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                                                                     EXHIBIT 4.2

                                 PROMISSORY NOTE

$75,000.00                                                    September 27, 2001

For value received, INTEGRATED SECURITY SYSTEMS, INC., a Delaware corporation
(hereinafter referred to as "Maker"), promises to pay to the order of FROST
NATIONAL BANK, CUSTODIAN, FBO RENAISSANCE US GROWTH & INCOME TRUST PLC, a public
limited company registered in England and Wales (hereinafter referred to as
"Payee"), the principal sum of Seventy-Five Thousand Dollars ($75,000.00). The
principal of and interest on this Note shall be due and payable in lawful money
of the United States of America, c/o Security Processing T-8, P. O. Box 2479,
San Antonio, Texas 78298. All correspondence and notices should be mailed to the
above address with a copy to the offices of Payee at 8080 N. Central Expressway,
Suite 210, Dallas, Texas 75206, or at such other place as the holder hereof may
from time to time designate by written notice to Maker.

         1. Interest. Interest shall accrue on the unpaid principal balance due
under this Note at an annual rate equal to eight percent (8%). Interest shall
accrue from and including the date of this Note until, but not including, the
day on which it is paid in full. In no event shall the interest charged
hereunder exceed the maximum rate of interest allowed from time to time by law.
Interest shall be due and payable monthly on the first (1st) day of each month,
commencing November 1, 2001.

         2. Payment of Note. The principal balance of, and all accrued unpaid
interest on, this Note shall be due and payable one hundred twenty (120) days
from the date hereof, except as otherwise provided herein. ("Maturity Date").

         3. Prepayment. This Note may be prepaid in whole or in part at any
time, at the option of Maker, without premium or penalty.

         4. Default, Enforcement. Upon default in payment of this Note, Payee
may pursue any and all rights and remedies to which Payee may be entitled under
applicable law.

         5. Limitation of Interest. All agreements between Maker and Payee,
whether now existing or hereafter arising and whether written or oral, are
expressly limited so that in no contingency or event whatsoever, whether by
reason of advancement of the proceeds hereof, acceleration of the maturity of
the unpaid principal balance hereof, or otherwise, shall the amount contracted
for, charged, received, paid or agreed to be paid to the holder hereof for the
use, forbearance, or detention of the money evidenced by this Note or for the
payment or performance of any covenant or obligation contained herein or in any
other document pertaining to the indebtedness evidenced by this Note exceed the
maximum amount permissible under applicable usury laws. If, from any
circumstance whatsoever, fulfillment of any provision hereof or of any other
agreement shall, at the time fulfillment of such provision be due, involve
transcending the limit of validity prescribed by law which a court of competent
jurisdiction may deem applicable


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hereto, then, ipso facto, the obligation to be fulfilled shall be reduced to the
limit of such validity; and if from any circumstance the holder hereof shall
ever receive as interest an amount which would exceed the maximum lawful rate,
any amount equal to any excessive interest shall (a) be applied to the reduction
of the unpaid principal balance due hereunder and not to the payment of
interest, or (b) if such excess interest exceeds the unpaid principal balance of
this Note, such excess shall be refunded to Maker. All sums contracted for,
charged or received hereunder for the use, forbearance or detention of the
indebtedness evidenced hereby shall, to the extent permitted by applicable law,
be amortized, prorated, allocated and spread throughout the full term of this
Note until payment in full so that the rate of interest on account of such
indebtedness is uniform throughout the term hereof. The terms and provisions of
this paragraph shall control and supersede every other provision of all
agreements between Maker and the holder hereof.

         6. Waiver. Except as otherwise expressly provided herein, Maker waives
demand, presentment for payment, notice of intent to accelerate, notice of
acceleration, notice of nonpayment or dishonor, grace, protest, notice of
protest, all other notices, and any and all diligence or delay in collection or
the filing of suit hereon.

         7. Governing Law and Venue. This Note shall be construed according to
and governed by the laws of the State of Texas. The obligations of Maker under
this Note are performable in Dallas County, Texas.

         8. Security Agreement. This Note is secured by the Security Agreement,
dated September 27, 2001, among Maker, B&B Electromatic, Inc. and Intelli-Site,
Inc., Payee and Renaissance Capital Growth & Income Fund III, Inc., and Payee is
entitled to the rights and benefits thereunder.

         9. Stock Pledge Agreement. This Note is secured by the Stock Pledge
Agreement dated as of September 27, 2001, among Maker, Payee, Renaissance
Capital Growth & Income Fund III, Inc., and Payee is entitled to the rights and
benefits thereunder.

         10. Successors and Assign. This Note shall bind Maker's successors and
assigns.

         11. Collection Costs. If this Note is collected by legal proceeding or
through a probate or bankruptcy court, or is placed in the hands of an attorney
for collection after default (whether or not suit is filed), Maker agrees to pay
all costs of collection and/or suit, including but not limited to reasonable
attorneys' fees and expenses incurred by Payee.

         12. Unenforceability. The invalidity, or unenforceability in particular
circumstances, of any provision of this Note shall not extend beyond such
provision or such circumstances, and no other provision of this Note shall be
affected thereby.

         13. Headings. The paragraph headings of the sections of this Note are
inserted for convenience of reference only and shall not affect the meaning or
interpretation of this Note.


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         IN WITNESS WHEREOF, Maker has duly executed this Note as of the day and
year first above written.

                                    INTEGRATED SECURITY SYSTEMS, INC.



                                    By:
                                         ---------------------------------------
                                         C. A. Rundell, Jr.
                                         Chairman and Chief Executive Officer


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