<PAGE>

                                                                EXHIBIT 10.28

                               THIRTEENTH AMENDMENT TO
                   AMENDED AND RESTATED LOAN AND SECURITY AGREEMENT
                                     AND CONSENT

     THIS THIRTEENTH AMENDMENT TO AMENDED AND RESTATED LOAN AND SECURITY
AGREEMENT AND CONSENT (the "Amendment") is dated as of January 8, 1999, and is
by and among AMERICABLE, INC., a Minnesota corporation ("Americable")
(Americable is hereinafter referred to as the "Original Borrower"), ENSTAR
NETWORKING CORPORATION, a Minnesota corporation ("ENC") (Americable and ENC are
hereinafter each individually referred to as an "Borrower" and collectively as
the "Borrowers") and U.S. BANK NATIONAL ASSOCIATION, f/k/a First Bank National
Association, (the "Bank").  Capitalized terms not otherwise expressly defined
herein shall have the meanings set forth in the Loan Agreement hereinafter
described.

                                      RECITALS

     WHEREAS, the Borrowers and the Bank are parties to an Amended and Restated
Loan and Security Agreement, dated as of June 1, 1993, as amended by a
First Amendment dated as of November 29, 1993, a Waiver, a Second Amendment
dated March 3, 1995, a Third Amendment dated May 31, 1996, a letter amendment
dated June 28, 1996, a letter amendment dated July 31, 1996, a Sixth Amendment
dated as of August 9, 1996, a Seventh Amendment dated as of May 29, 1997, an
Eighth Amendment dated as of September 30, 1997, a Ninth Amendment dated as of
March 5, 1998, a Tenth Amendment dated as of June 30, 1998, an Eleventh
Amendment dated as of August 31, 1998 and a Twelfth Amendment dated as of
November 30, 1998 (as so amended, the "Loan Agreement");

     WHEREAS, the Borrowers have requested and the Bank consent to ENC's sale of
a portion of its Accounts Receivable, Inventory and other Assets to Vicom, Inc.
("Vicom") pursuant to that certain Asset Purchase Agreement dated as of December
28, 1998 (the "Vicom Asset Purchase Agreement") between ENC and Vicom; and

     WHEREAS, the Bank is willing to do so provided that certain terms of the
Loan Agreement are further amended and the Borrowers are willing to futher
amend the Loan Agreement.  

     NOW, THEREFORE, in consideration of the foregoing and of the mutual
covenants and agreements contained herein, and for other good and valuable
consideration, the receipt and sufficiency of which is hereby acknowledged, the
parties hereto agree as follows:

                         ARTICLE 1 - AMENDMENTS TO THE LOAN AGREEMENT

     1.1  Amendments to Loan Agreement.  As of the effective date of this
Amendment, the Loan Agreement is amended as follows:

          (a)   The definitions of "Borrower" and "Borrowers" set forth in the
     preamble to the Loan Agreement are respectively amended in their entireties



                                 -1-






<PAGE>

     to read as follows:

                "Americable and ENC are hereinafter each individually referred
          to as a  'Borrower' and collectively as the 'Borrowers'."

          (b)   Section 6.2 of the Loan Agreement is amended in its entirety to
     read as follows:

                "6.2  Sale of Assets.  Sell, transfer, convey, lease, assign or
          otherwise dispose (with or without recourse) of: (a) any of the
          1,025,000 shares of CorVel Corporation's common stock (the 'CorVel
          Stock') owned by Americable; or (b) any of the shares of Vicom, Inc.
          common stock (the 'Vicom Stock') acquired by ENC as part of the
          consideration of the sale of certain of its assets to Vicom pursuant
          to that certain Asset Purchase Agreement dated as of December 28, 1998
          (the "Vicom Asset Purchase Agreement") between ENC and Vicom, which
          Vicom Stock includes the 1,350,000 shares of Vicom Stock originally
          acquired by ENC and any additional shares of Vicom Stock acquired by
          ENC pursuant to the antidilution provisions of the Vicom Asset
          Purchase Agreement; or (c) any of its other assets (including, without
          limitation, any Accounts Receivable, instruments or chattel paper)
          except for sales and leases of Inventory in the ordinary course of
          business."

     1.2  Construction.  All references in the Loan Agreement to "this
Agreement," "herein" and similar references shall be deemed to refer to the Loan
Agreement as amended by this Amendment.

                       ARTICLE II - REPRESENTATIONS AND WARRANTIES

          To induce the Bank to enter into this Amendment and to make and
maintain the Loans under the Loan Agreement as amended hereby, the Borrowers
hereby warrant and represent to the Bank that: (a) The execution, delivery
and performance by the Borrowers of this Amendment and any other documents to
which the Borrower is a party have been duly authorized by all necessary
corporate or partnership action, do not require any approval or consent of,
or any registration, qualification or filing with, any government agency or
authority or any approval or consent of any other person (including, without
limitation, any stockholder or partner), do not and will not conflict with,
result in any violation of or constitute any default under, any provision of
the Borrower's articles of incorporation or bylaws, any agreement binding on
or applicable to such Borrower or any of its property, or any law or
governmental regulation or court decree or order, binding upon or applicable,
to such Borrower or of any of its property and will not result in the creation
or imposition of any security interest or other lien or encumbrance in or on
any of its property pursuant to the provisions of any agreement applicable to
the Borrower or any of its property; (b) The Loan Agreement as amended by this
Amendment is the legal, valid and binding obligations of each Borrower and is






                                 -2-






<PAGE>

enforceable in accordance with its terms, subject only to bankruptcy,
insolvency, reorganization, moratorium or similar laws, rulings or decisions
at the time in effect affecting the enforceability of rights or creditors
generally and to general equitable principles which may limit the right to 
obtain equitable remedies; and (c) the Borrowers have provided the Bank with
true, correct and complete copies of the Vicom Asset Purchase Agreement and
all amendments, modifications or supplements thereto.

                      ARTICLE III - CONDITIONS AND EFFECTIVENESS

     This Amendment shall become effective on the date first set forth above,
provided, however, that the effectiveness of this Amendment is subject to the
satisfaction of each of the following conditions:

          3.1  Before and after giving effect to this Amendment, the
     representations and warranties in ARTICLE IV of the Loan Agreement shall
     be true and correct as though made on the date hereof except for changes
     that are permitted by the terms of the Loan Agreement.  The execution by
     the Borrowers of this Amendment shall be deemed a representation that the
     Borrowers have complied with the foregoing condition.

          3.2  Before and after giving effect to this Amendment, no Event of
     Default or no Default, shall have occurred and be continuing under the
     Loan Agreement except for those expressly waived by the terms hereof.  The
     execution by the Borrowers of this Amendment shall be deemed a
     representation that the Borrowers have complied with the foregoing 
     condition.

          3.3  The Bank shall have received the following documents
     appropriately completed and duly executed by the Borrowers and the other
     Obligors where appropriate and shall have received the other items 
     described herein:

               (a)   This Amendment appropriately completed and duly executed by
     the Borrowers, together with a resolution of each of the Borrowers
     authorizing the execution and delivery of this Amendment in form and
     substance satisfactory to the Bank;

               (b)   An Acknowledgement and Consent duly executed by ENStar in
     form and substance satisfactory to the Bank;

               (c)   Copies of the Vicom Asset Purchase Agreement and all
     amendments, modifications or supplements thereto certified by ENC's
     secretary to be true, correct and complete copies thereof, and

               (d)   Such other approvals, opinions or documents, as the Bank
     may reasonably request.






                                 -3-







<PAGE>

                              ARTICLE IV - GENERAL

          4.1  Expenses.  The Borrowers jointly and severally agree to reimburse
the Bank upon demand for all reasonable expenses, including reasonable fees of
attorneys (who may be employees of the Bank) and legal expenses incurred by the
Bank in the preparation, negotiation and execution of this Amendment and any
other document required to be furnished herewith, and in enforcing the
obligations of the Borrowers hereunder, and to jointly and severally pay and
save the Bank harmless from all liability for, any stamp or other taxes which
may be payable with respect to the execution or delivery of this Amendment,
which obligations of the Borrowers shall survive any termination of the Loan
Agreement.

          4.2  Counterparts.  This Amendment may be executed in as many
     counterparts as may be deemed necessary or convenient, and by the
     different parties hereto on separate counterparts, each of which, when so
     executed, shall be deemed an original but all such counterparts shall
     constitute but one and the same instrument.

          4.3  Severability.  Any provision of this Amendment which is
     prohibited or unenforceable in any jurisdiction shall, as to such
     jurisdiction, be ineffective to the extent of such prohibition or
     unenforceability without invalidating the remaining portions hereof or
     affecting the validity or enforceability of such provisions in any other
     jurisdiction.

          4.4  Law.  This Amendment shall be a contract made under the laws of
     the State of Minnesota, which laws shall govern all the rights and duties
     hereunder.

          4.5  Successors: Enforceability.  This Amendment shall be binding
     upon the Borrowers and the Bank and their respective successors and
     assigns, and shall inure to the benefit of the Borrowers and the Bank and
     the successors and assigns of the Bank.  Except as hereby amended, the
     Loan Agreement shall remain in full force and effect and is hereby
     ratified and confirmed in all respects.

          4.6  Recitals.  The recitals hereto are incorporated herein by
     reference and constitute and integral part of this Amendment.

          4.7  Acknowledgement and Release.  In order to induce the Bank
     to enter into this Amendment, the Borrowers: (a) represent and warrant
     to the Bank that no events have taken place and no circumstances exist
     at the date hereof which would give the Borrowers the right to assert a
     defense, offset or counterclaim to any claim by the Bank for payment of
     the Obligations; and (b) hereby releases and forever discharges the Bank
     and its successors, assigns, directors, officers, agents, employees and
     participants from any and all actions, causes of action, suits,
     proceedings, debts, sums of money, covenants, contracts, controversies,
     claims and demands, at law or in equity, which the Borrower ever had or
     now has against the Bank or its successors, assigns, directors, officers,
     agents, employees or participants by virtue of their relationship to the
     Borrower in connection with the Loan Documents and the transactions
     related thereto.

                                 -4-






<PAGE>

          4.8   Consent.  The Bank hereby consents to ENC's sale of a portion of
     its Accounts Receivable, Inventory and other Assets to Vicom pursuant to
     the Vicom Asset Purchase Agreement and in accordance with the terms thereof
     as set forth in the copies thereof delivered to the Bank pursuant to
     Section 3.3 hereof and as further amended, modified or supplemented with
     the Bank's prior written consent.  This Consent is limited to the specific
     transaction described above and is not intended, and shall not be
     construed, to be a consent to any other transaction or a general waiver of
     any term or provision of the Loan Agreement.

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be executed by their respective officers thereunto duly authorized as of the 
date first written above.

                                     AMERICABLE, INC., a Minnesota Corporation

                                     By:  /s/ Peter E. Flynn
                                          ----------------------
                                          Peter E. Flynn
                                          President


                                     ENSTAR NETWORKING CORPORATION,
                                     a Minnesota Corporation

                                     By:  /s/ Thomas S. Wargolet
                                          ----------------------
                                          Thomas S. Wargolet
                                          Chief Financial Officer


                                     U.S. BANK NATIONAL ASSOCATION,
                                     f/k/a First Bank National Association

                                     By:  /s/ Kim Leppanen
                                          ----------------------
                                          Kim Leppanen
                                          Assistant Vice President