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                                                                     EXHIBIT 4.2


                           CONVERTIBLE PROMISSORY NOTE

$150,000.00                                                      April 11, 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 FBO RENAISSANCE US GROWTH & INCOME TRUST PLC, a Texas
corporation (hereinafter referred to as "Payee"), the principal sum of One
Hundred Fifty Thousand Dollars ($150,000.00). 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 on interest allowed from time to time by law.
Interest shall be due and payable monthly on the first (1st) day of each month.

         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
after the date hereof, except as otherwise provided herein ("Maturity Date").

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

         4. Conversion. This Note shall be convertible, at the option of Payee
in its sole and absolute discretion, in whole or in part and at any time or from
time to time, into fully paid and nonassessable shares (the "Conversion Shares")
of Common Stock, $.01 par value (the "Common Stock"), of Integrated Security
Systems, Inc., a Delaware corporation (the "Company"), at the conversion price
of $0.20 per share. If Payee elects to exercise its option, then the following
shall occur:

                           (a) Payee shall deliver to Maker a notice of such
                           election (the "Conversion Notice"), indicating the
                           amount of principal of this Note to be converted
                           (such amount to be converted referred to herein as
                           the "Converted Amount").

                           (b) Promptly upon receipt of the Conversion Notice,
                           Maker shall deliver to the Company (i) a certificate
                           or certificates of Maker's Common Stock representing
                           at least the number of shares issuable to Payee upon
                           conversion of the Converted Amount, duly endorsed in
                           blank or accompanied by a stock transfer power
                           executed in like manner, and (ii) a copy of the
                           Conversion Notice.


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                           (c) Upon its receipt of Maker's endorsed Common Stock
                           certificate(s) and the Conversion Notice, the Company
                           shall immediately issue and deliver to Payee or its
                           designated affiliates a certificate or certificates
                           for the number of shares of Common Stock, registered
                           in Payee's or its designated affiliates' name(s), to
                           which Payee shall be entitled upon such conversion,
                           bearing such legends as may be required by applicable
                           state and federal securities laws. The Company shall
                           issue to Maker a certificate representing any shares
                           surrendered by Maker in excess of the shares issued
                           to Payee upon conversion.

                           (d) If this Note is converted in whole, Payee shall
                           deliver this Note to Maker marked "Canceled," and
                           Maker shall immediately pay to Payee all accrued and
                           unpaid interest then due and owing on the date of
                           such conversion. If this Note is converted in part,
                           Maker shall immediately pay to Payee all accrued and
                           unpaid interest then due and owing on the date of
                           such conversion, and Payee shall deliver to Maker a
                           replacement Note for any outstanding principal amount
                           not converted, dated the date of such conversion,
                           with the same Maturity Date and provisions as
                           contained in this Note.

                           (e) No fractional shares will be issued on conversion
                           of this Note.

         5. Adjustment for Issuance of Shares at Less Than the Conversion Price.
If and whenever any Additional Common Stock (herein defined) shall be issued by
Maker (the "Stock Issue Date") for a consideration per share less than the
Conversion Price, then in each such case the initial Conversion Price shall be
reduced to a new Conversion Price in an amount equal to the price per share of
the Additional Common Stock then issued, if issued in connection with a sale of
shares, or the value of the Additional Common Stock then issued, as determined
in accordance with general accepted accounting principles, if issued other than
for cash, and the number of shares issuable to Payee upon conversion shall be
proportionately increased; and, in the case of Additional Common Stock issued
without consideration, the initial Conversion Price shall be reduced in amount
and the number of shares issued upon conversion shall be increased in an amount
so as to maintain for the Payee the right to convert this Note into shares equal
in amount to the same percentage interest in the Common Stock of the Company as
existed for the Payee immediately preceding the Stock Issue Date.

         6. Sale of Shares. In case of the issuance of Additional Common Stock
for a consideration part or all of which shall be cash, the amount of the cash
consideration therefor shall be deemed to be the gross amount of the cash paid
to Maker for such shares, before deducting any underwriting compensation or
discount in the sale, underwriting or purchase thereof by underwriters or
dealers or others performing similar services for any expenses incurred in
connection therewith. In case of the issuance of any shares of Additional Common
Stock for a consideration part or all of which shall be other than cash, the
amount of the consideration therefor, other than cash, shall be deemed to be the
then fair market value of the property received.

         7. Stock Dividends. Shares of Common Stock issued as a dividend or
other distribution on any class of capital stock of Maker shall be deemed to
have been issued without consideration.

         10. Stock Splits, Subdivisions or Combinations. In the event of a stock
split or subdivision of shares of Common Stock into a greater number of shares,
the Conversion Price shall be proportionately decreased, and in the event of a
combination of shares of Common Stock into a smaller number of


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shares, the Conversion Price shall be proportionately increased, such increase
or decrease, as the case may be, becoming effective at the record date.

         11. Exceptions. The term "Additional Common Stock" herein shall mean
all shares of Common Stock hereafter issued by Maker (including Common Stock
held in the treasury of Maker), except for (A) Common Stock issued upon the
conversion of this Note; (B) Common Stock issued upon exercise of any
outstanding warrants, options or convertible debt instruments; and (C) Common
Stock issued upon exercise of outstanding employee stock options.

         10. Adjustment for Mergers, Sales and Consolidations. In the event of
any consolidation or merger of the Company with or into, or the sale of all or
substantially all of the properties and assets of the Company, to any person,
and in connection therewith, consideration is payable to holders of Common Stock
in cash, securities or other property, then as a condition of such
consolidation, merger or sale, lawful provision shall be made, and duly executed
documents evidencing the same shall be delivered to the Payee, so that the Payee
shall have the right at any time prior to the maturity of this Note to purchase,
at a total price equal to the Conversion Price immediately prior to such event,
the kind and amount of cash, securities or other property receivable in
connection with such consolidation, merger or sale, by a holder of the same
number of shares of Common Stock as were exercisable by the Payee immediately
prior to such consolidation, merger or sale. In any such case, appropriate
provisions shall be made with respect to the rights and interest of the Payee so
that the provisions hereof shall thereafter be applicable with respect to any
cash, securities or property deliverable upon exercise hereof. Notwithstanding
the foregoing, (i) if the Company merges or consolidates with, or sells all or
substantially all of its property and assets to, any other person, and
consideration is payable to holders of Common Stock in exchange for their Common
Stock in connection with such merger, consolidation or sale which consists
solely of cash, or (ii) in the event of the dissolution, liquidation or winding
up of the Company, then the Payee shall be entitled to receive distributions on
the date of such event on an equal basis with holders of Common Stock as if this
Note had been converted immediately prior to such event, less the Conversion
Price. Upon receipt of such payment, if any, the rights of the Payee shall
terminate and cease and this Note shall expire. In case of any such merger,
consolidation or sale of assets, the surviving or acquiring person and, in the
event of any dissolution, liquidation or winding up of the Company, the Company
shall promptly, after receipt of this surrendered Note, make payment by
delivering a check in such amount as is appropriate (or, in the case of
consideration other than cash, such other consideration as is appropriate) to
such person as it may be directed in writing by the Payee surrendering this
Note.

         11. Adequate Shares. Maker will at times reserve and keep available,
for the purpose of issuance upon conversion, a sufficient number of shares of
Common Stock owned by Maker deliverable upon Payee's exercise of its conversion
rights under this Note.

         12. Default, Enforcement. Upon default in payment of this Note, Payee
may pursue any and all remedies to which Payee may be entitled.

         13. 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 or 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


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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 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.

         14. 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.

         15. 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.

         16. Registration Rights. The shares of Common Stock issued upon
conversion of this Note shall be restricted from transfer by the Payee, unless
the shares are duly registered for sale pursuant to the Securities Act of 1933,
as amended, or the transfer is exempt from registration. The Payee has certain
rights with respect to the registration of shares of Common Stock issued upon
the conversion of this note pursuant to the terms of the Registration Rights
Agreement between Payee and maker dated October 20, 2000.

         17. Security Agreement This Note is secured by the Security Agreement
among Maker, B&B Electromatic, Inc., Golston Company, Inc., Innovative Security
Technologies, Inc. (now named Intelli-Site, Inc.) and Tri-Coastal Systems, Inc.
and Payee and Renaissance US Growth and Income fund III, Inc., dated as of
October 2, 1998, and Payee is entitled to the rights and benefits thereunder.

         18. Stock Pledge Agreement. This Note is secured by the Stock Pledge
Agreement dated as of December 31, 1996, as amended by the First Amendment to
Stock Pledge Agreement, dated May 5, 2000, among Maker, Payee and Renaissance
Capital Growth & Income Fund III, Inc., and Payee is entitled to the rights and
benefits thereunder.

         19. Successors and Assigns. This Note shall bind Maker's successors and
assigns.

         20. 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
attorney's fees incurred by Payee.


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         21. 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.

         22. 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.

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

                                   INTEGRATED SECURITY SYSTEMS, INC.

                                   BY:    /s/ C. A. RUNDELL, JR.
                                          ----------------------
                                          C. A. Rundell, Jr.
                                          President and CEO


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