EXHIBIT A
                                                                              to
                                                                      Securities
                                                                        Purchase
                                                                       Agreement


THIS  CONVERTIBLE  TERM  DEBENTURE AND THE SECURITIES  ISSUABLE UPON  CONVERSION
HEREOF HAVE NOT BEEN  REGISTERED  UNDER THE  SECURITIES  ACT OF 1933, AS AMENDED
(THE  "SECURITIES  ACT") OR THE  SECURITIES  LAWS OF ANY STATE.  THE  SECURITIES
REPRESENTED HEREBY MAY NOT BE OFFERED,  SOLD OR OTHERWISE TRANSFERRED UNLESS THE
SECURITIES  ARE  REGISTERED  UNDER  THE  SECURITIES  ACT  AND  APPLICABLE  STATE
SECURITIES  LAWS,  OR ANY SUCH OFFER,  SALE OR  TRANSFER IS MADE  PURSUANT TO AN
AVAILABLE EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF THOSE LAWS.


                           CONVERTIBLE TERM DEBENTURE

April 9, 1997                                                        $ _________
        
     FOR  VALUE  RECEIVED,   FASTCOMM  COMMUNICATIONS  CORPORATION,  a  Virginia
corporation  (hereinafter  called the  "Borrower" or the  "Corporation")  hereby
promises  to pay to the  order of  ______________  or  registered  assigns  (the
"Holder") the sum of ______________________Dollars  ($________) on April 9, 2001
(the "Scheduled  Maturity  Date"),  and to pay interest on the unpaid  principal
balance  hereof at the rate of five  percent (5%) per annum from the date hereof
(the "Issue Date") until the same becomes due and payable (which  interest shall
accrue on a daily  basis  based on a 360 day year),  whether at maturity or upon
acceleration  or  otherwise.  Any amount of  principal  of or  interest  on this
Debenture  which is not paid when due shall bear interest at the rate of fifteen
percent  (15%)  per  annum  from the due date  thereof  until  the same is paid.
Interest  shall  commence  accruing  on the Issue  Date and,  to the  extent not
converted  in  accordance  with the  provisions  of Article  IV below,  shall be
payable on the date the  principal  amount in respect of which it has accrued is
paid,  whether at maturity or upon  acceleration  or by prepayment or otherwise.
Except as otherwise  provided in Article VIII hereof,  all payments of principal
and  interest  shall be made in shares of Common Stock and  Warrants  (each,  as
defined  below) at a  conversion  price  equal to the average of the Closing Bid
Prices for the ten (10)  consecutive  trading  days  ending on the  trading  day
immediately   preceding  the  Scheduled  Maturity  Date  (subject  to  equitable
adjustment for any stock splits,  stock dividends,  reclassifications or similar
events during such ten (10) trading day period).  All payments  shall be made at
such address as the




Holder shall hereafter give to the Borrower by written notice made in accordance
with the provisions of this Debenture.

     This  Debenture  is  being  issued  by  the  Borrower  along  with  similar
convertible  term  debentures  (the "Other  Debentures"  and together  with this
Debenture,  the "Debentures") delivered to other holders (if any) (together with
the  Holder  referred  to  herein,  the  "Holders")  pursuant  to  that  certain
Securities  Purchase  Agreement,  dated as of the date hereof,  by and among the
Borrower and the Holders (the "Purchase Agreement").

                                    ARTICLE I

                                   PREPAYMENT

     A. Limited Right to Prepay.  Upon the occurrence of an Event of Default (as
defined  herein),  this Debenture shall be prepaid by the Borrower in accordance
with the provisions of Article VIII hereof. Except as provided in Paragraph B of
this  Article I, this  Debenture  may not be prepaid  without the prior  written
consent of the Holder.

     B. Prepayment at Borrower's Option.

          (i) So long as no Event of  Default  shall have  occurred,  during the
thirty (30) calendar day period beginning on the trading day following the first
ten (10) consecutive trading day period (if any) that the average of the Closing
Bid Prices (as defined  below) for the Common Stock is less than $3.02 per share
(subject  to  equitable   adjustments   for  stock  splits,   stock   dividends,
reclassifications  or  similar  events)  and  provided  the  Borrower  is not in
material  violation  of any of its  obligations  under the  Securities  Purchase
Agreement or the Registration Rights Agreement, then the Borrower shall have the
right to prepay ("Prepayment at Borrower's  Election") all or any portion of the
then  outstanding  Debentures  (other than Debentures which are the subject of a
Notice  of  Conversion  delivered  prior to the  delivery  date of the  Optional
Prepayment  Notice (as defined  below)) for the Optional  Prepayment  Amount (as
defined  below),  which right shall be  exercisable  one time during the term of
this Debenture by the Borrower in its sole discretion by delivery of an Optional
Prepayment Notice during such thirty (30) day period and otherwise in accordance
with the prepayment procedures set forth below. Any optional prepayment pursuant
to this  Paragraph B shall be made ratably  among the holders of  Debentures  in
proportion to the principal  amount of Debentures then  outstanding.  Holders of
Debentures  may  convert  all or any  part  of  their  Debentures  selected  for
prepayment  hereunder into Common Stock and Warrants at a conversion price equal
to the average of the Closing  Bid Prices for the ten (10)  consecutive  trading
days  ending on the  trading  day  immediately  preceding  the  Conversion  Date
(subject  to  equitable  adjustment  for  any  stock  splits,  stock  dividends,
reclassifications  or similar events during such ten (10) trading day period) by
delivering  a Notice of  Conversion  to the  Borrower  at any time  prior to the
Effective  Date of Prepayment as defined in  subparagraph  (iii).  The "Optional
Prepayment  Amount" with respect to each Debenture  means (a) 120% multiplied by
the sum of the principal amount thereof plus all


                                       2


accrued and unpaid interest and Conversion Default Payments (if any) through the
date of  prepayment  plus (b) Warrants to purchase  forty  percent  (40%) of the
number of shares of Common Stock which would have been issuable to the holder of
such  Debenture had such Debenture been converted into Common Stock and Warrants
in  accordance  with  Article  IV.A on the  date  of  delivery  of the  Optional
Prepayment Notice.

          (ii) The Borrower may not deliver an Optional  Prepayment  Notice to a
holder of Debentures unless on or prior to the date of delivery of such Optional
Prepayment Notice,  the Borrower shall have deposited with NationsBank,  N.A. or
another escrow agent  reasonably  satisfactory  to the Holder,  as a trust fund,
cash and Warrants  sufficient  in amount to pay all amounts to which the holders
of Debentures are entitled upon such prepayment  pursuant to subparagraph (i) of
this Paragraph B, with  irrevocable  instructions and authority to such transfer
agent or escrow agent to complete the prepayment thereof in accordance with this
Paragraph B. Any Optional  Prepayment  Notice  delivered in accordance  with the
immediately preceding sentence shall be accompanied by a statement executed by a
duly  authorized  officer of its transfer agent or escrow agent,  certifying the
amount of funds and Warrants  which have been deposited with such transfer agent
or escrow agent and that the transfer agent or escrow agent has been  instructed
and agrees to act as prepayment agent hereunder.

          (iii) The Borrower shall effect each prepayment under this Article I.B
by giving at least five (5)  business  days but not more than ten (10)  business
days prior written notice (the "Optional  Prepayment  Notice") of the date which
such prepayment is to become effective (the "Effective Date of Prepayment"), the
Debentures selected for prepayment and the Optional Prepayment Amount to (i) the
holders of  Debentures  selected  for  prepayment  at the address and  facsimile
number of such holder  appearing in the  Borrower's  register for the Debentures
and (ii) the transfer  agent for the Common  Stock,  which  Optional  Prepayment
Notice  shall be deemed to have been  delivered  on the  business  day after the
Borrower's  fax  (with a copy  sent  by  overnight  courier  to the  holders  of
Debentures) of such notice to the holders of Debentures.

          (iv) The Optional Prepayment Amount shall be paid to the holder of the
Debentures being prepaid within three (3) business days of the Effective Date of
Prepayment;  provided,  however,  that the  Borrower  shall not be  obligated to
deliver  any  portion  of  the  Optional  Prepayment  Amount  until  either  the
Debentures  being  prepaid are  delivered  to the office of the  Borrower or the
transfer  agent,  or the holder notifies the Borrower or the transfer agent that
such  Debentures   have  been  lost,   stolen  or  destroyed  and  delivers  the
documentation  in accordance with Article X.H hereof.  Notwithstanding  anything
herein to the contrary,  in the event that the Debentures  being prepaid are not
delivered to the  Borrower or the  transfer  agent prior to the 3rd business day
following the Effective  Date of  Prepayment,  the  prepayment of the Debentures
pursuant to this Article I.B shall still be deemed effective as of the Effective
Date of  Prepayment  and the  Optional  Prepayment  Amount  shall be paid to the
holder of Debentures being prepaid within five (5) business days of the date the
Debentures are actually delivered to the Borrower or the transfer agent.


                                       3


                                   ARTICLE II

                             [INTENTIONALLY OMITTED]


                                   ARTICLE III

                               CERTAIN DEFINITIONS

     The following terms shall have the following meanings:

     A. "Closing Bid Price" means,  for any security as of any date, the closing
bid price of such  security  on the  principal  securities  exchange  or trading
market  where  such  security  is  listed or traded  as  reported  by  Bloomberg
Financial  Markets (or a  comparable  reporting  service of national  reputation
selected by the Corporation  and reasonably  acceptable to holders of a majority
of the aggregate principal amount represented by the then outstanding Debentures
("Majority  Holders")  if  Bloomberg  Financial  Markets  is not then  reporting
closing  bid prices of such  security)  (collectively,  "Bloomberg"),  or if the
foregoing  does not apply,  the last reported sale price of such security in the
over-the-counter  market on the  electronic  bulletin board for such security as
reported by  Bloomberg,  or, if no sale price is reported  for such  security by
Bloomberg,  the average of the bid prices of any market makers for such security
as reported in the "pink sheets" by the National  Quotation Bureau,  Inc. If the
Closing Bid Price cannot be calculated  for such security on such date on any of
the foregoing  bases,  the Closing Bid Price of such security on such date shall
be the fair market value as reasonably  determined by an investment banking firm
selected by the Corporation and reasonably  acceptable to the Majority  Holders,
with the costs of such appraisal to be borne by the Corporation.

     B.  "Conversion  Amount" means the portion of the principal  amount of this
Debenture  being  converted,  plus the  accrued  interest  thereon  through  the
Conversion  Date as specified in the notice of  conversion  in the form attached
hereto (the "Notice of Conversion").

     C. "Conversion Date" means, for any Optional Conversion, the date specified
in the Notice of  Conversion  so long as the copy of the Notice of Conversion is
faxed (or  delivered  by other  means  resulting  in notice) to the  Corporation
before  Midnight,  New York City time, on the  Conversion  Date indicated in the
Notice of  Conversion.  If the Notice of Conversion is not so faxed or otherwise
delivered  before  such  time,  then the  Conversion  Date shall be the date the
holder faxes or otherwise delivers the Notice of Conversion to the Corporation.


                                       4


     D. "Conversion  Percentage"  shall have the following  meaning and shall be
subject to adjustment as provided herein:

If the Conversion Date is:                    Then the Conversion Percentage is:
- --------------------------                    ----------------------------------

Prior to October 6, 1997                                     100%

On or after October 6, 1997                                   90%

     E.  "Conversion  Price"  means,  (a) with  respect to any  Conversion  Date
occurring prior to October 6, 1997, the Variable  Conversion  Price and (b) with
respect to any Conversion  Date occurring on or after October 6, 1997, the lower
of the Fixed Conversion Price and the Variable  Conversion Price, each in effect
as of such date and subject to adjustment as provided herein.

     F. "Fixed Conversion Price" means $7.54, and shall be subject to adjustment
as provided herein.

     G. "N" means the number of days from,  but  excluding,  the  Issuance  Date
through and including the Conversion Date.

     H. "Variable Conversion Price" means, as of any date of determination,  the
amount obtained by multiplying  the Conversion  Percentage then in effect by the
average  of the  Closing  Bid  Prices  for the  Common  Stock  for the ten  (10)
consecutive  trading days ending on the trading day  immediately  preceding such
date of  determination  (subject to equitable  adjustment  for any stock splits,
stock  dividends,  reclassifications  or  similar  events  during  such ten (10)
trading day period), and shall be subject to adjustment as provided herein.

     I. "Warrant Coverage Percentage" shall have the following meaning and shall
be subject to adjustment as provided herein:

If the Conversion Date is:              Then the Warrant Coverage Percentage is:
- --------------------------              ----------------------------------------

Prior to October 6, 1997                                  0%

On or after October 6, 1997                              20%
and prior to April 4, 1998

On or after April 4, 1998                                40%


                                       5


                                  IV CONVERSION

     A.  Conversion at the Option of the Holder.  Subject to the  limitations on
conversions  contained in Paragraph C of this Article IV, the Holder may, at any
time and from time to time,  convert (an "Optional  Conversion") all or any part
of the outstanding principal amount of this Debenture, plus all accrued interest
thereon through the Conversion Date, into:

          (i) a number of fully paid and  nonassessable  shares of Common  Stock
determined in accordance with the following formula:

                                Conversion Amount
                                -----------------
                                Conversion Price
                                                         Plus

          (ii)  warrants,  in the form  attached as Exhibit B to the  Securities
Purchase Agreement,  to purchase a number of shares of Common Stock equal to the
Warrant Coverage  Percentage  multiplied by the number of shares of Common Stock
issuable pursuant to clause (i) above (the "Warrants") .

     B.  Mechanics  of  Conversion.  In order to effect an Optional  Conversion,
Holder shall: (x) fax (or otherwise deliver) a copy of the fully executed Notice
of Conversion to the Corporation for the Common Stock and (y) surrender or cause
to be surrendered, this Debenture duly endorsed, along with a copy of the Notice
of Conversion as soon as practicable thereafter to the Corporation. Upon receipt
by the  Corporation of a facsimile  copy of a Notice of Conversion  from Holder,
the Corporation shall immediately send, via facsimile,  a confirmation to Holder
stating that the Notice of Conversion has been received, the date upon which the
Corporation  expects to deliver the Common Stock and Warrants  upon a conversion
and the name  and  telephone  number  of a  contact  person  at the  Corporation
regarding the conversion. The Corporation shall not be obligated to issue shares
of Common Stock and Warrants  issuable upon such  conversion  unless either this
Debenture is  delivered  to the  Corporation  as provided  above,  or the holder
notifies the Corporation that this Debenture has been lost,  stolen or destroyed
(and the requirements of Article X.H are complied with).

          (i) Delivery of Common Stock and Warrants  Upon  Conversion.  Upon the
surrender of this Debenture and a Notice of Conversion,  the Corporation  shall,
no later than the later of the (a) second  business day following the Conversion
Date and (b) the date of such surrender  (or, if this Debenture is lost,  stolen
or destroyed certificates, after provision of indemnity pursuant to Article X.H)
(the  "Delivery  Period"),  issue and  deliver to the Holder (x) that  number of
shares of Common Stock and Warrants  issuable upon  conversion of the portion of
this  Debenture  being  converted  and (y) a new  Debenture  in the form  hereof
representing the balance of the principal amount hereof not being converted,  if
any.


                                       6


          (ii) Taxes.  The  Corporation  shall pay any and all taxes (other than
transfer  taxes)  which may be imposed  upon it with respect to the issuance and
delivery of the shares of Common Stock and Warrants upon the  conversion of this
Debenture.

          (iii) No Fractional  Shares. If any conversion of this Debenture would
result in the issuance of either a fractional share of Common Stock or a Warrant
to purchase a fractional  share of Common Stock,  such fractional share shall be
disregarded and the number of shares of Common Stock issuable upon conversion of
this Debenture or upon exercise of the Warrant shall be the nearest whole number
of shares.

          (iv) Conversion Disputes. In the case of any dispute with respect to a
conversion, the Corporation shall promptly issue such number of shares of Common
Stock and  Warrants as are not  disputed in  accordance  with  subparagraph  (i)
above.  If such dispute only involves the  calculation of the Conversion  Price,
the Corporation shall submit the disputed calculations to its outside accountant
via  facsimile  within  two  (2)  business  days of  receipt  of the  Notice  of
Conversion.   The  accountant  shall  audit  the  calculations  and  notify  the
Corporation  and the Holder of the results no later than two (2)  business  days
from  the  date  it  receives  the  disputed   calculations.   The  accountant's
calculation shall be deemed  conclusive,  absent manifest error. The Corporation
shall then issue the  appropriate  number of shares of Common Stock and Warrants
in accordance with subparagraph (i) above.

     C.  Limitations on Conversions.  The Conversions of this Debenture shall be
subject to the following limitations (each of which limitations shall be applied
independently):

          (i)  Volume  Limitations.  During  the  first 360 days  following  the
Closing Date  (except  during a Prepayment  Conversion  Period),  Holder may not
during any ninety (90) calendar day period ending on a Conversion Date,  convert
at the Variable  Conversion  Price more than  seventy-five  percent (75%) of the
original  principal  amount of this Debenture.  For the avoidance of doubt,  the
conversion  of any  portion of this  Debenture  into Common  Stock and  Warrants
subject to an Optional Prepayment Notice shall not be counted as a conversion at
the Variable Conversion Price for purposes of this subparagraph (i).

          (ii)  Cap  Amount.  Unless  permitted  by  the  applicable  rules  and
regulations  of the  principal  securities  market on which the Common  Stock is
listed or traded,  in no event shall the total  number of shares of Common Stock
issued upon  conversion of this  Debenture and the Other  Debentures  exceed the
maximum  number  of shares of Common  Stock  that the  Corporation  can so issue
pursuant  to Rule  4460(i)  of the Nasdaq  National  Market  ("Nasdaq")  (or any
successor  rule) (the "Cap  Amount")  which,  as of the Issue Date is  2,016,261
shares.  The  portion of the Cap Amount  allocable  to this  Debenture  shall be
40,325  shares and shall be subject to adjustment as provided in Article X.D. In
the event the Corporation is prohibited from issuing shares of Common Stock as a
result of the operation of this  subparagraph  (i), the Corporation shall comply
with Article VII.


                                       7


          (iii) No Five Percent Holders. In no event shall Holder be entitled to
receive  shares of Common Stock upon a conversion  to the extent that the sum of
(x) the number of shares of Common  Stock  beneficially  owned by Holder and its
affiliates  (exclusive  of shares  issuable upon  conversion of the  unconverted
portion of any Debentures or the unexercised or unconverted portion of any other
securities of the  Corporation  (including,  without  limitation,  the Warrants)
subject to a limitation on conversion or exercise  analogous to the  limitations
contained herein) and (y) the number of shares of Common Stock issuable upon the
conversion  of  the  portion  of  this  Debenture  with  respect  to  which  the
determination  of this  subparagraph  is being made,  would result in beneficial
ownership by the holder and its affiliates of more than 4.9% of the  outstanding
shares of Common Stock. For purposes of this subparagraph,  beneficial ownership
shall be determined in accordance with Section 13(d) of the Securities  Exchange
Act of 1934, as amended,  and Regulation 13 D-G thereunder,  except as otherwise
provided in clause (x) above.  The  restriction  contained in this  subparagraph
(iii) shall not be altered, amended, deleted or changed in any manner whatsoever
unless  the  holders  of a majority  of the  Common  Stock and the Holder  shall
approve such alteration, amendment, deletion or change.


                     V RESERVATION OF SHARES OF COMMON STOCK

     A. Reserved Amount.  On the Issue Date, the Corporation shall have reserved
60,000  authorized  but  unissued  shares of  Common  Stock  for  issuance  upon
conversion  of  this  Debenture  and  exercise  of the  Warrants  issuable  upon
conversion hereof and thereafter the number of authorized but unissued shares of
Common Stock so reserved  (the  "Reserved  Amount")  shall not be decreased  and
shall  at  all  times  be  sufficient  to  provide  for  the  conversion  of the
outstanding principal amount of this Debenture (and accrued interest thereon) at
the then current  Conversion  Price and exercise of the Warrants  then  issuable
upon conversion hereof.

     B. Increases to Reserved  Amount.  If the Reserved Amount for any three (3)
consecutive  trading  days (the last of such  three (3)  trading  days being the
"Authorization Trigger Date") shall be less than 135% of the number of shares of
Common Stock  issuable  upon  conversion  of this  Debenture and exercise of the
Warrants  issuable upon conversion  hereof on such trading days, the Corporation
shall  immediately  notify Holder of such  occurrence  and shall take  immediate
action (including,  if necessary,  seeking shareholder approval to authorize the
issuance of additional  shares of Common Stock) to increase the Reserved  Amount
to 200% of the number of shares of Common Stock then issuable upon conversion of
this Debenture and exercise of the Warrants issuable upon conversion  hereof. In
the event the Corporation fails to so increase the Reserved Amount within ninety
(90) days after an Authorization  Trigger Date, Holder shall thereafter have the
option,  exercisable  in whole  or in part at any time and from  time to time by
delivery of a Default Notice (as defined in Article VIII.C) to the  Corporation,
to require the Corporation to prepay for cash, at the Default Amount (as defined
in Article  VIII.B),  a portion of the principal  amount of this Debenture (plus
accrued interest thereon) such that, after giving effect to such prepayment, the
Reserved  Amount  exceeds  135% of the total  number  of shares of Common  Stock
issuable  to Holder  upon  conversion  of this  Debenture  (and  exercise of the
Warrants issuable upon conversion hereof) on the


                                       8


date of the Default Notice.  If the Corporation fails to pay such Default Amount
within five (5) business days after its receipt of a Default Notice, then Holder
shall be entitled to the remedies provided in Article VIII.C.


                        VI FAILURE TO SATISFY CONVERSIONS

     A.  Conversion  Default  Payments.  If, at any time,  (x) Holder  submits a
Notice of  Conversion  and the  Corporation  fails for any  reason  (other  than
because such issuance would exceed Holder's Reserved Amount or allocated portion
of the Cap Amount,  for which  failures  the Holder  shall have the remedies set
forth in Articles V and VII) to deliver,  on or prior to the fourth business day
following the expiration of the Delivery Period for such conversion, such number
of  Warrants  or freely  tradeable  shares of  Common  Stock to which  Holder is
entitled upon such  conversion,  or (y) the  Corporation  provides notice to any
Holder at any time of its  intention not to issue  Warrants or freely  tradeable
shares of Common Stock upon exercise by any Holder of its  conversion  rights in
accordance  with the terms of the  Debentures  (other than because such issuance
would  exceed such  Holder's  Reserved  Amount or  allocated  portion of the Cap
Amount) (each of (x) and (y) being a "Conversion Default"), then the Corporation
shall pay to Holder, payments for the first ten (10) business days following the
expiration of the Delivery Period, in the case of a Conversion Default described
in clause (x), and for the first ten (10) business days of a Conversion  Default
described  in  clause  (y),  an amount  equal to $500 per day.  In the event any
Conversion  Default  continues  beyond such ten (10)  business  day period,  the
Corporation shall pay to Holder an additional amount equal to:

                     (.24) x (D/365) x (the Default Amount)

where:

     "D" means the number of days after the  expiration of the ten (10) business
day period described above through and including the Default Cure Date;

     "Default  Amount" means the outstanding  principal amount of all Debentures
held by Holder plus all accrued and unpaid interest  thereon as of the first day
of the Conversion Default.

     "Default  Cure  Date"  means  (i)  with  respect  to a  Conversion  Default
described in clause (x) of its definition,  the date the Corporation effects the
conversion of the portion of this  Debenture  submitted for  conversion and (ii)
with respect to a Conversion  Default described in clause (y) of its definition,
the date the  Corporation  begins  to issue  freely  tradeable  Common  Stock in
satisfaction of all conversions of Debentures in accordance with their terms.

     The payments to which Holder shall be entitled pursuant to this Paragraph A
are referred to herein as  "Conversion  Default  Payments."  Holder may elect to
receive  accrued  Conversion  Default  Payments in cash or to convert all or any
portion of such accrued Conversion Default


                                       9


Payments,  at any time, into Common Stock and Warrants at the lowest  Conversion
Price in  effect  during  the  period  beginning  on the date of the  Conversion
Default  through the Conversion  Date for such  conversion.  In the event Holder
elects to receive any  Conversion  Default  Payments in cash, it shall so notify
the Corporation in writing. Such payment shall be made in accordance with and be
subject to the  provisions of Article X.J. In the event Holder elects to convert
all or any portion of the Conversion Default Payments,  Holder shall indicate on
a Notice of Conversion  such portion of the  Conversion  Default  Payments which
Holder elects to so convert and such  conversion  shall otherwise be effected in
accordance with the provisions of Article IV.

     B. Adjustment to Conversion Price. If Holder has not received  certificates
for all shares of Common Stock and Warrants  prior to the tenth (10th)  business
day after the expiration of the Delivery  Period with respect to a conversion of
any portion of any of Holder's  Debentures  for any reason  (other than  because
such issuance would exceed Holder's  Reserved Amount or allocated portion of the
Cap Amount,  for which  failures  Holder  shall have the  remedies  set forth in
Articles V and VII),  then the Fixed  Conversion  Price shall  thereafter be the
lesser of (i) the Fixed Conversion Price on the Conversion Date specified in the
Notice of  Conversion  which  resulted  in the  Conversion  Default and (ii) the
lowest Conversion Price in effect during the period beginning on, and including,
such  Conversion  Date through and including the day such shares of Common Stock
and  Warrants  are  delivered  to the Holder.  If there shall occur a Conversion
Default of the type  described  in clause (y) of Article  VI.A.,  then the Fixed
Conversion  Price with respect to any conversion  thereafter shall be the lowest
Conversion  Price in effect at any time  during  the  period  beginning  on, and
including,  the date of the occurrence of such  Conversion  Default  through and
including the Default Cure Date. The Fixed  Conversion Price shall thereafter be
subject to further adjustment for any events described in Article IX.

     C. Buy-In Cure.  Unless the  Corporation has notified the Holder in writing
that the  Corporation  is unable to honor  conversions,  if (i) the  Corporation
fails for any reason to  deliver  during the  Delivery  Period  shares of Common
Stock  to  Holder  upon a  conversion  of this  Debenture  and  (ii)  after  the
applicable Delivery Period with respect to such conversion, Holder purchases (in
an open market transaction or otherwise) shares of Common Stock to make delivery
upon a sale by Holder of the shares of Common  Stock (the "Sold  Shares")  which
Holder anticipated receiving upon such conversion (a "Buy-In"),  the Corporation
shall pay Holder (in  addition to any other  remedies  available  to Holder) the
amount  by  which  (x)  Holder's  total  purchase  price  (including   brokerage
commissions, if any) for the shares of Common Stock so purchased exceeds (y) the
net  proceeds  received  by the  Holder  from the sale of the Sold  Shares.  For
example,  if a holder  purchases  shares of Common Stock having a total purchase
price of $11,000 to cover a Buy-In with  respect to shares of Common  Stock sold
for $10,000,  the Corporation will be required to pay the Holder $1,000.  Holder
shall  provide  the  Corporation  written  notification  indicating  any amounts
payable to Holder pursuant to this Paragraph C. The  Corporation  shall make any
payments required pursuant to this Paragraph C in accordance with and subject to
the provisions of Article X.J.


                                       10


     D. Right to Require Prepayment.  If the Corporation fails, and such failure
continues  uncured for five (5)  business  days after the  Corporation  has been
notified  thereof in writing by Holder,  for any reason (other than because such
issuance would exceed Holder's  Reserved Amount or its allocated  portion of the
Cap Amount,  for which  failures  Holder  shall have the  remedies  set forth in
Articles V and VII) to issue shares of Common Stock or Warrants  within ten (10)
business days after the  expiration  of the Delivery  Period with respect to any
conversion of this Debenture, then Holder may elect at any time and from time to
time prior to the Default Cure Date for such Conversion  Default, by delivery of
a Default Notice (as defined in Article VIII.C) to the Corporation,  to have all
or any portion of Holder's outstanding Debentures prepaid by the Corporation for
cash,  the Default  Amount (as defined in Article  VIII.B).  If the  Corporation
fails to pay such Default Amount within five (5) business days after its receipt
of a Prepayment  Notice,  then Holder shall be entitled to the remedies provided
in Article VIII.C.

                   VII INABILITY TO CONVERT DUE TO CAP AMOUNT

     A. Obligation to Cure. If at any time the then unissued portion of Holder's
Cap  Amount  is less than 135% of the  number  of  shares of Common  Stock  then
issuable upon conversion of this Debenture (a "Trading  Market Trigger  Event"),
the  Corporation  shall  immediately  notify the Holders of such  occurrence and
shall take immediate action  (including,  if necessary,  seeking the approval of
its  shareholders  to  authorize  the  issuance  of the full number of shares of
Common Stock which would be issuable upon the  conversion of this  Debenture but
for the Cap Amount) to eliminate any  prohibitions  under  applicable law or the
rules or  regulations of any stock  exchange,  interdealer  quotation  system or
other self-regulatory organization with jurisdiction over the Corporation or any
of its securities on the  Corporation's  ability to issue shares of Common Stock
in excess of the Cap Amount. In the event the Corporation fails to eliminate all
such  prohibitions  within  ninety  (90) days after the Trading  Market  Trigger
Event, Holder shall thereafter have the option,  exercisable in whole or in part
at any time and from time to time by delivery of a Default Notice (as defined in
Article VIII.C) to the Corporation,  to require the Corporation to pay for cash,
at the Default Amount (as defined in Article VIII.B), a portion of the principal
amount of this  Debenture (and accrued and unpaid  interest  thereon) such that,
after giving effect to such prepayment,  Holder's  allocated  portion of the Cap
Amount  exceeds 135% of the total  number of shares of Common Stock  issuable to
Holder upon conversion of this Debenture on the date of such Default Notice.  If
the  Corporation  fails to pay the Default  Amount within five (5) business days
after its  receipt of a Default  Notice,  then  Holder  shall be entitled to the
remedies provided in Article VIII.C.

     B.  Remedies.   If  the  Corporation  fails  to  eliminate  the  applicable
prohibitions  within the ninety (90) day cure period  referred to in Paragraph A
of this Article VII and thereafter the  Corporation is prohibited,  at any time,
from  issuing  shares  of  Common  Stock or  Warrants  upon  conversion  of this
Debenture  because such issuance would exceed Holder's  allocated portion of the
Cap Amount  because of applicable  law or the rules or  regulations of any stock
exchange,  interdealer  quotation system or other  self-regulatory  organization
with jurisdiction  over the Corporation or its securities,  Holder may elect any
or both of the following additional remedies:


                                       11


          (i) to  require,  with  the  consent  of  the  Majority  Holders,  the
Corporation to terminate the listing of its Common Stock on Nasdaq (or any other
stock exchange, interdealer quotation system or trading market) and to cause its
Common Stock to be eligible for trading on the Nasdaq  SmallCap Market or on the
over-the-counter electronic bulletin board, at the option of the Holder; or

          (ii) to require the  Corporation  to issue  shares of Common Stock and
Warrants in accordance with Holder's Notice of Conversion at a conversion  price
equal to the average of the Closing Bid Prices of the Common  Stock for the five
(5)  consecutive  trading days  (subject to equitable  adjustment  for any stock
splits,  stock dividends,  reclassifications  or similar events during such five
(5) trading day period)  preceding  the date of Holder's  written  notice to the
Corporation  of its  election  to receive  shares of Common  Stock and  Warrants
pursuant to this subparagraph (ii).

                                  ARTICLE VIII

                                EVENTS OF DEFAULT

     A. Events of Default.  If any of the following  events of default (each, an
"Event of Default" ) shall occur:

          (i) the  Corporation  fails (i) to pay the principal  hereof when due,
whether at  maturity,  upon  acceleration  or  otherwise or (ii) to pay interest
hereon at maturity and such failure  continues for a period of five (5) business
days after the due date thereof,

          (ii) the Common  Stock  (including  any of the shares of Common  Stock
issuable upon conversion of this Debenture and exercise of the Warrants issuable
upon  conversion  hereof) is suspended  from trading on any of, or is not listed
(and  authorized)  for trading on at least one of, the New York Stock  Exchange,
the American  Stock Exchange or Nasdaq for an aggregate of ten (10) trading days
in any nine (9) month period,

          (iii) the Corporation  fails, and any such failure  continues  uncured
for five (5) business days after the  Corporation  has been notified  thereof in
writing by the Holder,  to remove any  restrictive  legend on any certificate or
any shares of Common Stock issued to the Holder upon conversion of any Debenture
or any  Warrant  as and when  required  by the  Debentures,  the  Warrants,  the
Securities Purchase Agreement or the Registration Rights Agreement,  dated as of
April 9, 1997, by and among the  Corporation and the other  signatories  thereto
(the "Registration Rights Agreement"),

          (iv) the Corporation provides notice to any of the Holders,  including
by way of public announcement, at any time, of its intention not to issue shares
of Common Stock or Warrants to any of the Holders upon  conversion in accordance
with the terms of the Debentures (other than


                                       12


due to the circumstances contemplated by Articles V or VII for which the Holders
shall have the remedies set forth in such Articles);

          (v) the Corporation  breaches any material  covenant or other material
term or  condition of this  Debenture  (other than as  specifically  provided in
subparagraphs  (i)-(iv) of this Paragraph A), the Securities Purchase Agreement,
the Warrants or the Registration  Rights Agreement and such breach continues for
a period of fifteen  (15)  business  days after  written  notice  thereof to the
Corporation;

          (vi) any  representation or warranty of the Corporation made herein or
in any agreement,  statement or certificate  given in writing pursuant hereto or
in connection herewith (including,  without limitation,  the Securities Purchase
Agreement and the Registration  Rights Agreement),  shall be false or misleading
in any material  respect when made and the breach of which would have a material
adverse effect on the Corporation or the rights of the Corporation  with respect
to any of the Debentures or the shares of Common Stock or Warrants issuable upon
conversion of the Debentures;

          (vii) the Corporation or any subsidiary of the Corporation  shall make
an  assignment  for the  benefit  of  creditors,  or apply for or consent to the
appointment  of a receiver  or trustee for it or for a  substantial  part of its
property  or  business;  or  such a  receiver  or  trustee  shall  otherwise  be
appointed, or

          (viii)   bankruptcy,   insolvency,   reorganization   or   liquidation
proceedings or other  proceedings for relief under any bankruptcy law or any law
for the relief of debtors shall be instituted by or against the  Corporation  or
any subsidiary of the Corporation (and such proceedings  shall continue unstayed
for thirty (30) days);

Then,  upon the occurrence and during the  continuation  of any Event of Default
specified in  subparagraphs  (i)-(vi) of this  Paragraph A, at the option of the
Holder  hereof,  and upon the  occurrence  of any Event of Default  specified in
subparagraph  (vii) or (viii) of this Paragraph A, the Corporation  shall pay to
the Holder,  in satisfaction of its obligation to pay the outstanding  principal
amount of this  Debenture  and accrued and unpaid  interest  thereon,  an amount
equal to the Default  Amount and such Default  Amount,  together  with all other
ancillary amounts payable  hereunder shall  immediately  become due and payable,
all without  demand,  presentment  or notice,  all of which hereby are expressly
waived, together with all costs, including, without limitation, reasonable legal
fees and  expenses of  collection,  and the Holder shall be entitled to exercise
all other rights and remedies available at law or in equity.


                                       13


     B. Definition of Default Amount.  The "Default  Amount" with respect to any
portion of this Debenture means an amount equal to:
    
                                        A            X        M
                                    ----------
                                       C P

where:

     "A"  means  the  principal  amount of this  Debenture  being  paid plus all
accrued and unpaid interest thereon through the payment date;

     "CP"  means  the  Conversion  Price in  effect  on the date of the  Default
Notice; and

     "M" means the  highest  Closing  Bid Price of the  Company's  Common  Stock
during the period  beginning on the date of the Default Notice and ending on the
payment date, as reported on the principal securities exchange or trading market
on which the Common Stock is traded.

     C.  Failure to Pay  Default  Amount.  If the  Corporation  fails to pay the
Default  Amount  within  five  (5)  business  days of its  receipt  of a  notice
requiring  such  payment  (a  "Default  Notice"),  then the  Holder (i) shall be
entitled  to  interest  on the  Default  Amount at a per annum rate equal to the
lower of  twenty-four  percent (24%) and the highest  interest rate permitted by
applicable  law from the date of the  Default  Notice  until the date of payment
hereunder,  and (ii) shall have the right, at any time and from time to time, to
require  the  Corporation,  upon  written  notice,  to  immediately  convert (in
accordance  with the terms of  Paragraph  A of Article IV) all or any portion of
the Default Amount, plus interest as aforesaid,  into shares of Common Stock and
Warrants at the lowest Conversion Price in effect during the period beginning on
the date of the Default Notice and ending on the Conversion Date with respect to
the conversion of such Default Amount.  In the event the Corporation is not able
to pay all amounts due and payable  with  respect to all  Debentures  subject to
Default  Notices,  the Corporation  shall pay the Holders such amounts pro rata,
based on the total amounts  payable to such Holder relative to the total amounts
payable to all Holders.

                                   ARTICLE IX

                       ADJUSTMENTS TO THE CONVERSION PRICE

     The  Conversion  Price shall be subject to adjustment  from time to time as
follows:

     A. Stock Splits, Stock Dividends,  Etc. If at any time on or after the date
of issuance of this Debenture,  the number of outstanding shares of Common Stock
is increased by a stock split, stock dividend, combination,  reclassification or
other  similar  event,  the  Fixed  Conversion  Price  shall be  proportionately
reduced,  or if the number of outstanding shares of Common Stock is decreased by
a reverse  stock split,  combination  or  reclassification  of shares,  or other
similar event, the Fixed


                                       14


Conversion  Price  shall  be  proportionately  increased.  In  such  event,  the
Corporation shall notify the  Corporation's  transfer agent of such change on or
before the effective date thereof.

     B. [Intentionally Omitted]

     C.  Adjustment  Due to Merger,  Consolidation,  Etc.  If, at any time there
shall be (i) any  reclassification or change of the outstanding shares of Common
Stock (other than a change in par value,  or from par value to no par value,  or
from no par value to par value, or as a result of a subdivision or combination),
(ii) any consolidation or merger of the Corporation with any other entity (other
than a merger in which the Corporation is the surviving or continuing entity and
its  capital  stock  is  unchanged),  (iii)  any  sale  or  transfer  of  all or
substantially  all of the assets of the  Corporation  or (iv) any share exchange
pursuant to which all of the  outstanding  shares of Common Stock are  converted
into other  securities or property,  then the Holder shall  thereafter  have the
right to receive  upon  conversion,  in lieu of the  shares of Common  Stock and
Warrants  immediately  theretofore  issuable,  such shares of stock,  securities
and/or other property as may be issued or payable with respect to or in exchange
for the number of shares of Common  Stock and Warrants  immediately  theretofore
issuable upon  conversion  had such merger,  consolidation,  exchange of shares,
recapitalization,  reorganization or other similar event not taken place, and in
any such case,  appropriate  provisions shall be made with respect to the rights
and interests of the Holder to the end that the  provisions  hereof  (including,
without limitation, provisions for adjustment of the Conversion Price and of the
number of shares of Common Stock and Warrants  issuable upon  conversion of this
Debenture)  shall  thereafter be applicable,  as nearly as may be practicable in
relation to any shares of stock or securities  thereafter  deliverable  upon the
conversion thereof.  The Corporation shall not effect any transaction  described
in this  Paragraph C unless (i) the Holder has received  written  notice of such
transaction at least thirty (30) days prior thereto,  but in no event later than
ten (10) days prior to the record  date for the  determination  of  shareholders
entitled to vote with respect thereto;  provided,  however, that the Corporation
shall not be required to disclose any material inside  information to the Holder
prior to the public  disclosure  thereof,  and (ii) the  resulting  successor or
acquiring  entity (if not the  Corporation)  assumes by written  instrument  the
obligations of this Debenture.  The above  provisions  shall apply regardless of
whether  or not there  would have been a  sufficient  number of shares of Common
Stock  authorized  and available for issuance upon  conversion of the Debentures
outstanding and the Warrants issuable upon conversion  thereof as of the date of
such  transaction,  and shall similarly  apply to successive  reclassifications,
consolidations, mergers, sales, transfers or share exchanges.

     D. Adjustment Due to Distribution. If the Corporation shall declare or make
any  distribution  of its assets (or rights to acquire its assets) to holders of
Common Stock as a partial liquidating  dividend,  by way of return of capital or
otherwise   (including  any  dividend  or  distribution  to  the   Corporation's
shareholders in cash or shares (or rights to acquire shares) of capital stock of
a subsidiary  (i.e. a spin-off))  (a  "Distribution"),  then the Holder shall be
entitled,  upon any  conversion of this  Debenture  after the date of record for
determining shareholders entitled to such Distribution, to receive the amount of
such  assets  which would have been  payable to the Holder  with  respect to the
shares of Common Stock issuable upon such conversion had Holder been the holder


                                       15


of such  shares of Common  Stock on the  record  date for the  determination  of
shareholders entitled to such Distribution.

     E. Issuance of Other Securities With Variable  Conversion Price. If, at any
time when prior to the first annual  anniversary of the Closing Date (as defined
in  the  Securities  Purchase  Agreement),   the  Corporation  shall  issue  any
securities   which  are  convertible  into  or  exchangeable  for  Common  Stock
("Convertible  Securities") at a conversion or exchange rate based on a discount
from the market price of the Common Stock at the time of conversion or exercise,
then the Variable  Conversion  Price in respect of any conversion of any portion
of this Debenture after such issuance shall be calculated utilizing the greatest
discount applicable to any such Convertible Securities.

     F. Purchase Rights. If the Corporation issues any Convertible Securities or
rights to purchase stock, warrants,  securities or other property (the "Purchase
Rights") pro rata to the record  holders of any class of Common Stock,  then the
Holder will be entitled to acquire,  upon the terms  applicable to such Purchase
Rights,  the aggregate  Purchase  Rights which the Holder could have acquired if
Holder had held the number of shares of Common Stock  acquirable  upon  complete
conversion of this  Debenture  immediately  before the date on which a record is
taken for the grant,  issuance or sale of such Purchase  Rights,  or, if no such
record is taken,  the date as of which the record holders of Common Stock are to
be determined for the grant, issue or sale of such Purchase Rights.

     G.  Notice  of  Adjustments.  Upon the  occurrence  of each  adjustment  or
readjustment  of  the  Conversion   Price  pursuant  to  this  Article  IX,  the
Corporation,   at  its  expense,  shall  promptly  compute  such  adjustment  or
readjustment  and prepare and furnish to the Holder a certificate  setting forth
such adjustment or readjustment  and showing in detail the facts upon which such
adjustment or readjustment is based.  The  Corporation  shall,  upon the written
request at any time of  Holder,  furnish  to Holder a like  certificate  setting
forth (i) such adjustment or readjustment, (ii) the Conversion Price at the time
in effect and (iii) the number of shares of Common  Stock and  Warrants  and the
amount,  if any,  of other  securities  or  property  which at the time would be
received upon conversion of this Debenture.

                                    ARTICLE X

                                  MISCELLANEOUS

     A. Failure or Indulgency Not Waiver. No failure or delay on the part of the
Holder in the exercise of any power, right or privilege  hereunder shall operate
as a waiver thereof, nor shall any single or partial exercise of any such power,
right or privilege  preclude other or further  exercise  thereof or of any other
right, power or privilege.

     B. Notices. Any notice herein required or permitted to be given shall be in
writing and may be personally served or delivered by courier and shall be deemed
to have been given upon


                                       16


receipt (which shall include  telephone line  facsimile  transmission).  For the
purposes  hereof,  the address of the Holder shall be as shown on the records of
the  Corporation;  and the  address of the  Corporation  shall be 45472  Holiday
Drive, Sterling,  Virginia 20166, Attention:  Chief Financial Officer; Facsimile
Number:  (703)  318-4315,  with a copy to Amon & Sabatini,  L.L.P.,  437 Madison
Avenue,  New York, New York 10022,  Attention:  Thomas Amon;  Facsimile  Number:
(212) 980- 3075.  Both the Holder and the  Corporation may change the address or
facsimile number for service by service of written notice to the other as herein
provided.

     C. Amendment Provision. This Debenture and any provision hereof may only be
amended by an instrument in writing signed by the  Corporation  and the Majority
Holders.  The term  "Debenture" and all references  thereto,  as used throughout
this instrument,  shall mean this instrument as originally executed, or if later
amended or supplemented, then as so amended or supplemented.

     D. Assignability.  This Debenture shall be binding upon the Corporation and
its  successors and assigns and shall inure to the benefit of the Holder and its
successors  and  assigns.  The  Holder  shall  notify the  Corporation  upon the
assignment  of this  Debenture.  In the event a Holder  shall sell or  otherwise
transfer any portion of this Debenture, each transferee shall be allocated a pro
rata portion of such transferor's Cap Amount and Reserved Amount. Any portion of
the Cap Amount or  Reserved  Amount  which  remains  allocated  to any person or
entity which does not hold any  Debentures  shall be allocated to the  remaining
holders  of  Debentures,  pro  rata  based  on the  total  principal  amount  of
Debentures  then  held by such  Holders.  Each  Holder  agrees  that it will not
transfer less than $200,000 of principal amount of Debentures to any transferee.

     E. Cost of  Collection.  If default is made in any manner  with  respect to
this Debenture, the Corporation shall pay the Holder hereof costs of collection,
including reasonable attorneys' fees.

     F.  Governing  Law.  This  Debenture  shall be governed by and construed in
accordance  with the laws of the State of New York  applicable to contracts made
and to be  performed  in the  State of New  York.  The  Corporation  irrevocably
consents to the  jurisdiction of the United States federal courts located in New
York,  New  York in any  suit or  proceeding  based  on or  arising  under  this
Agreement  and  irrevocably  agrees  that all  claims in respect of such suit or
proceeding may be determined in such courts. The Corporation  irrevocably waives
the  defense  of an  inconvenient  forum  to the  maintenance  of  such  suit or
proceeding.  The  Corporation  further  agrees that  service of process upon the
Corporation,  mailed  by first  class  mail  shall be  deemed  in every  respect
effective  service  of  process  upon  the  Corporation  in  any  such  suit  or
proceeding.  Nothing  herein shall affect the Holder's right to serve process in
any  other  manner  permitted  by  law.  The  Corporation  agrees  that a  final
non-appealable  judgment in any such suit or proceeding  shall be conclusive and
may be enforced in other  jurisdictions by suit on such judgment or in any other
lawful manner.

     G.  Denominations.  At the request of the Holder,  upon  surrender  of this
Debenture,  the Corporation shall promptly issue new Debentures in the aggregate
outstanding  principal amount hereof, in the form hereof, in such  denominations
of at least $100,000 as the Holder shall request.


                                       17


     H.  Lost or Stolen  Debentures.  Upon  receipt  by the  Corporation  of (i)
evidence of the loss,  theft,  destruction  or mutilation of this  Debenture and
(ii) (y) in the case of loss,  theft or  destruction,  of  indemnity  reasonably
satisfactory  to  the  Corporation,  or  (z) in the  case  of  mutilation,  upon
surrender and cancellation of this Debenture,  the Corporation shall execute and
deliver new Debentures,  in the form hereof,  in such  denominations of at least
$100,000  as the Holder  may  request.  However,  the  Corporation  shall not be
obligated   to   reissue   such  lost  or  stolen   Debentures   if  the  Holder
contemporaneously requests the Corporation to convert this Debenture.

     I. Statements of Available  Shares.  Upon request,  the  Corporation  shall
deliver to Holder a written report  notifying the Holder of any occurrence which
prohibits the  Corporation  from issuing  Common Stock or Warrants upon any such
conversion.  The report shall also specify (i) the total principal amount of all
outstanding  Debentures as of the date of the request,  (ii) the total number of
shares of Common Stock and Warrants  issued upon all  conversions  of Debentures
through  the date of the  request,  (iii) the  total  number of shares of Common
Stock issued upon exercise of all Warrants through the date of the request, (iv)
the total number of shares of Common Stock which are reserved for issuance  upon
conversion of Debentures  and exercise of Warrants as of the date of the request
and (v) the total  number of shares of  Common  Stock  which may  thereafter  be
issued by the Corporation upon conversion of Debentures and exercise of Warrants
before the Corporation would exceed the Cap Amount and the Reserved Amount.  The
Corporation  shall  provide,  within  fifteen  (15) days after  delivery  to the
Corporation of a written request by Holder, all of the information enumerated in
clauses (i) - (v) of this Paragraph I.

     J. Payment of Cash; Defaults.  Whenever the Corporation is required to make
any cash  payment  to Holder  under  this  Debenture  (as a  Conversion  Default
Payment,  Default Amount or  otherwise),  such cash payment shall be made to the
Holder  within  five (5)  business  days  after  delivery  by Holder of a notice
specifying that the Holder elects to receive such payment in cash and the method
(e.g.,  by check,  wire  transfer) in which such payment should be made. If such
payment is not delivered within such five (5) business day period,  Holder shall
thereafter  be entitled  to  interest  on the unpaid  amount at a per annum rate
equal to the lower of  twenty-four  percent (24%) and the highest  interest rate
permitted  by  applicable  law until such  amount is paid in full to the Holder.
Payment of interest under this Article X.J. is in lieu of and not in addition to
the interest provided for in clause (i) of Article VIII.C.

     K. Restrictions on Shares. The shares of Common Stock and Warrants issuable
upon conversion of this Debenture may not be sold or transferred unless (i) they
first shall have been registered  under the Securities Act and applicable  state
securities laws, (ii) the Corporation  shall have been furnished with an opinion
of legal counsel (in form,  substance  and scope  customary for opinions in such
circumstances)  to the  effect  that such sale or  transfer  is exempt  from the
registration  requirements of the Securities Act or (iii) they are sold pursuant
to Rule 144  under the Act.  Except  as  otherwise  provided  in the  Securities
Purchase  Agreement,  each  certificate  for shares of Common Stock and Warrants
issuable upon  conversion of this Debenture that have not been so registered and
that have not been sold  pursuant to an exemption  that  permits  removal of the
legend, shall bear a legend substantially in the following form, as appropriate:


                                       18



          THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
          REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED. THE
          SECURITIES  HAVE BEEN ACQUIRED FOR INVESTMENT AND MAY NOT BE
          SOLD, TRANSFERRED OR ASSIGNED IN THE ABSENCE OF AN EFFECTIVE
          REGISTRATION   STATEMENT  FOR  THE   SECURITIES   UNDER  THE
          SECURITIES ACT OF 1933, AS AMENDED, OR AN OPINION OF COUNSEL
          IN FORM,  SUBSTANCE AND SCOPE CUSTOMARY FOR OPINIONS IN SUCH
          CIRCUMSTANCES  THAT  REGISTRATION IS NOT REQUIRED UNDER SAID
          ACT OR UNLESS SOLD  PURSUANT TO RULE 144 UNDER SAID ACT. ANY
          SUCH SALE,  ASSIGNMENT  OR  TRANSFER  MUST ALSO  COMPLY WITH
          APPLICABLE STATE SECURITIES LAWS.

Upon the request of a holder of a certificate  representing any shares of Common
Stock or Warrants  issuable upon conversion of this  Debenture,  the Corporation
shall remove the foregoing legend from the certificate or issue to such holder a
new certificate  therefor free of any transfer legend, if (i) with such request,
the Corporation  shall have received either (A) an opinion of counsel,  in form,
substance and scope customary for opinions in such circumstances,  to the effect
that any such legend may be removed from such  certificate,  or (B) satisfactory
representations  from the Holder that  Holder is eligible to sell such  security
pursuant to Rule 144 or (ii) a registration  statement  under the Securities Act
covering such securities is in effect. Nothing in this Debenture shall (i) limit
the Corporation's  obligation under the Registration  Rights Agreement,  or (ii)
affect in any way the Holder's obligations to comply with applicable  securities
laws upon the resale of the securities referred to herein.

     L. Status as Debentureholder.  Upon submission of a Notice of Conversion by
Holder,  the principal amount of this Debenture and the interest thereon covered
thereby shall be deemed  converted  into shares of Common Stock and the holder's
rights with respect thereto shall cease and terminate,  excepting only the right
to receive  certificates  for such  shares of Common  Stock and to any  remedies
provided herein or otherwise  available at law or in equity to Holder because of
a  failure  by the  Corporation  to  comply  with the  terms of this  Debenture.
Notwithstanding the foregoing,  if Holder has not received  certificates for all
shares  of  Common  Stock  prior to the  tenth  (10th)  business  day  after the
expiration of the Delivery  Period with respect to a conversion  for any reason,
then (unless Holder  otherwise elects to retain its status as a holder of Common
Stock) the portion of the principal  amount and interest thereon subject to such
conversion shall be deemed  outstanding under this Debenture and the Corporation
shall,  as soon as  practicable,  return this  Debenture  to the Holder.  In all
cases,  Holder shall retain all of its rights and remedies  (including,  without
limitation,  (i) the right to receive  Conversion  Default Payments  pursuant to
Article VI.A to the extent required thereby for such Conversion  Default and any
subsequent  Conversion  Default and (ii) the right to have the Conversion  Price
with respect to subsequent  conversions  determined  in accordance  with Article
VI.B) for the Corporation's failure to convert this Debenture.


                                       19


     M. Remedies  Cumulative.  The remedies  provided in this Debenture shall be
cumulative and in addition to all other remedies available under this Debenture,
at law or in equity  (including  a decree of specific  performance  and/or other
injunctive  relief),  and nothing  herein shall limit  Holder's  right to pursue
actual  damages for any failure by the  Corporation  to comply with the terms of
this  Debenture.  The  Corporation  acknowledges  that  a  breach  by it of  its
obligations  hereunder  will cause  irreparable  harm to the Holder and that the
remedy at law for any such breach may be inadequate.  The Corporation  therefore
agrees, in the event of any such breach or threatened  breach,  the Holder shall
be  entitled,  in addition to all other  available  remedies,  to an  injunction
restraining  any breach,  without the  necessity  of showing  economic  loss and
without any bond or other security being required.

     N. Required Conversion Into Preferred Stock.

          (i) At any time during the first six (6) months following the Issuance
Date (the "Required Conversion Period") that the Required Conversion  Conditions
(as defined in subparagraph  (iii) below) are satisfied,  the Corporation  shall
have the right to require the  conversion  of all (but not less than all) of the
outstanding  principal  amount of this Debenture into that number of shares of a
to-be created series of the Company's  preferred stock having the  designations,
preferences and rights set forth in the Certificate of Designations, Preferences
and Rights  attached  to the  Securities  Purchase  Agreement  as Exhibit F (the
"Series  A  Preferred  Stock")  which  have a total  face  amount  equal to such
principal  amount (a "Required  Conversion").  The Corporation may exercise such
right by delivery of a Required  Conversion  Notice (as defined in  subparagraph
(ii)  below)  during  the  Required  Conversion  Period in  accordance  with the
procedures  set forth  below.  Holder  may  convert  all or any  portion of this
Debenture  into  Common  Stock by  delivering  a  Notice  of  Conversion  to the
Corporation at any time prior to the Effective  Date of Required  Conversion (as
defined in subparagraph (ii) below).

          (ii) The  Corporation  shall effect a Required  Conversion  under this
Paragraph N by giving at least five (5) business days but not more than ten (10)
business days prior written  notice (the  "Required  Conversion  Notice") of the
date which such Required  Conversion is to become effective (the "Effective Date
of Required  Conversion") to Holder,  which Required  Conversion Notice shall be
deemed to have been  delivered on the business day after the  Corporation's  fax
(with a copy sent by  overnight  courier)  of such  notice to  Holder.  Upon the
surrender of this Debenture,  the Corporation  shall issue and deliver to Holder
the shares of Series A  Preferred  Stock to which  Holder is  entitled  upon the
Required Conversion.

          (iii) The "Required Conditions" shall consist of the following:

               (a) no Event of Default shall have occurred;

               (b) the Common Stock shall be authorized  for quotation on Nasdaq
and  trading  in the  Common  Stock (or  Nasdaq  generally)  shall not have been
suspended;


                                       20


               (c) Holder shall have  received an officer's  certificate  in the
form attached as Exhibit A attached hereto;

               (d) the  Certificate of Designation  shall have been accepted for
filing with the  Secretary of State of Virginia and a copy thereof  certified by
the Secretary of State of Virginia shall have been delivered to Holder;

               (e) Holder  shall  have  received  an  opinion  of the  Company's
counsel,  dated as of the Effective Date of Required Conversion,  in form, scope
and substance reasonably satisfactory to Holder and in substantially the form of
Exhibit B attached hereto;

     O.  Force  Majeure.  Neither  the  Corporation  nor  the  Holder  shall  be
responsible  for any delay or failure to perform any part of this  Debenture  to
the  extent  that  such  delay or  failure  is  solely  caused  by fire,  flood,
earthquake,  explosion,  war, labor strike, riot, act of governmental,  civil or
military authority which imposes a moratorium on the performance of the specific
obligation  in  question  or other  comparable  extraordinary  event  beyond the
Corporation's  or Holder's  control.  Notice with full details of any such event
shall  be  given to the  other  party  as  promptly  as  practicable  after  its
occurrence.  The  affected  party  shall use its best  efforts to  minimize  the
effects  of or end any such event so as to  facilitate  the  resumption  of full
performance hereunder.

                  [Remainder of Page Intentionally Left Blank]


                                       21


     IN WITNESS WHEREOF,  Borrower has caused this Debenture to be signed in its
name by its duly authorized officer this 9th day of April, 1997.


                                       FASTCOMM COMMUNICATIONS CORPORATION


                                       By:______________________________________
                                            Name:
                                            Title:




                                       22


                                                                       Exhibit 1

                              NOTICE OF CONVERSION

The undersigned  hereby irrevocably  elects to convert  $____________  principal
amount of the  Debenture and all accrued and unpaid  interest on such  principal
amount  (i.e.,  $_________)  (the  "Conversion"),  into  shares of common  stock
("Common  Stock")  and  Warrants  of FastComm  Communications  Corporation  (the
"Corporation")  according to the  conditions of the  Convertible  Term Debenture
dated  April 9,  1997  (the  "Debenture"),  as of the  date  written  below.  If
securities are to be issued in the name of a person other than the  undersigned,
the undersigned will pay all transfer taxes payable with respect thereto. No fee
will be charged to the holder for any conversion,  except for transfer taxes, if
any. A copy of the Debenture is attached  hereto (or evidence of loss,  theft or
destruction thereof).

The  undersigned  represents  and  warrants  that all  offers  and  sales by the
undersigned of the securities  issuable to the  undersigned  upon  conversion of
this Debenture  shall be made pursuant to registration of the Common Stock under
the Securities Act of 1933, as amended (the "Act"),  or pursuant to an exemption
from registration under the Act.

In the event of partial exercise, please reissue an appropriate Debenture(s) for
the principal balance which shall not have been converted.

                               Date of Conversion:___________________________   
                               
                               Applicable Conversion Price:____________________
                               
                               Amount of Conversion Default Payments
                               to be Converted, if any:______________________
                               
                               Number of Shares of
                               Common Stock to be Issued:_____________________
                               
                               Number of Warrants to be Issued:_________________
                               
                               Signature:____________________________________
                               
                               Name:_______________________________________
                               
                               Address:______________________________________
                               
ACKNOWLEDGED AND AGREED:

FASTCOMM COMMUNICATIONS CORPORATION

BY:___________________________
NAME:________________________
TITLE:________________________                          DATE:___________________

* The  Corporation  is not  required to issue shares of Common Stock or Warrants
until the original Debenture (or evidence of loss, theft or destruction  thereof
and  reasonable  indemnity,  if  requested)  to be converted are received by the
Corporation  or its  transfer  agent.  The  Corporation  shall issue and deliver
shares of Common Stock and  Warrants to an overnight  courier not later than the
later  of (a) two  (2)  business  days  following  receipt  of  this  Notice  of
Conversion and (b) the date of surrender of this Debenture (or evidence of loss,
theft or destruction thereof), and shall make payments pursuant to the Debenture
for the failure to make timely delivery.