Document:

<PAGE>   1
                                                                    EXHIBIT 4.18

                                 PROMISSORY NOTE

$200,000.00                                                       April 12, 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 Two Hundred Thousand and no/100 Dollars ($200,000.00). 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 nine percent (9%). 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.

         2. Payment of Note. The principal balance of and all accrued unpaid
interest on this Note shall be due and payable on demand.

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

         4. 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 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.

<PAGE>   2

         5. 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.

         6. 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.

         7. Security Agreement This Note is secured by the accounts receivable
of Intelli-Site, Inc. due from Coken Company delineated as follows:

                        COKEN COMPANY ACCOUNTS RECEIVABLE

<TABLE>
<CAPTION>
             INVOICE DATE                           INVOICE NUMBER                         INVOICE AMOUNT
             ------------                           --------------                         --------------
<S>                                                 <C>                                    <C>
                4-9-99                                   5066                                $  29,368.73
                5-19-99                                  5072                                   12,000.00
                9-20-99                                  5119                                   20,000.00
               10-15-99                                  5136                                   12,053.82
               12-20-99                                  5166                                   25,000.00
                4-4-00                                   5065                                  122,500.00
                 Total                                                                       $ 220,922.55
</TABLE>

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

         9. 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.

         10. 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.

         11. 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:
                                     -------------------------------------------
                                     Gerald K. Beckmann
                                     President and CEO<PAGE>   1
                                                                    EXHIBIT 4.19

                                 PROMISSORY NOTE

$100,000.00                                                        June 28, 2000

         For value received, INTEGRATED SECURITY SYSTEMS, INC., a Delaware
corporation (hereinafter referred to as "Maker"), promises to pay to the order
of THE RUNDELL FOUNDATION, an individual (hereinafter referred to as "Payee"),
the principal sum of One Hundred Thousand and no/100 Dollars ($100,000.00). 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 nine percent (9%). 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.

         2. Payment of Note. The principal balance of and all accrued unpaid
interest on this Note shall be due and payable on demand.

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

         4. 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 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.

<PAGE>   2

         5. 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.

         6. 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.

         7. Security Agreement. This Note is secured by the accounts receivable
of Intelli-Site, Inc. due from Coken Company.

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

         9. 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.

         10. 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.

         11. 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:
                                     -------------------------------------------
                                     Gerald K. Beckmann
                                     President and CEO

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