<PAGE>   1
Exhibit 4.5


                    REVOLVING LINE OF CREDIT PROMISSORY NOTE
                    ----------------------------------------


$30,000,000.00 U.S.                 Manchester, NH                July 12, 1996

      FOR VALUE RECEIVED, the undersigned, WPI GROUP, INC., WPI ELECTRONICS,
INC., WPI MAGNETEC, INC., WPI MICRO PALM, INC., WPI POWER SYSTEMS, INC., WPI
TERMIFLEX, INC., MICRO PROCESSOR SYSTEMS, INC., and WPI DECISIONKEY, INC., each
a New Hampshire corporation with a principal office at 1155 Elm Street
Manchester, New Hampshire 03101 (all of such corporations are hereinafter
referred to individually and collectively as the "BORROWER"), hereby promise,
jointly and severally, to pay to the order of FLEET BANK - NH, a bank organized
under the laws of the State of New Hampshire with an address of Mail Stop NHNA
E02A, 1155 Elm Street, Manchester, New Hampshire 03101 (the "Bank"), at such
address, or such other place or places as the holder hereof may designate in
writing from time to time hereafter, the maximum principal sum of THIRTY MILLION
DOLLARS ($30,000,000.00), or so much thereof as may be advanced or readvanced by
the Bank to the Borrower from time to time hereafter (such amounts defined as
the "Debit Balance" below), together with interest as provided for hereinbelow,
in lawful money of the United States of America, all as provided hereinbelow.

      The Borrower's "Debit Balance" shall mean the debit balance in an account
on the books of the Bank, maintained in the form of a ledger card, computer
records or otherwise in accordance with the Bank's customary practice and
appropriate accounting procedures wherein there shall be recorded the principal
amount of all advances and readvances made by the Bank to the Borrower, all
principal payments made by the Borrower to the Bank hereunder, and all other
appropriate debits and credits to principal. The Bank shall render to the
Borrower a statement of account with respect thereto on a monthly basis. The
statement shall be considered correct and be considered accepted by the
Borrower, and shall conclusively bind the Borrower, unless Borrower notifies the
Bank to the contrary within thirty (30) days after the date of Borrower's
receipt of the statement.

      The Bank agrees to lend to the Borrower, and the Borrower may borrow, up
to the maximum principal sum provided for in this Note in accordance with and
subject to the terms, conditions, and limitations of this Note and the
Commercial Loan Agreement dated October 24, 1995, as amended by First Amendment
to Commercial Loan Agreement and Loan Documents dated March 20, 1996, and by
Second Amendment to Commercial Loan Agreement and Loan Documents of even date
herewith among the Bank and the Borrower, and as the same may be further amended
from time to time hereafter (as amended, the "Loan Agreement"). The holder of
this Note is entitled to all of the benefits and rights, and is subject to all
of the obligations, of the Bank under the Loan Agreement. However, neither this
reference to the Loan Agreement nor any provision thereof shall impair the
absolute and unconditional obligation of the Borrower to pay the principal and
interest of this Note as herein provided. Capitalized terms not otherwise
defined herein shall have the meanings ascribed to them in the Loan Agreement.


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      The Borrower agrees that the Bank may deliver all advances under this Note
by direct deposit to any demand account of the Borrower with the Bank or in such
other reasonable manner as may be designated in writing by the Bank to the
Borrower, and that all such advances shall represent binding obligations of the
Borrower.

      The Borrower acknowledges that this Note is to evidence the Borrower's
obligation to pay its Debit Balance, plus interest and any other applicable
charges as determined from time to time, and that it shall continue to do so
despite the occurrence of intervals when no Debit Balance exists because the
Borrower has paid the previously existing Debit Balance in full.

      Interest shall be calculated and charged daily, based on the actual days
elapsed over a three hundred sixty (360) day banking year, on the Debit Balance
outstanding from time to time at an annual variable rate equal to the Prime
Rate. The "Prime Rate" shall be the rate published by THE WALL STREET JOURNAL
from time to time under the category "Prime Rate: The Base Rate on Corporate
Loans posted by at least 75% of the Nation's 30 Largest Banks" (the lowest of
the rates so published if more than one rate is published under this category at
any given time) or such other comparable index rate selected by the Bank in its
sole discretion if THE WALL STREET JOURNAL ceases to publish such rate. The
BORROWER acknowledges that the Prime Rate is used for reference purposes only as
an index and is not necessarily the lowest interest rate charged by the BANK on
commercial loans. Each time the Prime Rate changes the interest rate hereunder
shall change contemporaneously with such change in the Prime Rate.
Notwithstanding the foregoing, the Borrower may elect from time to time a fixed
rate of interest equal to the Eurodollar Based Rate to apply to outstanding
principal hereunder, all in accordance with, and subject to the limitations of,
the provisions of the Loan Agreement.

      Outstanding principal hereunder shall be paid in accordance with the terms
and conditions of the Loan Agreement. Accrued interest only on the outstanding
Debit Balance hereunder shall be payable monthly in arrears commencing thirty
(30) days from the date hereof (or on any day within 30 days of the date hereof
agreed to by the Borrower and the Bank to provide for a convenient payment date)
and continuing on the same date of each month thereafter through and until the
Review Date as to which the Bank does not renew the Revolving Line of Credit
Loan, whereupon all outstanding principal and accrued and unpaid interest
hereunder shall be due and payable in full.

      The Borrower may prepay this Note in whole or in part at any time;
provided, however, that such prepayment shall be subject to the terms and
conditions of the Loan Agreement and the charges provided thereunder payable by
Borrower to Bank. In the event that any such prepayment shall be made by the
Borrower, the amount thereof shall be applied first to accrued interest and
thereafter to principal.

      At the option of the Bank, this Note shall become immediately due and
payable in full, without further demand or notice, if any installment of
principal or interest is not paid when due hereunder or upon the occurrence of
any other Event of Default under the terms hereof, of the Loan Agreement, or of
any other Loan Document.




<PAGE>   3

      The holder may impose upon the Borrower a delinquency charge of five
percent (5%) of the amount of the principal and/or interest not paid on or
before the tenth (10th) day after such installment is due. The entire principal
balance hereof, together with accrued interest, shall after maturity, whether by
demand, acceleration or otherwise, bear interest at the contract rate of this
Note plus an additional five percent (5%) per annum.

      The Borrower grants to Bank the right to set off and apply, upon an Event
of Default and without demand or notice of any nature, all, or any portion, of
deposits, credits and other property now or hereafter due from the holder to the
Borrower, against the indebtedness evidenced by this Note.

      The Borrower agrees to pay on demand all reasonable out-of-pocket costs of
collection hereof, including reasonable attorneys' fees, whether or not any
action is instituted by the holder in its discretion.

      No delay or omission on the part of the holder in exercising any right,
privilege or remedy shall impair such right, privilege or remedy or be construed
as a waiver thereof or of any other right, privilege or remedy. No waiver of any
right, privilege or remedy or any amendment to this Note shall be effective
unless made in writing and signed by the holder. Under no circumstances shall an
effective waiver of any right, privilege or remedy on any one occasion
constitute or be construed as a bar to the exercise of or a waiver of such
right, privilege or remedy on any future occasion.

      The acceptance by the holder hereof of any payment after any default
hereunder shall not operate to extend the time of payment of any amount then
remaining unpaid hereunder or constitute a waiver of any rights of the holder
hereof under this Note.

      All rights and remedies of the holder, whether granted herein or
otherwise, shall be cumulative and may be exercised singularly or concurrently.

      The Borrower hereby waives, to the fullest extent permitted by law,
presentment, notice, and protest. Borrower assents to any extension of the time
of payment or any other indulgence.

      This Note and the provisions, hereof shall be binding upon the Borrower
and the Borrower's successors and assigns and shall inure to the benefit of and
shall bind the holder, the holder's heirs, administrators, executors,
successors, legal representatives and assigns.

      The word "holder" as used herein shall mean the payee or endorsee of this
Note who is in possession of it, or the bearer, if this Note is at the time
payable to the bearer.

      This Note may not be amended, changed or modified in any respect except by
a written document which has been executed by each party. This Note constitutes
a New Hampshire contract to be governed by the laws of such state and to be paid
and performed therein.



<PAGE>   4


      This Note is executed and delivered in replacement of, but not in novation
or discharge of, the Revolving Line of Credit Promissory Note of WPI GROUP,
INC., WPI ELECTRONICS, INC., WPI MAGNETEC, INC., WPI MICRO PALM, INC., WPI POWER
SYSTEMS, INC., WPI TERMIFLEX, INC., MICRO PROCESSOR SYSTEMS, INC., and WPI
DECISIONKEY, INC., payable to the order of the Bank in the principal amount of
Twenty Million Dollars ($20,000,000.00) dated March 20, 1996, as amended to date
(collectively, as amended, the "Old Note"). All references to the Old Note in
the Loan Agreement or any other Loan Document shall be deemed to refer to this
Note.


      Executed and delivered this 12th day of July, 1996.


WITNESSES:                                      BORROWER:

                                                WPI GROUP, INC.


/s/ Michael Tule                                By: /s/ Michael Foster
-----------------------                             -----------------------
                                                    Signature and Title
                                                    Duly Authorized

                                                    Chairman and CEO
                                                    -----------------------
                                                    Print Name and Title


                                                WPI POWER SYSTEMS, INC.

/s/ Michael Tule                                By: /s/ Michael Foster
-----------------------                             -----------------------
                                                    Signature and Title
                                                    Duly Authorized

                                                    Chairman and CEO
                                                    -----------------------
                                                    Print Name and Title




<PAGE>   5



WITNESSES:                                      WPI MAGNETEC, INC.


/s/ Michael Tule                                By: /s/ Michael Foster
-----------------------                             -----------------------
                                                    Signature and Title
                                                    Duly Authorized

                                                    Chairman and CEO
                                                    -----------------------
                                                    Print Name and Title



                                                WPI ELECTRONICS, INC.


/s/ Michael Tule                                By: /s/ Michael Foster
-----------------------                             -----------------------
                                                    Signature and Title
                                                    Duly Authorized

                                                    Chairman and CEO
                                                    -----------------------
                                                    Print Name and Title


                                                WPI TERMIFLEX, INC.


/s/ Michael Tule                                By: /s/ Michael Foster
-----------------------                             -----------------------
                                                    Signature and Title
                                                    Duly Authorized

                                                    Chairman and CEO
                                                    -----------------------
                                                    Print Name and Title


                                                WPI MICRO PALM, INC.


/s/ Michael Tule                                By: /s/ Michael Foster
-----------------------                             -----------------------
                                                    Signature and Title
                                                    Duly Authorized

                                                    Chairman and CEO
                                                    -----------------------
                                                    Print Name and Title


<PAGE>   6

WITNESSES:                                      WPI PROCESSOR SYSTEMS, INC.


/s/ Michael Tule                                By: /s/ Michael Foster
-----------------------                             -----------------------
                                                    Signature and Title
                                                    Duly Authorized

                                                    Chairman and CEO
                                                    -----------------------
                                                    Print Name and Title



                                                WPI DECISIONKEY, INC.


/s/ Michael Tule                                By: /s/ Michael Foster
-----------------------                             -----------------------
                                                    Signature and Title
                                                    Duly Authorized

                                                    Chairman and CEO
                                                    -----------------------
                                                    Print Name and Title