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                                                                    EXHIBIT 4.18

               FIRST AMENDMENT TO CREDIT AUTHORIZATION AGREEMENT


         THIS FIRST AMENDMENT TO CREDIT AUTHORIZATION AGREEMENT, dated as of
December 10, 1997 ( this "Amendment" ) by and between INTERFACE SYSTEMS, INC., a
Delaware Corporation, the ( "Borrower" ) and NBD BANK, a Michigan banking
corporation ( the "Bank" ).

                                    RECITALS

         The  Borrower and the Bank are parties to a Credit Authorization
              Agreement dated as of August 31, 1997, ( the "Agreement" ).

         A.   The Borrower has defaulted under the Agreement due to a breach of
              the NET WORTH covenant in Section 9.3(h), for the period ended
              September 30, 1997.

         B.   The Borrower has requested that the Bank waive such covenant
              default, and the Bank is willing to do so strictly in accordance
              with the terms hereof, and provided the Agreement is amended as
              set forth herein, and the Borrower has agreed to such amendment.

                                    AGREEMENT

                         Based upon these recitals, the parties agree as
follows:

         1.   Upon satisfaction of the conditions set forth in paragraph 4
              hereof, the Agreement shall hereby be amended as of the effective
              date hereof as follows:

                  A.  The definition of " Net Worth " in section 9.3(h) shall be
                      deleted in its entirety and the following shall be
                      inserted in place thereof:

                  Net Worth . Permit its Net Worth at any time to be less than
$9,700,000 and increasing by the sum of (A) $9,700,000 plus (B) 75% of quarterly
net income, without reduction for any loss periods, for each fiscal quarter,
commencing with the quarterly period ending December 31, 1997.

         2. From and after the effective date of this Amendment, references to
the "Agreement" in the Credit Authorization Agreement, the Note, the Security
Documents, and all other documents executed pursuant to the Credit Authorization
Agreement shall be deemed references to the Credit Authorization Agreement as
amended hereby.

         3. The Borrower represents to the Bank that:

               (a) (i) The execution, delivery and performance of this
                   amendment by the Borrower and all agreements and documents
                   delivered pursuant hereto by the Borrower have been duly
                   authorized by all necessary action and do not and will not
                   require any consent or approval of its shareholders,
                   violate any provision of any law, rule, regulation, order,
                   writ, judgment, injunction, decree, determination or award
                   presently in effect having applicability to it or of its
                   articles of incorporation or by-laws; (ii) no
                   authorization, consent, approval, license, exemption of or
                   filing a registration with any court or governmental
                   department, commission, board, bureau, agency or
                   instrumentality, domestic or foreign, is or will be
                   necessary to the valid execution, delivery, or performance
                   by the Borrower of this amendment and all agreements and
                   documents delivered pursuant hereto and (iii) this
                   Amendment and all agreements and documents delivered
                   pursuant hereto by the Borrower are the legal, valid
                   binding obligations of the Borrower enforceable against it
                   in accordance with the terms thereof.



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               (a) After giving effect to the amendment contained herein and
                   effected pursuant hereto, the representations and
                   warranties contained in Section 10.0 of the Agreement are
                   true and correct on and as of the effective date hereof
                   with the same force and effect as if made on and as of
                   such effective date.

               (a) Other than the Existing Default, as defined in and to be
                   waived pursuant to paragraph 5, no Events of Acceleration
                   ( as defined in Section 11.0 of the Agreement) and no
                   default shall have occurred and be continuing or will
                   exist under the Agreement as of the effective date hereof.

         4.   This Amendment shall not become effective until it shall be duly
              executed by the Borrower and the Bank and the Borrower shall have
              paid to the Bank an amendment fee of $10,000, which the Borrower
              acknowledges has been earned by the Bank .

         5.   The Borrower acknowledges that an Event of Acceleration has 
              occurred because the Borrower has breached a covenant contained in
              Section  9.3(h) of the  Agreement  for the period of the  Borrower
              ended September 30, 1997 ( the "Existing  Default" ). The Borrower
              acknowledges  that the  Bank has the  ability  to  accelerate  all
              indebtedness and exercise all of its rights and remedies under the
              Agreement. In consideration of the execution of this Amendment and
              subject to the satisfaction of the condition required by Paragraph
              4 hereof, the Bank agrees to waive the Existing Default,  provided
              that such waiver  shall waive only the  Existing  Default and does
              not waive any other  Events  of  Acceleration,  including  without
              limitation  any  future  Events  of  Acceleration  caused  by  any
              violation of Section 9.3(h). This waiver shall not be deemed to be
              a waiver,  or a consent to any  modification or amendment,  of any
              other term or condition of the  Agreement or any term or condition
              of any agreement,  instrument,  or document referred to therein or
              executed pursuant  thereto,  or to prejudice any present or future
              right or rights which the Bank now has or may have hereunder.

         6.   The terms used but not defined herein shall have the respective
              meanings ascribed thereto in the Agreement. Except as expressly
              contemplated hereby, the Agreement, and all related notes,
              guaranties, certificates, instruments and other documents, are
              hereby ratified and confirmed and shall remain in full force and
              effect, and the Borrower acknowledges that it has no defense,
              offset, or counterclaim thereunder.

         7.   This Amendment shall be governed by and in accordance with the 
              laws of the State of Michigan.

              IN WITNESS WHEREOF,  the parties hereto have caused this Amendment
to be duly executed and delivered as of the day and year first written above.





NBD BANK                                  INTERFACE SYSYEMS,  INC.

By: /S/ Mike Kelly                        By: /s/ John R. Ternes
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Its: First Vice President                 Its: Vice President Finance
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