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                                                                    EXHIBIT 4.20

                           CONVERTIBLE PROMISSORY NOTE

$50,000.00                                                     November 17, 2000


         For value received, INTEGRATED SECURITY SYSTEMS, INC., a Delaware
corporation (hereinafter referred to as "Maker"), promises to pay to the order
of C.A. RUNDELL, JR. an individual (hereinafter referred to as "Payee"), the
principal sum of Fifty Thousand and No/100 Dollars ($50,000.00). The principal
of and interest on this Note shall be due and payable in lawful money of the
United States of America, at the offices of Payee at 2200 Ross Avenue, Suite
4660 West, Dallas, Texas 75201, 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 $.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.

               (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


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               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, 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 cashed, the amount of the cash
consideration therefore 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 therefore, 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.

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

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         9. 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
provision 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 exceed the
maximum amount permissible under

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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 limits of validity prescribed by
law which a court of competent jurisdiction may deem applicable hereto the 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, 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.

         17. Security Agreement. This Note is secured by the accounts receivable
of Intelli-Site, Inc.

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

         19. 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.

         20. 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.

         21. 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:
                                      ------------------------------------------
                                      Holly J. Burlage
                                      Executive Vice President and
                                      Chief Financial Officer