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                                                                    EXHIBIT 10.1

                THIRD AMENDMENT TO CREDIT AUTHORIZATION AGREEMENT
                -------------------------------------------------

         THIS THIRD AMENDMENT TO CREDIT AUTHORIZATION AGREEMENT, dated as of May
20, 1998 ( this "Amendment" ) by and between INTERFACE SYSTEMS, INC., a Delaware
Corporation, the ("Borrower" ), and NBD BANK, a Michigan banking corporation 
(the "Bank" ).

                                    RECITALS
                                    --------

         A.   The Borrower and the Bank are parties to a Credit Authorization
              Agreement dated as of August 31, 1997, as amended by a First
              Amendment To Credit Authorization Agreement dated as of December
              10, 1997 and a Letter Agreement dated March 11, 1998 (as amended
              the "Agreement" ).

         B.   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
              March 31, 1998.

         C.   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.  Section 3.1 "Borrowing Base" definition shall be amended
                      to include the following subsection 3.1(C):

                  C.  100% of the Borrower's "refundable income taxes" as
                      recorded on its balance sheet as of the fiscal quarter
                      ended March 31, 1998. "Refundable income taxes" will cease
                      to be an addition to the Borrowing Base upon collection of
                      the amount recorded at March 31, 1998.

                  B.  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 $7,300,000 and increasing by the sum of (A)
                      $7,300,000 plus (B) 75% of quarterly net income, without
                      reduction for any loss periods, for each fiscal quarter,
                      commencing with the quarterly period ending June 30,
                      1998.

                  C.  Section 4.2 "Out-of-Pocket Expenses is amended to
                      include the addition of the following sentences:

                  "Furthermore, Borrower also acknowledges and agrees that it
                  shall be responsible to pay all fees, expenses, and
                  out-of-pocket disbursements incurred by NBD, including legal
                  fees and expenses, including, without limitation: (1) fees
                  incurred in connection with audits by NBD of 


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                  the Borrower's facilities, books and records or the
                  collateral, which audits NBD may conduct as frequently as it
                  reasonably desires; (2) all fees and expenses of NBD and
                  counsel for NBD for the preparation, examination, approval,
                  negotiation, execution, and delivery of this Agreement; (3)
                  all fees and out-of-pocket disbursements incurred, in any way
                  arising from or in connection with any action taken by NBD to
                  monitor, advise, administer, enforce or collect any of the
                  obligations of the Borrower under the Agreement; and (4) all
                  costs, expenses, and fees incurred by NBD or its counsel in
                  connection with consultants, expert witnesses or other
                  professionals retained by NBD or its counsel , to assist,
                  advise, appraise, or give testimony with respect to any matter
                  relating to the collateral, the obligations, the Loan
                  Documents, or the business relationship between NBD, on the
                  one hand, and the Borrower, on the other hand. All of the
                  foregoing costs, expenses, reimbursement obligations , and
                  indemnification obligations are part of the borrowings and
                  other obligations and are secured by all of the Collateral.

         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.

                  (b) 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.

                  (c) 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.


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         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 March 31, 1998 ( 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/ Michael Kelly                 By:     /S/ John Ternes
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     Its:   First Vice President              Its:     Chief Financial Officer
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