<PAGE>
 
                                                                   EXHIBIT 10.30
                                PROMISSORY NOTE
                                  (BASE RATE)

BORROWER NAME: RESEARCH ENGINEERS, INC
BORROWERS ADDRESS
22700 SAVI RANCH PARKWAY
YORBA LINDA, CA 92887

ORANGE, CALIFORNIA                  $500.000.00          DATE: SEPTEMBER 2, 1996

FOR VALUE RECEIVED, ON APRIL 1, 2000, THE UNDERSIGNED ("DEBTOR") PROMISES TO PAY
TO THE ORDER OF UNION BANK OF CALIFORNIA, N.A. ("BANK"), AS INDICATED BELOW, THE
PRINCIPAL SUM OF FIVE HUNDRED THOUSAND AND N0/100 DOLLARS ($500.000.00), OR SO
MUCH THEREOF AS IS DISBURSED, TOGETHER WITH INTEREST ON THE BALANCE OF SUCH
PRINCIPAL FROM TIME TO TIME OUTSTANDING, AT THE PER ANNUM RATES AND AT THE TIMES
SET FORTH BELOW.
1. PAYMENTS.

   PRINCIPAL PAYMENTS. Principal shall be payable in 36 equal consecutive
   monthly installments, each installment in an amount sufficient to fully
   amortize the principal balance by the final maturity date, beginning MAY 1,
   1997 and continuing on the 1ST day of each consecutive month.

   INTEREST PAYMENTS. Debtor shall pay interest on the 1ST day of each MONTH
   (commencing DECEMBER 1, 1996). Should interest not be paid when due, it shall
   become part of the principal and bear interest as herein provided. All
   computations of interest under this note shall be made on the basis of a year
   of 360 days, for actual days elapsed.

   a. BASE INTEREST RATE. At Debtor's option, amounts outstanding hereunder in
      increments of at least $10.000 shall bear interest at a rate to be
      selected by Debtor which is 2.25% per annum in excess of Bank's Adjusted
      LIBOR-Rate for the Interest Period so selected by Debtor.

      Any Base Interest Rate selected by Debtor may not be changed, altered or
      otherwise modified until the expiration of the Interest Period for which
      it was selected. The exercise of interest options by Debtor shall be as
      recorded in Bank's records, which records shall be prima facie evidence of
      the amount borrowed under either interest option and the interest rate;
      provided, however, that failure of Bank to make any such notation in its
      records shall not discharge Debtor from its obligations to repay in full
      with interest all amounts borrowed. In no event shall any Interest Period
      extend beyond the maturity date of this note.

      To select a Base Interest Rate, Debtor may, from time to time with respect
      to principal outstanding on which a Base Interest Rate has not been
      selected and on the expiration of any Interest Period with respect to
      principal outstanding on which a Base Interest Rate has been selected,
      select a Base Interest Rate by telephoning an authorized lending officer
      of Bank located at the banking office identified below prior to 10:00
      a.m., California time on any Business Day and advising that officer of the
      Base Interest Rate, the Interest Period and the Origination Date selected
      (which Origination Date, for a Base Interest Rate Loan based on the
      Adjusted LIBOR-Rate, shall follow the date of such election by no more
      than two (2) Business Days).

      Bank will confirm the terms of the election in writing by mail to Debtor
      promptly after the election is made. Failure to send such confirmation
      shall not affect Bank's rights to collect interest at the rate selected.
      If on the date of the election, the Base Interest Rate selected is
      unavailable for any reason, the selection shall be void. Bank reserves the
      right to fund the principal from any source of funds notwithstanding any
      Base Interest Rate selected by Debtor.

   b. VARIABLE INTEREST RATE. All principal outstanding hereunder which is not
      bearing interest at a Base Interest Rate shall bear interest at a rate per
      annum of .50% in excess of the Reference Rate, which rate shall vary as
      and when the Reference Rate changes. Debtor shall pay all amounts due
      under this
<PAGE>
 
     note in lawful money of the United States a Bank's ORANGE COVNTY COMMERCIAL
     BANKING Office, or such other office as may be designated by Bank, from
     time to time.

2. LATE PAYMENTS.  If any payment required by the terms of this note shall
   remain unpaid ten days after same is due, at the option of the Bank. Debtor
   shall pay a fee of $100 to Bank.

3. INTEREST RATE FOLLOWING DEFAULT. In the event of default, at the option of
   Bank, and, to the extent permitted by law, interest shall be payable on the
   outstanding principal under this note at a per annum rate equal to five
   percent (5%) in excess of the interest rate specified in paragraph l.b,
   above, of this note, calculated from the date of default until all amounts
   payable under this note are paid in full.

4. PREPAYMENT.

  a. Amounts outstanding under this note bearing interest at a rate based on the
     Reference Rate may be prepaid in whole or in part at any time, without
     penalty or premium. Amounts outstanding at a Base Interest Rate under this
     note may only be prepaid, in whole or in part provided Bank has received
     not less than five (5) Business Days prior written notice of an intention
     to make such prepayment and Debtor pays a prepayment fee to Bank in an
     amount equal to: (i) the difference between (a) the Base Interest Rate
     applicable to the principal amount which Debtor intends to prepay, and (b)
     the return which Bank could obtain if it used the amount of such prepayment
     of principal to purchase at bid price regularly quoted securities issued by
     the United States having a maturity date most closely coinciding with the
     relevant Base Rate Maturity Date and such securities were held by Bank
     until the relevant Base Rate Maturity Date ("Yield Rate"); (ii) the above
     difference. if greater than zero. is multiplied by a fraction. the
     numerator of which is the number of days in the period between the date of
     prepayment and the relevant Base Rate Maturity Date and the denominator of
     which is 360 days; (iii) the above product is multiplied by the amount of
     the principal so prepaid (except in the event that principal payments are
     required and have been made as scheduled under the terms of the Base
     Interest Rate Loan being prepaid. then the amount multiplied in this
     section shall be the lesser of the amount prepaid or 50% of the total of
     the amount prepaid and the amount of principal scheduled under the terms of
     the Base Interest Rate Loan being prepaid to be outstanding at the relevant
     Base Rate Maturity Date): and (iv) the above product is then discounted to
     present value using the Yield Rate as the annual discount factor.

  b. In no event shall Bank be obligated to make any payment or refund to
     Debtor, nor shall Debtor be entitled to any setoff or other claim against
     Bank, should the return which Bank could obtain under the above prepayment
     formula, exceed the interest that Bank would have received if no prepayment
     had occurred. All prepayments shall include payment of accrued interest on
     the principal amount so prepaid and shall be applied to payment of interest
     before application to principal. A determination by Bank as to the
     prepayment fee amount, if any, shall be conclusive. In the event of partial
     prepayment, such prepayments shall be applied to principal payments in the
     inverse order of their maturity.

  c. Such prepayment fee, if any, shall also be payable if prepayment occurs as
     the result of the acceleration of the principal of this note by Bank
     because of any default hereunder. If, following such acceleration, all or
     any portion of a Base Interest Rate Loan is satisfied, whether through sale
     of property encumbered by a security agreement or of her agreement securing
     this note, if any, at a foreclosure sale held thereunder or through the
     tender of payment any time following such acceleration. but prior to such a
     foreclosure sale, then such satisfaction shall be deemed an evasion of the
     prepayment conditions set forth above, and Bank shall, automatically and
     without notice or demand. be entitled to receive, concurrently with such
     satisfaction the prepayment fee set forth above, and the obligation to pay
     Each prepayment fee shall be added to the principal. DEBTOR HEREBY
     ACKNOWLEDGES AND AGREES THAT BANK WOULD NOT LEND TO DEBTOR THE LOAN
     EVIDENCED BY THIS NOTE WITHOUT DEBTOR'S AGREEMENT. AS SET FORTH ABOVE. TO
     PAY BANK A PREPAYMENT FEE UPON THE SATISFACTION OF ALL OR ANY PORTION OF
     THE PRINCIPAL BEARING INTEREST AT A BASE INTEREST RATE FOLLOWING THE
     ACCELERATION OF THE MATURITY DATE HEREOF BY REASON OF A DEFAULT. DEBTOR HAS
     CAUSED THOSE 
<PAGE>
 
     PERSONS SIGNING THIS NOTE ON ITS BEHALF TO SEPARATELY INITIAL THE AGREEMENT
     CONTAINED IN THIS PARAGRAPH BY PLACING THEIR INITIALS BELOW:

5. DEFAULT AND ACCELERATION OF TIME FOR PAYMENT. Default shall include. but not
   be limited to, any of the following: (a) the failure of Debtor to make any
   payment required under this note when due; (b) any breech, misrepresentation
   or other default by Debtor, any guarantor, co-maker, endorser, or any person
   or entity other than Debtor providing security for this note (hereinafter
   individually and collectively referred to as the "Obligor") under any
   security agreement. guaranty or other agreement between Bank and any Obligor;
   (c) the insolvency of any Obligor or the failure of any Obligor generally to
   pay such Obligor's debts as such debts become due; (d) the commencement as to
   any Obligor of any voluntary or involuntary proceeding under any laws
   relating to bankruptcy, insolvency, reorganization, arrangement, debt
   adjustment or debtor relief; (e) the assignment by any Obligor for the
   benefit of such Obligor's creditors; (f) the appointment, or commencement of
   any proceeding for the appointment of a receiver, trustee, custodian or
   similar official for all or substantially all of any Obligor's property; (g)
   the commencement of any proceeding for the dissolution or liquidation of any
   Obligor; (h) the termination of existence or death of any Obligor; (i) the
   revocation of any guaranty or subordination agreement Given in connection
   with this note; (j) the failure of any Obligor to comply with any order,
   judgement, injunction, decree, writ or demand of any court or other public
   authority; (k) the filing or recording against any Obligor. or the property
   of any Obligor of any notice of levy, notice to withhold, or other legal
   process for taxes other than property taxes; (l) the default by any Obligor
   personally liable for amounts owed hereunder on any obligation concerning the
   borrowing of money; (m) the issuance against any Obligor. or the property of
   any Obligor, of any writ of enactment, execution, or other judicial lien; or
   (n) the deterioration of the financial condition of any Obligor which results
   in Bank deeming itself, in good faith, insecure. Upon the occurrence of any
   such default, Bank, in its discretion, may cease to advance funds hereunder
   and may declare all obligations under this note immediately due and payable;
   however, upon the occurrence of an event of default under d, e, f, or g, all
   principal and interest shall automatically become immediately due and
   payable.

6. ADDITIONAL AGREEMENTS OF DEBTOR. If any amounts owing under this note are not
   paid when due. Debtor promises to pay all costs end expenses including
   reasonable attorneys' fees, incurred by Bank in the collection or enforcement
   of this note. Debtor and any endorsers of this note, for the maximum period
   of time and the full extent permitted by law, (a) waive diligence,
   presentment, demand, notice of nonpayment, protest, notice of protest, and
   notice of every kind; (b) waive the right to assert the defense of any
   statute of limitations to any debt or obligation hereunder; and (c) consent
   to renewals and extensions of time for the payment of any amounts due under
   this note. If this note is signed by more than one party, the term "Debtor"
   includes each of the undersigned and any successors in interest thereof; all
   of whose liability shall be joint end several. Any married person who signs
   this note agrees that recourse may be had against the separate property of
   that person for any obligations hereunder. The receipt of any check or other
   item of payment by Bank, at its option, shall not be considered a payment on
   account until such check or other item of payment is honored when presented
   for payment at the drawee bank.  Bank may delay the credit of such payment
   based upon Bank's schedule of funds availability, and interest under this
   note shall accrue until the funds are deemed collected. In any action brought
   under or arising out of this note. Debtor and any Obligor, including their
   successors or assigns, hereby consent to the jurisdiction of any competent
   court within the State of California, as provided in any alternative dispute
   resolution agreement executed between Debtor and Bank, and consent to service
   of process by any means authorized by California law. The term "Bank"
   includes, without limitation, any holder of this note. This note shall be
   construed in accordance with and governed by the laws of the State of
   California.  This note hereby incorporates any alternative dispute resolution
   agreement previously, concurrently or hereafter executed between Debtor and
   Bank.

7. DEFINITIONS. As used herein, the following terms shell have the meanings
   respectively set forth below: "Adjusted LIBOR-Rate" shall mean the LIBOR Base
   Rate as adjusted for reserve requirements imposed on Bank from time to time.
   "Base Interest Rate" shall mean a rate of interest based on the Adjusted
   LIBOR-Rate. "Base Interest Rate Loan" shall mean amounts outstanding under
   this note that bear interest at a Base Interest Rate. "Base Rate Maturity
   Date" shall mean the last day of the Interest Period with respect to
   principal outstanding on which a Base Interest Rate has been selected by
   Debtor. "Business Day" shall mean a day which is not a Saturday or Sunday on
   which Bank is open for business in California and on which 
<PAGE>
 
   dealings in U.S. dollar deposits outside of the United States may be carried
   on by Bank. "Interest Period" shall mean any calendar period of one, three,
   six, nine or twelve months. In determining an Interest Period, a month means
   a period that starts on one Business Day in a month and ends on and includes
   the day preceding the numerically corresponding day in the next month. For
   any month in which there is no such numerically corresponding day, then as to
   that month, such day shall be deemed to be the last calendar day of such
   month. Any Interest Period which would otherwise end on a non-Business Day
   shall end on the next succeeding Business Day unless that is the first day of
   a month, in which event such Interest Period shall end on the next preceding
   Business Day. "LIBOR Base Rate" shall mean for each Interest Period the rate
   per annum (rounded upward, if necessary to the nearest 1/100 of 1%) at which
   dollar deposits, in immediately available funds and in lawful money of the
   United States would be offered to Bank, outside of the United States, for a
   term coinciding with such Interest Period and for an amount equal to the
   amount of principal covered by Debtor's interest rate election. "Origination
   Date" shall mean the Business Day on which funds are made available to Debtor
   relating to Debtor's selection of a Base Interest Rate. "Reference Rate"
   shall mean the rate announced by Bank from time to time at its corporate
   headquarters at its "Reference Rate." The Reference Rate is an index rate
   determined by Bank from time to time as a means of pricing certain extensions
   of credit and is neither directly tied to any external rate of interest or
   index not necessarily the-lowest rate of interest charged by Bank at any
   given time.

  RESEARCH ENGINEERS, INC.

  By:  /S/ Brian Paul

  Title: CFO