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                                                                    EXHIBIT 10.3


                     AMENDED AND RESTATED PROMISSORY NOTE


$12,500,000.00                     Norwood, Massachusetts
                                   December 13, 1995
                                   (Originally dated March 22, 1995)


        For value received, the undersigned, MICROCOM, INC. a Massachusetts
corporation (the "BORROWER"), promises to pay to BayBank, N.A. (the "BANK") at
the office of the Bank located at 7 New England Executive Park, Burlington, MA
01803, or to its order, the lesser of Twelve Million Five Hundred Thousand
Dollars ($12,500,000.00) or the outstanding principal amount hereunder, on
November 5, 1996 (the "MATURITY DATE"), together with interest on the principal
amount hereof from time to time outstanding at a fluctuating rate per annum
equal to the Prime Rate (as defined below) until the Maturity Date, payable
monthly in arrears on the fifth day of each calendar month occurring after the
date hereof and on the Maturity Date.  The Borrower promises to pay on demand
interest at a per annum rate of interest equal to the Prime Rate plus 4% on any
overdue principal (and to the extent permitted by law, overdue interest).  The
"Prime Rate" is the per annum rate of interest from time to time announced and
made effective by the Agent (as defined in the Credit Agreement) as its Prime
Rate (which rate may or may not be the lowest rate available from the Bank at
any given time).

        Computations of interest shall be made by the Bank on the basis of a
year of 360 days for the actual number of days occurring in the period for
which such interest is payable.

        This promissory note amends and restates the terms and conditions of
the obligations of the Borrower under the promissory note dated March 22, 1995
(the "ORIGINAL NOTE") by the Borrower to the Bank.  This promissory note is one
of the Notes referred to in the amended and restated credit agreement dated as
of March 22, 1995, as previously amended as of April 28, 1995, June 21, 1995,
September 14, 1995 and December 13, 1995, (together with all related schedules,
as the same may be amended, modified or supplemented from time to time, the
"CREDIT AGREEMENT") by and among the Borrower, the Lenders named therein
(including the Bank), and Silicon Valley Bank in its capacity as agent for the
Lenders (together with its successors in such capacity, the "Agent"), and is
subject to optional and mandatory prepayment as provided therein, and is
entitled to the benefits thereof and of the other Loan Documents referred to
therein.  The Credit Agreement provides for the acceleration of the maturity of
this Note in certain events.

        Payment of principal and interest on this Note is unconditionally
guaranteed pursuant to the Guaranty dated as of March 22, 1995 by Microcom
Systems, Inc., a Delaware corporation, in favor of the Agent for the benefit of
the Lenders.

        Each reference in each Loan Document (as defined in the Credit
Agreement) to "the Note", "thereof", "therein", "thereunder", or words of like
import referring to the Original Note, shall mean and be a reference to the
Original Note, as amended and restated hereby.

        The Bank shall keep a record of the amount and the date of the making
of each advance pursuant to the Credit Agreement and each payment of principal
with respect thereto by maintaining a computerized record of such information
and printouts of such computerized record, which computerized record, and the
printouts thereof, shall constitute PRIMA FACIE evidence of the accuracy of the
information so endorsed.

        The undersigned agrees to pay all reasonable costs and expenses of the
Agent and the Bank (including, without limitation, the reasonable fees and
expenses of attorneys) in connection with the enforcement of this Note and the
other Loan Documents and the preservation of their respective rights hereunder
and thereunder.





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        No delay or omission on the part of the Bank in exercising any right
hereunder shall operate as a waiver of such right or of any other right of the
Bank, nor shall any delay, omission or waiver on any one occasion be deemed a
bar to or waiver of the same or any other right on any future occasion.  The
Borrower and every endorser or guarantor of this note regardless of the time,
order or place of signing waives presentment, demand, protest and notices of
every kind and assents to any one or more extensions or postponements of the
time of payment or any other indulgences, to any substitutions, exchanges or
releases of collateral for this note, and to the additions or releases of any
other parties or persons primarily or secondarily liable.

        THE BORROWER HEREBY EXPRESSLY WAIVES ANY RIGHT IT MAY NOW OR HEREAFTER
HAVE TO A JURY TRIAL IN ANY SUIT, ACTION OR PROCEEDING  WHICH ARISES OUT OF OR
BY REASON OF THIS NOTE, ANY LOAN DOCUMENT (AS DEFINED IN THE CREDIT AGREEMENT),
OR THE TRANSACTIONS CONTEMPLATED HEREBY.

        BY ITS EXECUTION AND DELIVERY OF THIS NOTE, THE BORROWER ACCEPTS FOR
ITSELF AND IN CONNECTION WITH ITS PROPERTIES, GENERALLY AND UNCONDITIONALLY,
THE NON-EXCLUSIVE JURISDICTION OF ANY STATE OR FEDERAL COURT OF COMPETENT
JURISDICTION IN THE COMMONWEALTH OF MASSACHUSETTS OR THE STATE OF CALIFORNIA IN
ANY ACTION, SUIT OR PROCEEDING OF ANY KIND AGAINST IT WHICH ARISES OUT OF OR BY
REASON OF THIS NOTE, ANY LOAN DOCUMENT (AS DEFINED IN THE CREDIT AGREEMENT), OR
THE TRANSACTIONS CONTEMPLATED HEREBY, IN ADDITION TO ANY OTHER COURT IN WHICH
SUCH ACTION, SUIT OR PROCEEDING MAY BE BROUGHT, IRREVOCABLY AGREES TO BE BOUND
BY ANY FINAL JUDGMENT RENDERED BY ANY SUCH COURT IN ANY SUCH ACTION, SUIT OR
PROCEEDING IN WHICH IT SHALL HAVE BEEN SERVED WITH PROCESS IN THE MANNER
HEREINAFTER PROVIDED, SUBJECT TO EXERCISE AND EXHAUSTION OF ALL RIGHTS OF
APPEAL AND TO THE EXTENT THAT IT MAY LAWFULLY DO SO, WAIVES AND AGREES NOT TO
ASSERT, BY WAY OF MOTION, AS A DEFENSE OR OTHERWISE, IN SUCH ACTION, SUIT OR
PROCEEDING ANY CLAIMS THAT IT IS NOT PERSONALLY SUBJECT TO THE JURISDICTION OF
SUCH COURT, THAT ITS PROPERTY IS EXEMPT OR IMMUNE FROM ATTACHMENT OR EXECUTION,
THAT THE ACTION, SUIT OR PROCEEDING IS BROUGHT IN AN INCONVENIENT FORUM OR THAT
THE VENUE THEREOF IS IMPROPER, AND AGREES THAT PROCESS MAY BE SERVED UPON IT IN
ANY SUCH ACTION, SUIT OR PROCEEDING IN THE MANNER PROVIDED BY CHAPTER 223A OF
THE GENERAL LAWS OF MASSACHUSETTS, RULE 4 OF THE MASSACHUSETTS RULES OF CIVIL
PROCEDURE OR RULE 4 OF THE FEDERAL RULES OF CIVIL PROCEDURE.

        ALL RIGHTS AND OBLIGATIONS HEREUNDER SHALL BE GOVERNED BY THE LAW OF
THE COMMONWEALTH OF MASSACHUSETTS AND THIS NOTE SHALL BE DEEMED TO BE UNDER
SEAL.

                                MICROCOM, INC.



                                By:     
                                   ------------------------------------
                                   Name:
                                   Title:







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