United States Patent [19J
`Cummings et al.
`
`lllllllllll~llllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`US005406260A
`5,406,260
`[II] Patent Number:
`[45] Date of Patent:
`Apr. 11, 1995
`
`[75]
`
`(54) NETWORK SECURITY SYSI'EM FOR
`DETECfiNG REMOVAL OF ELECrRONIC
`EQUIPMENf
`Inventors: Mlli'Shall B. Cummings, Ann Arbor,
`Mich.; Christopher R. Young, Austil),
`Tex.
`(73) Assignee: CbriMar Systems, Inc . ., Ann Arbor,
`Mich.
`(21] Appl. No.: 992,.924
`[22] Filed:
`Dec. 18, 1992
`Int. C1,6 .............................................. GOSB 21/00
`[51]
`(52) u.s. a ...................................... 340!568; 340/687
`[58] Field of Search ............... 340/568, 571, 572, 652,
`340/664, 687
`
`[56)
`
`Rerercnces Cited
`U.S. PATENT DOCUMENTS
`3,618,065 11/1971 Trip ..................................... 340/568
`3,932,857 1/1976 Way et al. ........................... 340/572
`4,654,640 3/1987 Carll el ai. .. ......................... 340/568
`4,686,514 8/1987 Liptak, Jr. et aJ .................. 340/S71
`4,736,195 4/1988 McMurtry et al. ................. 340/568
`4,760,382 7/1988 Faulkller ............................. 340/572
`5,034,723 7/1991 Maman ................................ 340/568
`5,059,948 1/1991 Oesmeules .......................... 340/568
`5,066,942 11/1991 Matsuo ................................ 340/568
`5,136,580 8/1992 Vide1ock et al ...................... 370/60
`5,231,375 7/1993 Sanders et al ....................... 340/568
`
`5,243,328 9/1993 Lee et al ............................. 340/568
`
`FOREIGN P ATENT D OCUMENTS
`357482 3/1990 European Pat. Off ............. 340/568
`4203304 8/1992 Germany ............................ 340/568
`Primary Examiner-John K. Peng
`Assistant Examiner-Thomas J. Mullen, Jr.
`Attorney, Agent, or Firm-Harness, Dickey & Pierce
`(57)
`ABSTRACf
`A system and method are provided for monitoring the
`connection of electronic equipment, such as remote
`computer workstations, to a network via a communica(cid:173)
`tion link and detecting the disconnection of such equip(cid:173)
`ment from the network. The system includes current
`loops internally coupled to protected pieces of equip(cid:173)
`ment so that each piece of associated equipment has an
`associated current loop. A low current power signal is
`provided to each of the current loops. A sensor moni(cid:173)
`tors the current flow through each current loop to de(cid:173)
`tect removal of the equipment from the network. Re(cid:173)
`moval of a piece of hardware breaks the current flow
`through the associated current loop which in tum may
`activate an alarm. This invention is particularly adapted
`to be used with an existing lOBaseT communication link
`or equivalent thereof, employing existing wiring to
`form the current loops.
`
`19 Qaims, 3 Drawing Sheets
`
`-------------------~----~
`
`16
`
`JO
`
`OP-AMP
`VOLTAGE TO
`CURRENT
`CONVERTER
`
`34
`
`Page 1
`
` Dell Inc.
` Exhibit 1004
`
`

`
`0
`0'\
`N
`...
`0'\
`0
`~
`U1
`
`w
`g,
`....
`00 r ....
`
`Ul
`~
`....
`.!"""
`~ ....
`
`~ ~
`~ a
`•
`00
`~
`
`~-------------------------------1
`I
`I
`1
`1
`I
`I
`1
`I
`I
`I
`I
`I
`1
`1
`I
`I
`:
`I
`I
`1
`:
`
`Fig-1
`
`LOGIC
`ALARM
`DIGITAL
`
`38
`
`34
`
`32
`
`\
`
`I
`
`I ----· .. __ ,
`
`VOLTAGE TO
`
`CONVERTER
`CURRENT
`
`OP-AMP
`
`I
`
`1
`
`24
`
`: I ISOLATION ~26
`--------------~-----------------
`
`POWER SUPPLY
`
`28
`
`I
`
`)
`
`I~
`
`• 0 0
`
`18
`
`•
`
`•
`
`:
`-------
`~10
`______ _/ __ _
`
`(
`
`16
`
`NETWORK BACKBONE
`
`•
`
`NETWORK I
`
`SEAVER
`FILE
`
`-------------
`
`Page 2
`
`

`
`N g
`...
`0'\
`Q
`~
`(11
`
`w
`~ a.
`t:r !
`
`00
`
`""" ~
`!"'"
`"""
`~ :"'
`
`~ f'+
`~ a
`00 .
`d
`
`42~
`CRE§IT)
`
`V+
`44b
`
`44c
`
`12d
`
`52~ Fig-2 44d~4a
`
`ALA~~
`
`40
`
`II 2-1 36b
`~tt:Sa
`
`38
`
`§
`
`I
`34~~-_....!----.
`
`.=::::::::! -1:/
`
`I
`
`':f -f f-} '!: 4 fa-d)
`
`(§)25
`
`HUB-XMIT
`
`'--
`
`20
`
`.ft:j1-50d
`
`I')Sd
`
`*c3+:C2-f.C1
`
`46c 44d -I
`
`50c
`
`12c
`
`SZc
`
`12b
`
`12a
`
`~ 52b
`
`--c46a
`
`I
`44a
`
`Page 3
`
`

`
`U.S. Patent
`
`Apr. 11, 1995
`
`Sheet 3 of 3
`
`5,406,260
`
`Page 4
`
`

`
`1
`
`5,406,260
`
`NETWORK SECURITY SYSTEM FOR DETECITNG
`REMOVAL OF ELECTRONIC EQUIPMENT
`
`2
`closes a theft alarm system for use with a digital signal
`PBX telephone system. This method includes a plural(cid:173)
`ity of electronic tethers which are connected to individ(cid:173)
`·ual pieces of protected equipment by way of connectors
`BACKGROUND OF THE INVENTION
`which in turn are bonded to the surface of the protected
`1. Technical Field
`equipment. Each tether includes a pair of conductors
`This invention relates generally to theft protection
`which are connected together to form a closed current
`loop via a series resistor and conductive foil which is
`security systems and, more particularly, to a network
`adhesively bonded to the outside of the equipment.
`security system for detecting the unauthorized removal
`of remotely located electronic equipment from a net- 10 However, this method requires the addition of an exter-
`work.
`nally mounted current loop, and it is conceivable that
`2. Discussion
`the current loop may be carefully removed without any
`There has been an ever increasing need to provide
`detection.
`security for electronic equipment against the unautho-
`It is therefore desirable to provide for an enhanced
`rized removal or theft thereof. Computer systems have IS network security system which detects unauthorized
`become a major capital expenditure for users which
`removal of remotely located pieces of hardware from a
`commonly include businesses, educational institutions
`network. More particularly, it is desirable to provide for
`and gove~ental entities, amon~ c;>ther users. Ad-
`such a security system which feasibly employs separate
`van~ments 10 ~chnology have Significantly ~educed
`current loops provided through an existing data com-
`the sJZe and weight of complex computer equipment, 20 munication link to monitor the presence of remotely
`located computer equipment. In addition, it is desirable
`thus making expensive computer equipment more easily
`portable. As a consequence, modem computer equip-
`to provide for a security network system which may be
`ment is generally more compact and more easily trans-
`easily and inexpensively implemented in an existing
`portable, which further makes it more difficult to secure
`network system and may not be easily physically re-
`2s moved or detached from tile system without detection.
`against the unauthorized removal or theft thereof.
`Today, computer network systems are frequently
`SUMMARY OF THE INVENTION
`employed to provide efficient computing capabilities
`throughout a large work area. Existing computer net-
`In accordance with the teachings of the present in-
`work systems generally include a number of remotely
`vention, a security system is provided for detecting
`located work stations coupled via a data communica- 30
`unauthorized removal of electronic equipment from a
`tion link to a central processing center. For instance,
`network. The system includes current loops internally
`many educational institutions such as universities com-
`monly provide a large number of individual work sta-
`coupled to protected pieces of equipment so that each
`piece of associated equipment has an associated current
`tions at different locations throughout the university
`campus so as to allow easy computing access to the 3S loop. A low current power signal is applied to each of
`computer network system. However, the wide disserni-
`the current loops. A detector monitors current flow
`~hrough each of th~ current loops so as to detect .a drop
`nation of such equipment at remote locations has made
`m current flow which represents removal of eq~pment
`the equipment an accessible target for computer thieves.
`froi? the network. Detec~on of removal o~ ~ p1ece. of
`Accordingly, a number of methods have been devel-
`oped for guarding against the unauthorized removal of ~ ~Ulpi?ent may 10 tum activate an .alarm .. This. mve~tton
`IS particularly adapted I? be used .10. conjunction .Wit~ a
`electronic equipment. Early methods of protection have
`included the physical attachment of a security cord to
`c~~puter network ~avmg an ~XIstmg co.mmurucauon
`each piece of protected equipment. However, the secu-
`WIIlDg scheme co~pling each ptece of eqwpment to the
`rity cord generally may be cut or physically detached
`network, and which may be used to form the current
`from its secured position and is usually considered to be 4S loops.
`a non-appealing aesthetic ~ddi.tion to the equipment.
`BRIEF DEsCRIPTION OF THE DRAWINGS
`Another method of protection mcludes the attachment
`Other objects and advantages of the present invention
`of a non-removal tag to the equipment which also re-
`quires cooperating sensing devices responsive to the tag
`will become apparent to those skilled in the art upon
`which are properly located at exit locations from the so reading the following detailed description and upon
`premises. However, this approach requires rather ex-
`reference to the drawings in which:
`pensive sensing devices and is generally not very feasi-
`FIG. 1 is a block diagram which illustrates a network
`ble especially when multiple exit points exist.
`security system coupled in to a computer network in
`Other methods of theft protection have included
`accordance with the present invention;
`installing a special electronic card inside each computer 55
`FIG. 2 is a circuit diagram which illustrates the net-
`machine which responds to polls from an external moni-
`work security system coupled to the computer network
`toeing station. Upon removal of the machine, the card
`in accordance with the present invention; and
`stops responding to the polling of the central station and .
`FIG. 3 is a schematic diagram which illustrates instal-
`an alarm is initiated. Another approach involves mount-
`lation of the network security system into an existing
`ing a sensing device on or into the machine to detect 60 computer network in accordance with the present in-
`movement of the machines. These approaches, how-
`vention.
`ever, are generally undesirable since they require the
`DETAILED DESCRIPTION OF THE
`incorporation of additional components into each ma(cid:173)
`PREFERRED EMBODIMENT
`chine. ·
`More recent methods of theft protection have in- 65
`Turning now to FIGS. 1 and 2 a network security
`eluded the sensing of a current loop coupled to the
`system 24 is provided therein for achieving theft protec(cid:173)
`protected equipment. One such method is discussed in
`tion of electronic computer equipment associated with a
`U.S. Pat. No. 4,654,640 issued to Carll et al which dis-
`computer network 10. In general, the network security
`
`Page 5
`
`

`
`5,406,260
`
`4
`3
`through C. operate as a power supply filter to filte.r out
`system 24 momtors remotely located electronic work
`any undesirable AC signals such as network operating
`stations such as personal computers t 2a through 12d via
`current loop continuity so as to detect the removal of
`signals. Each of power supply lines 28a through 28d is
`any of the personal computers 12a through 12d from the
`further coupled in series to one of resistors R1(a-d) and
`computer network 10. The network security system 24 5 one of inductors L1(a-d), respectively. Each of resistors
`described herein is particularly adapted to be easily
`R1(a- d) has a preferred resistance of about one kiloohm
`implemented in conjunction with an existing computer
`(I kO.) which ensures a low current flow thereacross.
`network 10 without the need for substantial modifica-
`Accordingly, inductors L1(a-d) provide isolation to
`tions and while realizing minimal interference to the
`power supply 26 by blocking unwanted AC signals
`10 from transmitting through lines 28a through 28d. Ac-
`computer network 10.
`cording to one embodiment, capacitors c, through C3
`Remotely located personal computers 12a through
`have respective values of 100 pF, 0.1 JA.F and 1.0 J.I.F,
`12d are each connected to the computer network 10 so
`as to provide widespread remote user access to the
`while capacitors C4(a-d) each have values of O.l J.I.F and
`inductors L1(a-d) each have values of 120 mH.
`computer network 10. The computer network 10 shown
`herein is of the conventional type which includes a 15 The power supply lines 28a through 28d each are
`network file server 18 connected to a network back-
`electrically coupled to respective transmit wires 44a
`through 44d found within data communication link 14.
`bone 16. The computer network 10 may include most
`any type of backbone such as, for instance, an Ether-
`Receive power lines 30a through 30d are likewise elec-
`net ® backbone manufactured by Xerox Corporation.
`trically coupled to transmit wires 46a through 46d also
`A plurality of hubs such as hubs 20, 21, 22 and 23 are 20 found within the data communication link 14. Transmit
`wires 44a through 44d and 46a through 46d are existing
`generally coupled to the network file server 18 or back-
`bone 16 to provide communication links therewith. The
`wires found within data communication link 14 that are
`remotely located personal computers 12a through 12d
`selectively tapped as pairs in accordance with the pres-
`ent invention to provide current loops 50a through SOd.
`are shown conn~ted to hub 20 via a data communica-
`tion link 14. Data communication link 14 includes a 25 As a consequence, power supply line 28a continu-
`plurality of transmit and receive data communication
`ously supplies a low current DC power signal to remote
`lines for communicating infonnation between each of
`personal computer 12a via transmit wire 44a. The low
`remotely located personal computers 12a through 12d
`current power signal flows through an internal path
`and the network ftle server 18 via network backbone 16
`provided by existing circuitry in personal computer
`and hub 20.
`30 12a. The low current power signal then exits the remote
`The invention descn"bed herein is particularly suited
`personal computer 12a via transmit wire 46a and in turn
`to be implemented in conjunction with a computer
`is picked up by receive power line 30a. The low current
`power signal is continuously supplied to current loops
`network 10 which preferably employs a conventional
`50a through SOd at all times regardless of whether the
`wiring approach of the type which may include lOBa-
`seT wiring. Wiring schemes of the lOBaseT type are 35 computer network 10 or any personal computers 12a
`through 12d are operating or not. In addition, the very
`commonly employed to provide data communication
`lines for electromc computer equipment. In accordance
`low current DC power signal is so small that it does not
`with conventional wiring approaches, data communica-
`interfere with or adversely effect the operation of the
`tion link 14 generally includes a plurality of pairs of
`associated computers 12a through 12d or computer
`transmit wires 44 and 46 as well as a plurality of pairs of 40 network 10. To prevent the now of DC current to or
`receive wires (not shown) connected to each of per-
`from hub 20, each of transrmt wires 44a through 44d
`and 46a through 46d are further coupled to DC block-
`sonal computers 12a through 12d. Each pair of transrmt
`ing capacitors Cs between each of current loops 50a
`wires 44 and 46 are internally coupled to an associated
`through SOd and hub 20. DC blocking capacitors Cs
`personal computer 12 via one winding 53 of an inter-
`nally located isolation transformer 52. Each pair of 45 thereby prevent unwanted DC current paths through
`hub 20.
`transrmt wires 44 and 46 along with isolation trans-
`former 52 thereby form a current loop 50 through the
`The return power signals tapped from transmit wires
`46a through 46d via receive power lines 30a through
`personal computer 12 which is advantageously em-
`30d are then applied to a signal isolation device 32. The
`played in accordance with the approach described
`herein. However, the same approach could be imple- 50 signal isolation device 32 includes an RLC circuit made
`mented with the pairs of receive wires without depart-
`up of inductors Lz(a-d) coupled in parallel to grounded
`ing from the scope of this invention.
`pairs of parallel connected resistors R2 and capacitors
`C6 which are coupled to each of receive power lines 30a
`The network security system 24 includes an isolation
`power supply 26 which supplies a continuous direct
`through 30d. Accordingly, the signal isolation device 32
`current (DC) power signal to each of current loops 50a 55 helps to prevent network operating signals from inter-
`through SOd. The DC power signal has a low current
`fering with one another. According to one preferred
`preferably on the order of magnitude of less than one
`embodiment, resistors Rz and capacitors C6 each have
`preferred values of 100 kO. and 0.33 JA.F, respectively,
`milliamp (I mA) and, more specifically includes a pre-
`ferred current of approximately fifty rmcroamps (50
`while inductors Lz(a-d) each have preferred values of
`JA.A). The isolation power supply 26 includes an input 60 120 mH each.
`terrmnal 25 for receiving a low voltage signal VJN.
`Op.amp voltage to current converters 34a through
`34d are further connected to receive power lines 30a
`which has a magnitude of approximately five (5) volts.
`A plurality of parallel connected capacitors Ct, C2, and
`through 30d, respectively. The voltage to current con-
`c 3 are connected to input terrmnal 25. In addition, a
`verters 34A through 340 each convert the return
`plurality of power supply lines 28a through 28d are 65 power signal to a desired magnitude current signal via
`an operational transconductance amplifier. A signal
`provided, each of which has one of capacitors C4(a-d)
`coupled thereto, and all of which are coupled to parallel
`conditioning unit 36 in tum is connected to the output
`connected capacitors c, through C3. Capacitors Ct
`of the voltage to current converter 34. The signal condi-
`
`Page 6
`
`

`
`5,406,260
`
`5
`6
`In operation, the isolation power supply 26 supplies a
`tioni.ng unit 36 includes Schmidt trigger buffers 36a
`through 36d which further ensure a smooth DC signal
`continuous low current DC power signal to each of
`power supply lines 28a through 28d. The low current
`response.
`power signal nows through current loops SOa through
`The signal conditioning unit 36 bas an output con-
`nected to digital alarm logic 38 which essentially in- 5 SOd via pairs of transtnit wires 44 and 46 and existing
`eludes a NAND gate 38. The NAND gate 38 bas four
`circuitry such as isolation transformers 52 within each
`inputs for receiving a signal from each of receive power
`of the remote personal computers 12a through l2d
`Jines 30(a-d) and generates a NAND logic operation in
`being monitored. The return signal in each of current
`loops 50a through SOd is applied to a ~ignal isolation
`response thereto. The output of the NAND gate 38 in
`turn provides an alarm output signal to an alarm 40. 10 device 32 for preventing signal interference among the
`separate communication channels and then is further
`Accordingly, a "high" signal on each NAND gate input
`which is indicative of unbroken current loop continuity
`coupled to an op-amp voltage to current converter 34.
`will result in a "low" alarm output signal. Whereas, a
`Voltage to current converter 34 converts the voltage to
`"low" signal on any input which is indicative of a cur-
`a desired current level which in turn is applied to a logic
`rent loop discontinuity will result in a "high" alarm IS NAND gate 38. The logic NAND gate 38 detects dis-
`continuit ies in the current loops SOa through SOd being
`output signal. The alarm 40 includes a reset 42 for dis-
`abling the alarm 40 when so desired. In addition, the
`monitored and provides an output indication to an
`alarm output signal may be further used to activate the
`alarm 40 which indicates removal of one or more of
`operation of additional security related functions which
`remote personal computers ~ through ~2d from the
`may include alarm status notification 10 designated au- 20 computer n~twor~ 1.0. In addttion, d~tectton of a c~-
`·
`link
`ther possible
`rent flow dtScontmutty further energtzcs the appropn-
`b "ti .
`1 1 h
`1
`amongs 0
`t on. es vta a e ep one ·
`ate light emitting diodes 44a through 44d associated
`funchons. ~own througho~t the field..
`with the disconnected personal computer 12.
`In addltton, each of recetve power ~es 30a. ~oug~
`While this invention has been described herein in
`30d is further coupled to one end of light. enutt~~~- 25 connection with a network security system 24 for de-
`odes 48a through 48d. The other end of light enu
`g
`tecting continued connection of remotely located per-
`sonal computers 12a through l2d to a computer net-
`diodes 48a through 48d are coupled to a voltage·P?wer
`s~pply V +. As a consequen~ each of ligb~ enut~ng work 10, it is conceivable that other electronic equip-
`~~~o~es. 48a through 48d pro~des an energized hg~t ment may likewise be detected without departing from
`mdicatton whe~ev~r the ~ctated current loop SO LS 30 the spirit of this invention. In addition, any number of
`pieces of equipment may be monitored with the net-
`broken so as to mdtcate wh!ch. of the personal comput-
`ers 12a through l2d are diSCOnnected from the com-
`work security system 24 and any number of network
`puter net~ork 10.
`.
`security systems may be coupled to a given network or
`FIG. 3 illustrates the connectton of the network secu-
`a plurality of networks to handle large numbers of re-
`rity system 24 ~an existing ~mputer ne~work 10. The 35 motely located pieces of equipment.
`In view of the foregoing, it can be appreciated that
`network secunty system 24 IS substantially enclosed
`within a housing 60 which is connected bet~een data
`the present invention enables the user to achieve anti-
`communication link 14 and hub 20. The houSUig.~ has
`theft protection for remotely located electronic equip-
`one or more female receptacles 62 each for recetvtng a
`ment connected to an existing network system. Thus,
`male plug 66 that is connected to one end of the data 40 while this invention has been disclosed herein in combi-
`communication ~ 14. The housing 60 further inclu~es
`nation with a particular example thereof, no limitation is
`one or more addttional female receptacles 64 for recetv-
`intended thereby except as defined in the following
`claims. This is because a skilled practitioner recognizes
`ing a male plug 68 from an additional data communica-
`tion extension line 70 which in tum connects to female
`that other modifications can be made without departing
`receptacle 74 in bub 20 via male plug 72. For purpo~ 45 from the spirit of this invention after studying the speci-
`of maintaining a secure system, the network secunty
`fication and drawings.
`system 24 is preferably located in a secure area separate
`What is claimed is:
`from personal computers 12a through 12d. This further
`1. A security system for detecting disconnection of
`ensures against unwanted tampering with the network
`electronic equipment from a network, said security
`security system 24.
`·so system comprising:
`To implement the present invention, the network
`current loop means including separate current loops ·
`security system 24 is easily installed into an existing
`associated with different pieces of monitored
`computer network 10 such as that employing a lOBaseT
`equipment. each of said current loops employing a
`hub to workstation communication link 14. In doing so,
`pair of data communication lines which connect
`the housing 60 enclosing the network security system 24 ss
`one of the associated pieces of equipment to the
`is connected between data communication link 14 and
`network and which are coupled to existing internal
`hub 20 so that male plug 66 is removed from female
`circuitry within the associated piece of monitored
`receptacle 74 in hub 20 and inserted into female recepta-
`equipment, and wherein respective pairs of data
`cle 62 in housing 60. The additional data communica-
`communication lines are associated with different
`tion extension link 70 is in turn connected between 60
`ones of the associated pieces of equipment;
`housing 60 and hub 20. As a consequence, powe.r supply
`source means for supplying a low DC current signal
`lines 28a through 28d and receive power lines 30a
`to each of said current loops; and
`through 30d are easily tapped into selected pairs of
`detector means for monitoring the current signal
`existing transmit wires 44(a-d) and 46(a-d) found in
`through each of said current loops and detecting a
`data communication link 14. The selected pairs of trans- 65
`change in said current signal through one of said
`mit wires 44 and 46 enable current to flow through
`current loops which represents disconnection of
`current loops 50a through SOd internally coupled to
`said associated piece of equipment from the net-
`personal computers 12a through 12d, respectively.
`work.
`
`Page 7
`
`

`
`5,406,260
`
`8
`7
`10. The security system as defined in claim 9 wherein
`2. The security system as defined in claim 1 wherein
`each of said pairs of data communication lines are cou-
`said electronic equipment comprises computer worksta-
`pled between one of said associated pieces of equipment
`tions each connected to a network flle server and lo-
`and a network file server.
`cated remote from the network file server.
`11. The security system as defined in claim 9 wherein
`3. The security system as delined in claim 1 wherein S
`said data communication lines are provided via IOBa-
`each of said current loops includes existing pairs of data
`seT wiring.
`communication lines used by said network for commu-
`U . The security system as defmed in claim 9 wherein
`nicating data between the associated pieces of equip-
`said internal circuitry within said associated equipment
`ment and a network file server.
`4. The security system as defined in claim 1 wherein 10 comprises a first winding of a transformer which is
`coupled between each of said respective pairs of data
`said network includes an Ethernet® network and said
`respective pairs of data communication lines include
`communication lines to form a circuit path through
`existing JOBaseT wiring connecting the different ones
`each of said pairs of data communication lines.
`of the associated pieces of equipment to said network.
`13. The sec~ty system as defined in claim 9 wherein
`5. The security system as defmed in claim 1 wherein 15 said D C current signal has a current of Jess than I milli-
`amp.
`said existing internal circuitry includes an isolation
`transformer having a first winding coupled between
`14. A method for detecting unauthorized discormec-
`said pair of data communication lines so as to aUow said
`tion of remotely located electronic equipment which
`has existing data communication lines connecting the
`current signal to flow therethrough when the associated
`piece of equipment is connected to the network.
`20 equipment to a network, said method comprising:
`6. The security system as defmed in claim 1 wherein
`selecting respective pairs of the existing data commu-
`nication lines for associated pieces of monitored
`said system further comprises high frequency filter
`means coupled to each of said current loops for provid-
`equipment so that each of said selected pairs of data
`ing isolation to each of said current loops.
`communication lines forms a current loop through
`7. The security system as defined in claim 1 further 25
`the associated pieces of monitored equipment,
`comprising DC blocking capacitors coupled to each of
`wherein said respective pairs of data communica-
`tion lines are associated with different ones of the
`said current loops for preventing said current signal
`associated pieces of equipment;
`through one of said current loops from interfering with
`supplying a low DC current signal to each current
`other of said current loops.
`8. The security system as defined in claim 1 further 30
`loop so as to achieve continuous current flow
`through each current loop while each of said asso-
`comprising alarm indicator means responsive to said
`current detection for providing an alann signal indica-
`ciated pieces of equipment is physically connected
`tive of a disconnected piece of said equipment when
`to said network via the data communication lines;
`said change in the current signal through one of said
`and
`sensing said DC current signal in each of said current
`current loops is detected.
`loops so as to detect a change in current flow indic-
`9. A security system for detecting unauthorized dis-
`ative of disconnection of one of said pieces of asso-
`connection of electronic equipment that is connected to
`a network communication link having existing pairs of
`ciated equipment.
`15. The method as defined in claim 14 further com-
`data communication lines interconnecting said elec-
`tronic equipment to a network, said system comprising: 40 prising the step of providing an alarm signal when said
`disconnection of one of said pieces of equipment is
`current loop means including separate current loops
`associated with different pieces of protected equip-
`detected.
`ment and internally coupled to the associated
`16. The method as defined in claim 14 further com-
`pieces of protected equipment, each of said current
`prising the step of:
`loops using said existing pair of data communica- 45
`selectively tapping into each of said selected pairs of
`lion lines which are coupled together via existing
`existing data communication lines at a location
`which is remote from said associated pieces of
`internal circuitry within said associated equipment
`to fonn a complete circuit path therethrough, and
`equipment.
`17. The method as defmed in claim 14 wherein said
`wherein respective pairs of data communication
`Jines are associated with different ones of the asso- SO existing data communication lines comprise lOBaseT
`wiring.
`ciated pieces of equipment;
`source means for supplying a low DC current signal
`18. The method as defined in claim 14 wherein each
`to each of said current loops;
`of said pieces of electronic equipment comprises a com-
`puter workstation connected to an Ethernet ® net-
`sensing means for sensing current flow through each
`of said current loops and detecting a change in said 55 work.
`19. The method as defined in claim 14 wherein each
`current flow through one of said current loops
`which is indicative of disconnection of one of the
`of said current loops includes existing circuitry within
`associated pieces of equipment; and
`the associated piece of equipment and coupled between
`the one of said associated pairs of data communication
`output means for providing an alarm output signal so
`as to indicate detection of a disconnected one of 60 lines to provide a circuit path therebetween.
`• * • *
`said pieces of equipment.
`•
`
`35
`
`65
`
`Page 8