                                          $
--------------------------------------------------------------------   -----------------------   -------------------
5.        Total Inventory                                              $
--------------------------------------------------------------------   -----------------------   -------------------
6.        Inventory excluded from the Borrowing Base due to the        $
          exclusions set forth in the definition of "Eligible
          Inventory"
--------------------------------------------------------------------   -----------------------   -------------------
7.        Total Eligible Inventory - Line 5 minus Line 6               $
--------------------------------------------------------------------   -----------------------   -------------------
8.        50% of Line 7                                                                          $
--------------------------------------------------------------------   -----------------------   -------------------
9.        Market Value of Borrower's Permitted Investment in           $
          Detection Systems, Inc.
--------------------------------------------------------------------   -----------------------   -------------------
10.       50% of Line 9                                                                          $
--------------------------------------------------------------------   -----------------------   -------------------
11.       Market Value of Borrower's Permitted Public Company          $
          Holdings (a complete listing of such holdings and their
          current Market Values is attached hereto) plus the
          balance in the Money Market Account (a statement for
          such account dated the last day of the Subject Month is
          attached hereto)
--------------------------------------------------------------------   -----------------------   -------------------
12.       80% of Line 11                                                                         $
--------------------------------------------------------------------   -----------------------   -------------------
13.       Balance in the Money Market Account                          $
--------------------------------------------------------------------   -----------------------   -------------------
14.       100% of Line 13                                                                        $
--------------------------------------------------------------------   -----------------------   -------------------
15.       Borrowing Base -- Sum of Line 3, Line 8, Line 10, Line                                 $
          12 and Line 14
--------------------------------------------------------------------   -----------------------   -------------------
16.       Total Commitments                                                                      $50,000,000
--------------------------------------------------------------------   -----------------------   -------------------
17.       Commitment usage plus LC Exposure                                                      $
--------------------------------------------------------------------   -----------------------   -------------------
18.       Lesser of either Line 15 or Line 16 minus Line 17                                      $
--------------------------------------------------------------------   -----------------------   -------------------
19.       Availability -- either                                                                 $

         (a)      If Line 18 is negative -- the amount to be repaid
--------------------------------------------------------------------   -----------------------   -------------------
         (b)      if Line 18 is positive -- the amount available                                 $
====================================================================   =======================   ===================
</TABLE>


<PAGE>   10


         On behalf of, and in my capacity as an officer of, Borrower, I certify
that, as of the last day of the Subject Month and on the date of this report (a)
I am the undersigned officer of Borrower, (b) no Material Adverse Event, Event
of Default or Potential Default exists, (c) a review of the activities of
Borrower during the Subject Month has been made under my supervision with a view
to determining the amount of the current Borrowing Base, (d) the accounts
receivable of the Domestic Companies included in the Borrowing Base above meet
all conditions in the Credit Agreement to qualify for inclusion in the Borrowing
Base, (e) the inventory of the Domestic Companies included in the Borrowing Base
above meet all conditions in the Credit Agreement to qualify for inclusion in
the Borrowing Base, (f) the Permitted Investments and Permitted Public Company
Holdings included in the Borrowing Base above meet all conditions in the Credit
Agreement to qualify for inclusion in the Borrower Base, (g) all of the
representations and warranties of the Companies in the Credit Documents are true
and correct in all material respects (unless they speak to a specific date or
are based on facts which have changed by transactions contemplated or expressly
permitted by the Credit Documents), (h) the Companies have otherwise complied
with all of their obligations under the Credit Documents, and (i) the attached
accounts receivable aging and inventory listings are true, correct and complete.


                                             ULTRAK, INC.


                                             By:
                                                --------------------------------
                                             Name:
                                                  ------------------------------
                                             Title:(1)
                                                   -----------------------------


--------
(1)       Must be Borrower's president, chief financial officer or treasurer.


<PAGE>   11


                                     Annex B
                                       to
                                 First Amendment
                                       to
                   First Amended and Restated Credit Agreement

                                    EXHIBIT J

                  PERMITTED ACQUISITION COMPLIANCE CERTIFICATE

ADMINISTRATIVE AGENT:    Bank One, Texas, N.A.         DATE:____________________

BORROWER:                Ultrak, Inc.

================================================================================

         This certificate is delivered under the First Amended and Restated
Credit Agreement dated as of August 12, 1999 (as renewed, extended and amended,
the "CREDIT AGREEMENT") between Borrower, certain Lenders and Administrative
Agent. Terms defined in the Credit Agreement have the same meanings when used
unless otherwise defined in this certificate.

         __________________[name of Company] intends to acquire _____________
[the "SUBJECT ACQUISITION"], on ________________, ____ (the "ACQUISITION DATE").
In connection with such Subject Acquisition, Borrower hereby confirms the
following:

         10       all of the representations and warranties under the Credit
                  Documents are true and correct immediately prior to and after
                  giving effect to the Subject Acquisition;

         20       the consent of Required Lenders to the Subject Acquisition has
                  been obtained and Borrower has delivered to Administrative
                  Agent all information regarding the Acquisition requested by
                  Administrative Agent.

         30       after giving effect to the Subject Acquisition, any Debt (if
                  any) incurred or assumed by the Companies in connection with
                  the Subject Acquisition will be permitted by SECTION 9.2.

         40       prior to the consummation of the Subject Acquisition, Borrower
                  has delivered to Administrative Agent all supplements to, or
                  revisions of, Schedules 7.3 and 7.8 to of the Credit Agreement
                  which are required to make the disclosures in such Schedules
                  accurate after giving effect to the Subject Acquisition, and
                  has obtained the consent of Required Lenders with respect to
                  such revised or supplemental Schedules (|_| check here if no
                  such revised or supplemental Schedules are required as a
                  result of the Subject Acquisition).

                                             ULTRAK, INC.

                                             By:
                                                --------------------------------
                                             Name:
                                                  ------------------------------
                                             (1)Title:
                                                      --------------------------
--------
(1)      Must be a Responsible Officer of Borrower.