1



                                                                     EXHIBIT 5.1

                         Testa, Hurwitz & Thibeault, LLP
                                 125 High Street
                                Boston, MA 02110

                                                                 August 30, 2000

NETsilicon, Inc.
411 Waverley Oaks Road, Bldg. 227
Waltham, MA 02452

         Re:  Registration Statement on Form S-8
              ----------------------------------

Ladies and Gentlemen:

     We are acting as counsel for NETsilicon, Inc., a Massachusetts corporation
(the "Company"), in connection with the registration on a Registration Statement
on Form S-8 (the "Registration Statement") under the Securities Act of 1933, as
amended, of the offer and sale of up to 500,000 shares of common stock, par
value $.01 per share (the "Common Stock"), of the Company under the 2000
Employee Stock Purchase Plan (the "Plan").

     In rendering our opinion, we have examined, and are familiar with, and have
relied as to factual matters solely upon originals or copies certified or
otherwise identified to our satisfaction, of such documents, corporate records
or other instruments as we have deemed necessary or appropriate for the purposes
of the opinion set forth herein, including, without limitation, the Company's
(a) Plan, (b) Restated Articles of Organization, (c) Amended and Restated
By-Laws, (d) minute books and (e) and stock records of the Company.

     Based upon and subject to the foregoing, we are of the opinion that the
Shares are duly authorized and, when issued and delivered pursuant to the terms
of the Plan and the terms of any agreement relating to any of the options
granted thereunder, will be validly issued, fully paid and nonassessable.

     We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement.

                                           Very truly yours,

                                           /s/  Testa, Hurwitz & Thibeault, LLP
                                           ------------------------------------
                                           TESTA, HURWITZ & THIBEAULT, LLP