Trunk busy test circuit
United States Patent Kennedy 5] Apr. 1, 1975 [5 1 TRUNK BUSY TEST CIRCUIT 2.771.519 ll/l956 Nance 179/175: 1)  
  3. 3 1 [75] Inventor: James Blakely Kennedy, Bexley, 770 898 11/197 wmdwqrd 179/18 AB Ohio Primary E.\&#39;tmu&#39;nerWilliam C. Cooper 173] Assignee: Bell Telephone Laboratories, Assistant Examiner-Douglas W. Olms Incorporated, Murruy Hill, Berkeley Attorney, Agent, or Firm-D. A. Marshall Heights. NJ. 122 Filed: Jan. 9. 1974 I571 ABSTRACT A test circuit for enabling a test position to access and Appl&#39; 432083 test the trunk circuits of a telephone central office. The test circuit distinguishes between idle subscriber [52] U5. Cl. i, 179/1753 R, 179/1751 R busy, and trouble busy ones of the trunk circuits and [51] Int. Cl. H041 3/46 enables the test position to access both idle and trou- 1581 Field of Search... 179/1753 R 1753 R 18 AB ble busy trunk circuits. The test circuit responds to the test position by controlling make-busy plug circuitry [56] References Cited to remove and restore a trunk circuit for use on sub- UNITED STATES PATENTS Cans- 2.75u.4511 6/1956 Murray 179 1752 D 1 Claims. 1 Drawing Figure STA! 11.111 E T1.Rl TELEPHONE CENTRAL OFFICEI T.R 1 51 I REMOTE DFFICE TEST LINE l0 1 71-3 TBT-Z &#34;JR 2L CL svr-s oMeA-a 12,112 @57 1 i .8? A2 1 l M1 RUQPRMB 011113 T J12 TTES&#39;T 1 TT2,TR2 svr-z 1 752 i Q l post nou I 1 13.113 13,11 1 f 1 &#34;&#34;1 53 TBT-fi 1 1 TT3,TR3 one-1 gg-g L 1 753 T5 11- X 11451;  
 &#39; nzsr Tss- T52 12 RLMB-l 1.1a PLUS 1 my I I4 i RI 2 I 1x13 -1 JKIZ an v- 9-4 V&#34; V R2 T Dl4l ans-1i TRUNK BUSY TEST CIRCUIT BACKGROUND OF THE lNV &#34;ION lv Field of the Invention This in\ ention relates to telephone switching systems. In particular it relates to an arrangement for accessing and testing the trunk circuits of a telephone central office.  
 1. Description of the Prior Art Telephone central offices of the telephone switching network are interconnected by trunk circuits arranged to establish and provide call communication paths from calling telephone stations to called telephone stations. Each telephone central office is interconnected by a number of trunk circuits. oftentimes referred to simply as trunks. with certain other telephone central offices and telephone calls between subscribers are established over interconnected ones of the trunks.  
  Arrangements have been disclosed in the prior art for selecting ones of the trunks at a telephone central office and conducting supervision and transmission tests thereon. These types of tests are required in order to determine the suitability of a trunk for use in providing a high standard of communication service to calling and called telephone subscribers,  
  The trunks of telephone central offices are often times tested from a common test position that is located remote to the telephone central ol&#39;lices. During a testing sequence the test position selects a particular central office trunk and examines the current opera tional status thereof. When the selected trunk is found to be idle. the test position accesses the trunk and sub&#39; sequently conducts supervisory and transmission tests thereon. If the selected trunk is found to be busy. the test position releases the busy trunk and selects another.  
  Telephone central office maintenance personnel routinely remove trunks that fail on subscriber calls from service by making the trunks appear busy to incoming subscriber calls. Since the central office made-busy trunks have failed on subscriber calls it is highly desirable that the test position obtain access to these trunks for the purpose of locating and correcting the source of the trunk failures. Although the test position detects idle and busy trunks it does not have the capability of distinguishing between trunks busy handling subscriber calls and trunks that have been made trouble busy by maintenance personnel.  
  Accordingly. a need exists in the art for an arrangement capable of enabling a test position to distinguish between subscriber busy and trouble busy trunks of a telephone central oflice. A need also exists for an arrangement capable of enabling a test position located remote to a telephone central office to remove from subscriber service trunks found to have a trouble condition and to restore the trunks to subscriber service when the trouble has been cleared.  
 SUMMARY OF THF. INVENTION In the embodiment of the invention a test circuit intended for use with a test position is arranged for enabling the test position to conduct operational tests on trunks utilized by a telephone central office to establish subscriber calls between telephone stations. The test circuit responds to signals generated from the test position by selecting ones of the telephone central office trunks on which it is intended to conduct operational tests thereon and establishing a connection with a make-busy lead of the selected trunks. Apparatus tests the make-busy lead and identifies busy and idle ones of the selected trunks from ground and the absence thereof appearing on the make-busy lead. Additional apparatus is subsequently connected to the make-busy leads of a trunk identified as busy and distinguishes between ones of the selected trunks busy handling subscribers calls and ones of the trunks made trouble busy from the appearance of direct and diode grounds appearing on the selected trunks make-busy leads. The test circuit establishes test connections from the trunk test position with idle ones and trouble made-busy ones of the selected trunks in order that the test position may conduct operational tests thereon.  
  In accordance with one feature of the invention. apparatus is provided for enabling maintenance personnel of a telephone central oflice to remove a trunk experiencing trouble from subscriber service by inserting a make-busy plug in the make-busy jack of the trunk to return the trunk sleeve lead through a directional conducting diode to ground.  
  Another feature of the invention is the provision of apparatus for enabling a remote test position to remove a trunk from subscriber service by making the trunk trouble busy and subsequently restoring the trunk to subscriber service by removing the trouble busy. The apparatus includes a makebusy plug comprising a latch relay in series with xener diodes connected in parallel with a directional conducting diode in series with a contact of the latch relay. The make-busy plug inserted in a trunk make-busy jack responds to first and second polarity voltages applied to a trunk sleeve lead from the test position by enabling the latch relay to connect and disconnect ground through the directional conducting diode with the trunk sleeve lead.  
  in accordance with another feature of the invention apparatus included in a remote office test line enables a test position to distinguish between the idle. subscriber busy. and trouble busy states of selected trunks. The apparatus responds to the absence of ground on a trunk sleeve lead and to the appearance of ground through a diode on the trunk sleeve lead by connecting the trunk to the test position in order that operational tests may he performed thereon. The apparatus under control of the test position applies first and second polarity voltages to a trunk sleeve lead for the purpose of enabling a relay equipped make-busy plug inserted in the make-busy jack of a trunk to remove and restore the trunk to subscriber service.  
 DESCRIPTION OF THE DRAWING The foregoing objects. features and advantages. as well others of the invention. will be more apparent from well as consideration of an illustrative embodiment now to be described with reference to the accompanying schematic drawing. Shown therein is an illustrative system for testing telephone central office trunks remotely selected from a test position.  
  It will be noted that the drawing employs a type of notation referred to as &#34;Detached Contact&#34; in which an X. shown intersecting a conductor. represents a normally open make&#34;contact of a relay. and a bar. shown intersecting a conductor at right angles. represents a normally closed break&#34; contact of a relay. norm-ally referring to the unoperated condition of the relay. The principles of this type of notation are described in an article entitled &#34;An lmpro\ ed Detached(&#39;tintact-&#39;lype of Schematic (ircuit Drawing&#34; by F. T. Meyer in the Sept. I955 publication of the American Int-limit of llft&#39;t&#39;ll&#39;ft&#39;tl/ Engineers &#39;li&#39;uusurlimir. CTHIIIIIIl/lft(ll/HIM um] lz&#39;lt&#39;rlmnitzr. \&#39;olume 74. pages Stl5-5 l 3.  
 GENERAL DESCRIPTION Referring now to the drawing. it is intended that telephone central office I shown thereon be a conventional type of telephone switching system equipped with a number of trunks II. I2. I3 employed to establish and provide connnunication paths between subscribers. Each of the trunks I I. I2. I3 is equipped with a corre spondingly numbered jack .lKI I. JKI2. JKI3 that is used by maintenance personnel of telephone central office I to make the trunks appear busy to subscriber calls. Telephone central office I is also equipped with remote office test line III for the purpose of enabling test position 2 to select ones of trunks II. I2. I3 and conduct operational tests thereon.  
  During a subscriber call sequence telephone central office I seiles a trunk for subscriber use. such as trunk I3. by placing a ground on trunk sleeve lead S3. After trunk I3 has been seized. a connection is established in the well-known manner with telephone station I in order that a communication path may be established through switching equipment over interconnected conductors T. R. and T3. R3 between calling and called subscribers. The ground appearing on trunk sleeve lead S3 maintains the connection during the calling interval and informs the switching equipment ot&#39;telephone central oflice I that trunk I3 is busy handling a subscriber call.  
  Maintenance personnel of telephone central office I removes a trunk. such as trunk I2. from service by in serting make-busy plug I4 into jack I2. (iround ap pearing on trunk sleeve lead TSZ through diode DI4I prevents the switching equipment of telephone central oflice I from selecting trunk I2 for use on a subscriber call. Trunk I2 may be restored to service by having the maintenance personnel remove make-busy plug I4 from jack .lKIZ.  
  &#39;lest position 2. used to test the trunks of a number oftelephone central offices. selects a particular trunk of telephone central office I to be tested. for example. trunk I2. by transmitting signals over control lead (L to remote office test line Ill. The received signals, in the well-known manner, direct test connector I6 to connect test lead TS of remote office test line III with trunk sleeve lead T52 of trunk 12. In the absence of a ground signal appearing on test lead TS. remote office test line Ill determines that selected trunk I2 is idle and proceeds to connect test leads TT. TR with leads TTZ &#39;lR2 in order that test position 2 can perform the necessary tests.  
  Ifground appears on test lead TS selected trunk I2 is identified by remote office test line I&#34; as being busy. When selected trunk I2 is found busy, test position 2 enables remote office test line I0 to conduct a subscriber busy and trouble busy test by examining the type of ground appearing on trunk sleeve lead TS2. Diode ground appearing on trunk sleeve lead T52 informs the circuitry of remote office test line Ill that trunk I2 has been made trouble busy and is therefore available for testing. Test leads TT. TR are then con nectcd with trunk leads T2. R2, via leads TT2, TRZ. in  
 Ill  
 order that test position 2 may conduct tests on madetrouble busy trunk l2.  
  Direct ground appearing on trunk sleeve lead TS2 informs remote office test line III that trunk I2 is busy handling a subscriber call and cannot be tested at this time. Test position 2 thereby instructs remote office test line [0 to release from the subscriber busy trunk without interrupting the subscriber call.  
  Developments in trunk testing have made it highly desirable that a remote make-busy and a remote makebusy release capability be provided for a test position such as test position 2. The present embodiment of the invention provides this capability with makebusy plug IS. Makebusy plug 15 is equipped with latch relay M that is arranged to connect diode DISI in series with the sleeve lead of a trunk. Assuming that make-busy plug I5 has been inserted intojack .IKI I. trunk I I can be made busy by having test position 2 direct remote office test line 10 to apply a voltage of a first polarity to test lead TS. This voltage appearing on trunk sleeve lead TSI operates latch relay M of make-busy plug I5 to connect trunk sleeve lead TS] to ground through diode DISI. After the voltage has been removed the contacts of latch relay M remain operated in order that diode ground may be continued on trunk sleeves TSI for the purpose of making trunk I I appear trouble busy to subsequent subscriber calls.  
  Test position 2 may restore made-busy trunk II to subscriber service by directing remote office test line It) to apply a voltage of a second polarity to trunk sleeve lcad TSI. The voltage appearing on trunk sleeve TS] releases latch M to remove diode ground from trunk sleeve TS] and thereby return trunk II to subscriber use.  
  In summary. the disclosed embodiment of the invention enables a test position to identify the idle and busy states of selected trunks and to distinguish subscriber busy trunks from trouble busy trunks. In additiom the test position has the capability of removing a trunk experiencing trouble from subscriber service and the capability of restoring the trunk for subscriber usage after the trouble has been located and cleared.  
 DETAILED DESCRIPTION A. Selecting ldle Trunks It is assumed for the present embodiment of the invention that make-busy plugs I4 and I5 shown in the drawing has been inserted by the maintenance personnel of telephone central office I into jacks JKIZ and JKI I respectively. It is further assumed that trunk I3 is currently in an idle state and that make contacts (C-3) are open thereby preventing ground from appearing on trunk sleeve lead T53.  
  Test position 2 selects a trunk for testing. for example. trunk 13, by seizing control line CL and transmitting identifying signals thcreover to remote office test line I0. The identifying signals appearing on control line CL direct test connector I6 to connect leads TS. TT. TR with trunk leads T53, TT3. TR3 respectively. Test position 2 initiates the test sequence on selected trunk I3 by operating trunk busy test relay TBT.  
  Operation of relay TBT closes make contacts TBT-I to establish an operate path for relay Tl from ground through closed make contacts TBT-I. normal contacts TIA-I and the secondry winding of relay TI to a 48 volt battery source. The resultant closing of contacts TI-l extends trunk sleeve lead T83 through test connector 16 met lcad TS and the make portion of transfer contacts Tl-l. through the normal portion oi translcr contacts OMB-l and the primary winding of relay Tl to the -lt volt battery source. The operation of relay Tl also closes make contacts Tl-4 to provide an ob\ious operate path for relay TlA which locks operated to ground through the make portion ol translcr contacts TlA-2 and closed make contacts TBT-4. Relay TlA. in operating. opens normal contacts TlA-l to interrupt the secondary winding operate path of relay Tl.  
  Since ground is not present on trunk sleeve lead T53 of idle trunk 13. relay TI releases and closes normal contacts Tl-2. With normal contacts Tl-Z closed. a ground signal is transmitted. through closed make contacts TlA-3 and over control line CL to test position 2 as an indication that trunk 13 is idle and is available tor testing. Test position 2 is then enabled to conduct tests on idle trunk [3 on a connection extending over leads TT. TR through make contacts TBT-Z. normal contacts Tl-3. and leads TT3. TR3 to trunk conductors T3. R3.  
  At the end of the test sequence tcst position 2 initiates a disconnect by releasing rclay TBT. The resultant opening of make contacts TBT-4 releases relay TIA to restore the trunk test circuitry of remote office tcst line to an idle state. Subsequently. test position 2 may. in the well-known manner. direct tcst connector [6 to release trunk l3 and select another trunk for testing.  
 B. Selection of a Subscriber Busy Trunk In the event trunk I3 is busy handling a subscriber call. make contacts (C-3] are closed to connect ground to trunk sleeve lcad T53 and telephone station 1 con ductors T. R are interconnected in the well-known manner with trunk conductors T3. R3. Test position 2 initiates the test sequence on selected trunk l3 by operating relay TBT. In the aforementioned manner rclay TBT closes make contacts TBT-l to operate rclay TI and thereby connect the primary winding of relay Tl over lead TS through the normal portion oi translcr contacts OMB-l and the make portion of operated transfer contacts Tl-I with trunk sleeve lead TS3. Current l&#39;lowing over this path from ground appearing on trunk sleeve lcad TS3 and through the primary winding of relay Tl maintains relay Tl operated when rclay TlA operates to open normal contacts TlA-l.  
  Relay Tl. maintained in the operated state from the ground connected to lead TS by subscriber busy trunk 13. prevents the closing olnormal contacts Tl-Z to return ground over control line CL. The absence of ground on control line (L informs test position 2 that selected trunk 13 is busy and that an additional test must be conducted to determine if trunk 13 is busy on a subscriber call or has been made trouble busy by maintenance personnel.  
  &#39;l&#39;cst position 2 initiates the additional test by operat ing rclay OMB over control line CL. Operation of relay OMB closes the make portion of transfer contacts OMB-l to establish a path from lead TS through the normal portion of transfer contacts SVT-l to the winding of relay SVT. Relay OMB also closes make contacts OMB-5 in order that a hold path for operated rclay Tl may be maintained to ground through closed make contacts TBT-S. Also occurring with the operation of relay OMB is the establishment of an operate path for slowoperate relay OMBA to ground through the normal portion of transfer contacts OMBA-l and closed make contacts OMB-4.  
  Current flows from a +48 volt battery source through the winding of relay SVT. the break portion of transfer contacts SVT-l. the make portion ot transfer contacts OMB-l. oyer lead TS through closed make contacts Tl-l. trunk sleeve lead T53 and closed make contacts (C-3) to ground. The resultant operation of relay SVT opens the break portion of transfer contacts SVT-1 to interrupt this current path. however. rclay SVT does not release in that a hold path is established from ground through make portion of operated transfer contacts SVT-l and make contacts TBT-fi.  
  The operation of relay SVT. which is tollowcd by the operation of slow-operate relay ()MBA. opens break contacts SVT-2 prior to the closing ol make contacts OMBA-2 to prevent a ground signal from being returned over control line CL to test position 2.  
  Since a ground signal is not received subsequent to the operation of relay OMB. test position 2 ascertains that selected trunk I3 is subscriber busy and proceeds to release relays OMB and TBT. Release ol relays OMB and TBT opens their respective make contacts OM 8-4. OMB-S. TBT-l. TBT-4. TBT-S. and TBT-6 to initiate release ol relays SVT. Tl. TlA. OMBA to restore the trunk tcst circuitry of remote office tcst line It) to the idle state.  
 C. Selection of a Trouble Made Busy Trunk Maintenance personnel of telephone central office 1 busy a trunk. for example trunk 12. by inserting makebusy plug l4 into jack JKIZ. The ground from jack .IKIZ is applied. via the center lcad ol make-busy plug 14 and diode Dl4l. to trunk sleeve lead T52 and prevents the switching equipment of telephone central oil&#39;icc I from selecting trunk T2 for use on a subscriber call. Thus. the use of make-busy plugs enables maintenance personnel to make trouble busy and remove from subscriber use those trunks that tailcd in establishing and handling subscriber calls.  
  in the aforementioned manner tcst position 2 selects trunk l2 by transmitting signals over control line CL to direct tcst connector 16 to connect test conductors TT. TR, TS with trunk conductors TTZ. TRZ. T52 respectively. Following the operation of relay TBT. relay Tl operates and closes the make portion ol transfer contacts Tl-l to connect the primary winding of relay Tl with trunk sleeve lead TSZ. A path is established from ground through diode D141, over trunk sleeve lead TSZ. through test connector 16, the make portion of transfer contacts TL 1. the normal portion of transl&#39;cr contacts OMB-l and the primary winding of relay TI to maintain relay Tl in the operate state when relay TlA subsequently opens break contacts TlA-l to interrupt the initial operate path of relay TI. The failure to apply ground through normal contacts Tl2 to control line CL int&#39;orms test position 2 that selected trunk 12 is busy.  
  Test position 2 initiates a trunk busy test sequence by operating relay OMB. The operation of relay OMB closes make contacts OM 8-4. OMB-5 to provide operate and hold paths for slow operate relay OMBA and relay Tl. respectively. and closes the make portion oitransfer contacts OMB-l to connect rclay SVT to test lead TS. Since the winding of relay SVT is connected to at +48 volt battery source and operated transfer contacts OMB-l opened the connection of lead T5 with relay Tl. diode DH] is reversed biased and thereby prevents the operation of relay SVT.  
  Subsequently. slow-operate relay ()MBA operates and locks operated to ground appearing through closed make contacts TBT -land the make portion of operated transfer contacts ()MBAJ. The operation of relay ()MBA closes make contacts ()MBA2 and transmits a ground signal through released break contacts S\&#39;T2 and make contacts ()MBA-Z. &#34;rms over control line (L to test position 2. In addition conducts TT. TR are connected with trunk conductors T2. R2 through closed make contacts &#39;l&#39;ll&#39;l -Z, ()MHA-l and released break contacts SVI Z.  
  The ground signal appearing on control line (L notifies test position 2 that selected trunk 2 is trouble busy and that the trunk has been sci/ed and is currently a\ailable for testing. After tests have completed on trunk conductors T2. R2 o\er the path comprising interconnected conductors TTZ. TRZ. and TT. TR. test position 2 releases relays ()MB. TMB to restore the trunk test circuitry of relnote office test line H) to the idle state.  
  I). Remote Makediusy and Make-Busy Release of a Trunk When make-busy plug I5 is inserted into jack .llx&#39;ll. test position 2 is enabled to remotely busy and restore trunk H to subscriber service. Make-busy plug comprises diode DISI in series with make contacts M-l of latch relay M. The winding of latch relay M is connected in series with zcner diode [D15] and the combination is connected in parallel with diode DlSl and series make contacts M4. Latch relay M is arranged in the wellknown manner so that momentary current flow in one direction will operate the relay to close the associated make contacts M-l. The o erated make contact M-l remains in a closed state until current llow in a reverse direction enables latch relay M to release the contacts. Zener diodes ZDISI are selected so that a voltage in excess of 60 volts appearing across the lener diodes enables the lener diodes to conduct current.  
  Test position 2. having selected idle trunk ll. initiates the sequence to busy selected trunk H by momentarily operating remote makebusy relay RMB over control line (1. to open the break portion of transfer contacts RMB-l and remove ground from trunk sleeve lead TSI. Operation of relay RMB closes the make portion oltransfer contacts RMB-I to apply I 3t) volt battery to test lead TS. (urrent flowing from ground through vener diodes ZDISI and the winding of latch relay M over leads &#39;l&#39;Sl. TS through the break portion of released relays &#39;ll-I. RLMB! and the make portion of operated relay RMH-l to l3(l volt battery source enables ltch relay M to operate and maintain in a closed state, make contacts M-l. Ground is maintained on trunk sleeve lead TS] through diode DIS] and operated make contacts M-l after the release of remote office test line Ill in order to make trunk ll appear busy to the switching equipment of telephone central office I.  
  Made-busy trunk I l is returned to subscriber service by test position 2 directing remote office test line It) to access made-busy trunk H and. operating release make-busy relay RLMB. Operation of relay RLMB closes the make portion of transfer contacts RLMB-l to place +l3t) volt battery on test lead TS. Current flows from the l volt battery through the make portion of transfer contacts RLMB-l. the normal portion of transfer contacts Tl-l. over leads TS. TS]. through Ill the winding of make-busy plug 15. latch relay M and /.ener diodes [D15] to ground. Latch relay M responds by opening make contacts M-l to interrupt the path from ground through diode DIS] to trunk sleeve lead &#39;lSl. After the release of remote office test line It) by test position 2. trunk ll. hzoing diode ground removed from trunk sleeve lead TS by make-busy plug I5. is restored to subscriber service.  
 Summary It is obvious frotn the foregoing that the tlexibilitv. economy. and efficiency ol telephone switching s tents may be enhanced by the provision of apparatus arranged for enabling a remote test position to select and test the trunks of telephone central offices. lt is further ob\ ious from the foregoing that the aforesaid apparatus arrangement for distinguishing between idle. subscriber busy. and trouble busy trunks ofa telephone central office enables a test position to select. remove from service. conduct test thereon. and restore to service after trouble has been corrected. ones of the telephone central office trunks utilized to establish and handle subscriber calls between telephone stations.  
  While the apparatus has been disclosed for use with the switching equipment of a telephone central office. it is to be understood that such an embodiment is intended to be illustrative of the principles of the invention and that numerous other arrangements may be directed by those skilled in the art without departing from the spirit and scope of the invention.  
 What is claimed is:  
  l. A test circuit for enabling a test position to conduct operational tests on telephone central office trunks arranged to establish subscriber calls between telephone stations comprising means responsive to signals generated by the test position for selecting ones of the trunks.  
 means enabled by the test position for identifying idle and busy ones of the selected trunks.  
 means for distinguishing between ones of the busy trunks handling subscriber calls and ones of the busy trunks made trouble busy. and  
 means for connecting both the selected idle and trottble made-busy trunks with the test position.  
  2. The test circuit set forth in claim I further coinprising make-busy plug apparatus arranged to busy one of the trunks by connecting a make-busy lead of the trunk to ground through a diode.  
  3. The test circuit set forth in claim 2 wherein said make-busy plug apparatus comprises a latch relay having a coil winding responsive to current How in one direction for closing make contacts and responsive to current flow in another direction for releasing said make contacts,  
 a pair of yener diodes connected in series with said latch relay coil winding. and  
 control path means including said make contacts and connected in parallel with said latch relay and said zener diodes for connecting said diode between ground and the trunk make-busy lead.  
  4. The test circuit set forth in claim 3 wherein said identifying means comprises a first test relay connected to a negative voltage source.  
 means responsive to the test position for operating said first test relay and connecting said operated first test relay to the make-busy lead ofthe selected trunks. and  
 means enabled by the subsequent release of said first test relay connected to the make-busy lead of a selccted one of the trunks for indicating an idle state of the selected trunk to the test position.  
  5. The test circuit set forth in claim 4 wherein said distinguishing means comprises a slow-operate relay.  
 a second test relay connected to a positive voltage source.  
 means responsive to the test position for initiating operation of said slow-operate relay and for connecting said second test relay to the make-busy lead of the identified busy trunlss. and  
 means responsive to release of said second test relay and the operation of said slow-operate relay for determining that ones of the identified busy trunks having make-busy leads connected to ground through said diode are trouble made-busy trunks.  
  6. The test circuit set forth in claim 5 wherein said connecting means comprises means enabled by said operating and connecting means in combination with the release of said first test relay for coupling the test position with the selected idle trunks. and  
 means enabled by the release of said second test relay in combination \\ith operation of said slow-operate relay and said operating and connecting means for coupling the test position \\ith the trouble madebusy trunks.  
  7. The test circuit set forth in claim 6 further coniprising means enabled by the test position for connecting another negative voltage source to the make-busy lead of a selected tt&#39;tlllls equipped with said makebusy plug apparatus to make the selected trunk trouble busy. and  
 means enabled by the test position for connecting another positive voltage source to the make-busy lead ofa selected trouble busy trunk equipped with said make-busy plug apparatus to restore the selected trouble busy trunk for use in establishing the subscriber calls.  
  8. A test circuit for enabling a test position to conduct operational tests on telephone central office trunks arranged to establish subscriber calls between telephone stations comprising first and second test relays.  
 means responsive to signals generated by the test position for selecting ones of the trunks.  
 means responsive to the test position for operating said first test relay and connecting said operated first test relay to a make-busy lead of the selected lti trunks.  
 means enabled by the subsequent release of said first test relay connected to the make busy lead of one of the selected trunks for indicating an idle state of the selected trunk to the test position.  
 means enabled by the test position for connecting said second test relay with the make-busy lead of a selected one ofthe trunks identified by continued operation of said connected first test relay as busy.  
 means responsive to operate and release states of said connected second test relay for distinguishing be tween ones of the identified busy trunks handling subscriber calls and ones of the identified busy trunks having been made trouble busy. and  
 means for connecting both the selected idle and made trouble busy trunks with the test position 9. The test circuit set forth in claim 8 further comprising make-busy plug apparatus for connecting the make busy lead ofone of the trunks to ground through a diode comprising a latch relay responsive to current flow in one direction for closing make contacts and responsive to current llovx in another direction for releasing said make contacts.  
 a first control path for enabling said latch relay and comprising /ener diodes connected in series with said latch relay. and  
 a second control path connected in parallel with said first control path for busying the one trunk and comprising the diode connected in series with said make contacts.  
  I&#34;. The test circuit set forth in claim 9 further comprising circuitry for remotely controlling said makebusy plug apparatus to remove and restore the one trunk to subscriber service by applying positive and negative voltages to the make busy lead for the purpose of enabling said latch relay to close and release said make contacts.  
  I l. A test circuit for use with a remote office test line to enable a test position to conduct operational tests on selected telepone central office idle and busy trunks comprising test apparatus for distinguishing ones of the selected busy trunks handling subscriber calls from-ones of the selected busy trunks made trouble busy.  
 path connect apparatus for connecting the selected idle trunks and the selected made trouble busy trunks with the test position. and  
 make-busy plug apparatus controllable from the test position for placing the selected idle trunks in a trouble busy state and the selected trouble busy trunks in an idle state.