US3828315A - Method and arrangement for protecting correed matrix contacts - Google Patents

Abstract

An arrangement and method for protecting the correed matrix contacts of a matrix, when connecting two sections of cabling together, including a near ground potential source which is coupled to the cabling at the beginning of disconnect before the connection is released, to discharge the inherent capacitance of the cabling.

Description

United States Patent [191 Mila 3,828,315 Aug. 6, 1974 METHOD AND ARRANGEMENT FOR PROTECTING CORREED MATRIX CONTACTS SERVICE ClRCU/T E I 3,713,103 l/l973 Risky 340/166 R Primary ExaminerDnald J. Yusko 7] ABSTRACT An arrangement and method for protecting the correed matrix contacts of a matrix, when connecting two sections of cabling together, including a near ground potential source which is coupled to the cabling at the beginning of disconnect before the connection is released, to discharge the inherent capacitance of the cabling.

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METHOD AND ARRANGEMENT FOR PROTECTING CORREED MATRIX CONTACTS This invention relates to a common control communication switching system and, more particularly, to an improved centralized automatic message accounting system. More particularly still, it relates to a method and arrangement for; protecting the correed matrix contacts of a matrix used in such types of systems.

In the hereinafter described centralized automatic message accounting system, a'correed matrix is used to connect incoming trunks to the service circuits during call set up. Because of the inherent characteristics of a dry reed switch, care must be taken to protect its contacts which arevery susceptible to damage due to large currents switched when sections of cable connected to each side of the'reed switch and having different potentials on them are connected together. In this system, as well as other similar common control communication switching systems, such cabling cannot be avoided due to its physical size.

Accordingly, it is an object of the present invention to provide an improvedarrangement and method for protecting correed contacts, when connecting two sections of cabling together.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to'each of the others and the apparatus embodying features of construction, combination'of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

For a fuller understanding .of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a block diagram schematic of the centralized automatic message accounting system; and

FIGS. 2a and 2b show a schematic of a portion of one of the service circuits of the system.

Similar reference characters refer to similar parts throughout the several views of the drawings.

DESCRIPTION OF THE INVENTION Referring now to the drawings, in FIG. 1 the centralized automatic message accounting system is illustrated in block diagram, and the functions of the principal equipment elements can be generally described as follows. The trunks 10, which may be either multifrequency (MF) trunks or dial pulse (DP) trunks, provide an interface between the originating office, the toll switching system, the marker 11, the switching network 12, and the billing unit 14. The switching network 12 consists of three stages of matrix switching equipment between its inlets and outlets. A suitable distribution of links between matrices are provided to insure that every inlet has full access to every outlet for any given size of the switching network. The three stages, which consist of A, B and C crosspoint matrices, are interconnected by AB and BC links. The network provides a minimum of 80 inlets, up to a maximum of 2,000 inlets and 80 outlets. Each inlet extends into an A matrix and is defined by an inlet address. Each outlet extends from a C matrix to a terminal and is defined by an outlet address.

Each full size network is divided into a maximum of 25 trunk grids on the inlet side of the network and a service grid with a maximum of 16 arrays on the outlet side of the network. The trunk grids and service grid within the networks are interconnected by the BC link sets of 16 links per set. Each MF trunk grid is provided for inlets. Each DP trunk grid is provided for 40 inlets. The service grid is provided for a maximum of 80 outlets. A BC link is defined as the interconnection of an outlet of a B matrix in a trunk grid and an inlet 0 a C matrix in the service grid.

The marker 11 is the electronic control for establishing paths through the electromechanical network. The marker constantly scans the trunks for a call for service. When the marker 11 identifies a trunk with a call for service, it determines the trunk type, and establishes a physical connection between the trunk and a proper receiver 16 in the service circuits 15.

The trunk identity and type, along with the receiver identity, are temporarily stored in a marker buffer 17 in the call processor 18 which interfaces the marker 11 and the call processor 18.

When the call processor 18 has stored all of the information transmitted from a receiver, it signals the marker 11 that a particular trunk requires a sender 19. The marker identifies an available sender, establishes a physical connection from the trunk to the sender, and informs the call processor 18 of the trunk and sender identities.

The functions of the receivers 16 are to receive MF 2/6 tones or DP signals representing the called number, and to convert them to an electronic 2/5 output and present them to the call processor 18. A calling number is received by MP 2/6 tones only. The receivers will also accept commands from the call processor 18, and interface with the ONI trunks 20.

The function of the MF senders are to accept commands from the call processor 18, convert them to MF 2/6 tones and send them to the toll switch.

The call processor 18 provides call processing control and, in addition, provides temporary storage of the called and calling telephone numbers, the identity of the trunk which is being used to handle the call, and other necessary information. This information forms part of the initial entry for billing purposes in a multientry system. Once this information is passed to the billing unit 14, where a complete initial entry is formated, the call will be forwarded to the toll switch for routing.

The call processor 18 consists of the marker buffer 17 and a call processor controller 21. Thee are 77 call stores in the call processor 18, each call store handling one call at a time. The call processor 18 operates on the 77 call stores on a time-shared basis. Each call store has a unique time slot, and the access time for all 77 call stores is equal to 39.4 MS, plus or minus 1 percent.

The marker bufier 17 is the electronic interface between the marker 11 and the call processor controller 21. Its primary functions are to receive from the marker 11 the identities of the trunk, receiver or sender, and the trunk type. This information is forwarded to the appropriate call store.

The operation of the call process controller revolves around the call store. The call store is a section of memory allocated for the processing of a call, and the call process controller 21 operates on the 77 call stores sequentially. Each call store has eight rows and each row consists of 50 bits of information. The first and second rows are repeated in rows 7 and 8, respectively. Each row consists of two physical memory words of 26 bits per word. Twenty-five bits of each word are used for storage of data, and the 26th bit is a parity bit.

The call processor controller 21 makes use of the information stored in the call store to control the progress of the call. It performs digit accumulation and the sequencing of digits to be sent. It performs fourth digit l'blocking on a 6 or 10 digit call. It interfaces with the receivers 16, the senders 19, the code processor 22, the billing unit 14, and the marker buffer 17 to control the call.

The main purpose of the code processor 22 is to analyze call destination codes in order to perform screening, prefixing and code conversion operations of a nature which are originating point dependent. This code processing is peculiar to the needs of direct distance dialing (DDD) originating traffic and is not concerned with trunk selection and alternate routing, which are regular translation functions of the associated toll switching machine. The code processor 22 is accessed only by the call processor 18 on a demand basis.

The billing unit 14 receives and organizes the call billing data, and transcribes it onto magnetic tape. A multi-entry tape format is used, and data is entered into tape via a tape transportoperating in a continuous recording mode. After the calling and called director numbers, trunk identity, and class of service information is checked and placed in storage, the billing unit 14 is accessed by the call process controller 21. At this time, the call record information is transmitted into the billing unit 14 where it is formated and subsequently recorded on magnetic tape. The initial entry will include the time. Additional entries to the billing unit 14 contain answer and disconnect information.

The trunk scanner 25 is the means of conveying the various states of the trunks to the billing unit 14. The trunk scanner 25 is connected to the trunks by a highway extending from the billing unit 14 to each trunk. Potentials on the highway leads will indicate states in the trunks.

Each distinct entry (initial, answer, disconnect) will contain a unique entry identity code as an aid to the electronic data processing (EDP) equipment in consolidating the multi-entry call records into toll billing statements. The billing unit 14 will provide the correct entry identifier code. The magnetic tape unit 26 is comprised of the magnetic tape transport and the drive, storage and control electronics required to read and write data from and to the 9 channel billing tape. The read function will allow the tape unit to be used to update the memory.

The'recorder operates in the continuous mode at a speed of 5 inches per second, and a packing density of 800 bits per inch. Billing data is recorded in a multientry format using a nine bit EBCDlC character (extended binary coded decimal interchange code). The memory subsystem 30 serves as the temporary storage of the call record, as the permanent storage of the code tables for the code processor 22, and as the alterable storage of the trunk status used by the trunk scanner 25.

The core memory 31 is composed of ferrite cores as the storage elements, and electronic circuits are used to energize and determine the status of the cores. The core memory 31 is of the random access, destructive readout type, 26 bits per word with 16 K words.

For storage, data is presented to the core memory data registers by the data selector 32. The address generator 33 provides the address of core'storage locations which activate the proper read/write circuits representing one word. The proper clear/write command allows the data selected by the data selector 32 to be transferred to the core storage registers for storage into the addressed core location.

For readout, the address generator 33 provides the address or core storage location of the word which is to be read out of memory. The proper read/restore command allows the data contained in the-word being read out, to be presented to the read buffer 34. With a read/restore command, the data being read out is also returned to core memory for storage at its previous location.

The method of operation of a typical call in the system, assuming the incoming call is via an MP trunk can be described as follows. When a trunk circuit 10 recog nizes the seizure from the originating office, it will provide an off-hook to theoriginating office and initiate a call-for-service to the marker 11. The-marker 11 will check the equipment group and position scanners to identify the trunk that is requesting service. Identification will result in an assignment of a unique four digit 2/5 coded equipment identity number. Through a trunk-type determination, the marker 11 determines the type of receiver 16 required and a receiver/sender scanner hunts for an idle receiver 16. Having uniquely identified the trunk and receiver, the marker 11 makes the connection through the three-stage matrix switching network 12 and requests the marker buffer 17 for service.

The call-for-service by the marker 11 is recognized by the marker buffer 17 and the equipment and receiver identities are loaded into a receiver register of the marker buffer 17. The marker buffer 17 now scans the memory for an idle call store to be allocated for processing the call, under control of the call process controller 21. Detection of an idle call store will cause the equipment and receiver identities to be dumped into the call store. At this time, the call process controller 21 will instruct the receiver 16 to remove delay dial and the system is now ready to receive digits.

Upon receipt of a digit, the receiver 16 decodes that digit into 2/5 code and times the duration of digit presentation by the calling end. Once it is ascertained that the digit is valid, it is presented to the call processor 18 for a duration of no less than 50 milliseconds of digit and 50 milliseconds of interdigital pause for storage in the called store. After receipt of ST, the call processor controller 21 will command the receiver 16 to instruct the trunk circuit 10 to return an off-hook to the calling office, and it will request the code processor 22.

The code processor 22 utilizes the called number to check for EAS blocking and other functions. Upon completion of the analysis, the code processor 22 will send to the call processor controller 21 information to route the call to an announcement or tone trunk, at up to four prefix digits if required, or provide delete information pertinent to the called number. If the call processor controller 21 determined that the call is an ANI call, it will receive, accumulate and store the calling number in the same manner as was done with the called number. After the call process controller 21 receives ST, it will request the billing unit 14 for storage of an initial entry in the billing unit memory. It will also command the receiver 16 to drop the trunk to receiver connection. The call processor controller2l now initiates a request to the marker 11 via the marker buffer 17 for a trunk to sender connection. Once the marker 11 has made the connection and has transferred the identities to the marker buffer 17, the marker buffer will dump this information into the appropriate call store. The call processor controller 21 now interrogates the sender 19 for information that delay dial has been removed by the routing switch (crosspoint tandem or similar). Upon receipt of this information the call processor controller 21 will initiate the sending of digits including KP and ST. The call process controller 21 will control the duration of tones and interdigital pause. After sending of ST, the call processor 18 will await the receipt of the matrix release signal from the sender l9. Receipt of this signal will indicate that the call has been dropped. At this time, the sender and call store are returned to idle, ready to process a new call.

The initial entry information when dumped from the call store is organized into the proper format and stored in the billing unit memory. Eventually, the call answer and disconnect entries will also be stored in the billing unit memory. The initial entry will consist of approximately 40 characters and trunk scanner 25 entries for answer or disconnectcontain approximately 20 characters. These entries will be temporarily stored in the billing unit memory until a sufficient number have been accumulated to comprise one data block of 1,370 characters. Once the billing u'nit memory is filled, the magnetic tape unit 26 is called and the contents of the billing unit memory is recorded onto the magnetic tape.

The final result of actions taken by the system on a valid call will be a permanent record of billing information stored on magnetic tape in multi-entry format consisting of initial, answer, and disconnect or forced disconnect entries.

Answer timing, force disconnect timing and other timing functions such as, for example, a grace period timing interval on answer, in the present system, are provided by the trunk timers. These trunk timers are memory timers, and an individual timer is provided for each trunk in a trunk scanner memory which comprises a status section and a test section.

The status section contains one word per ticketed trunk. Each word contains status, instruction, timing and sequence information. The status section also provides one word per trunk group which contains the equipment group number, and an equipment position tens word that identifies the frame. A fully equipped status section requires 2,761 words of memory representing 2,000 trunks spread over 60 groups plus a status section start word. As each status word is read from memory, it is stored in a trunk scanner read buffer (not shown). The instruction is read by a scanner control to identify the contents of the word. The scanner control logic acts upon the timing, sequence and status information, and returns the updated word to the trunk scanner memory and it is written into it for use during the next scanner cycle.

The test section contains a maximum of 83 words: a start word, a last programmed word, 18 delay words, two driver test words, one end-test word and one word for each equipment group. The start test word causes a scan point test to begin. Thedelay words allow time for; scan point filters to charge before the trunk groups are scanned, with the delay words containing only instructional data. The equipment group words contain a two digit equipment group identity and five trunk frame equipped bits. The trunk frame equipped bits (one per frame) indicates whether or not a frame exists in the position identified by its assigned bit. The delay words following the equipment group allow the scan point filters to recharge before the status section of memory is accessed again for normal scanning. The Last Program word inhibits read and write in the trunk scanner memory until a trunk scanner address generator has advanced through enough addresses to equal the scanner cycle time. When the cycle time expires, the trunk scanner address generator returns to the start of the status section of memory and normal scanning recommences.

The trunk scanner memory and the trunk scanner read buffer are not part of the trunk scanner 25, however, the operation thereof is controlled by a scanner control which forms a part of the trunk scanner 25 of the billing unit 14. The trunk scanner 25 maintains an updated record of the status of each ticketed trunk, determines from this status when a billing entry is required, and specifies the type of entry to be recorded. The entry includes the time it was initiated and the identification of its associated trunk.

Scanning is performed sequentially, by organizing the memory in such a manner that when each word is addressed, the trunk assigned to that address is scanned. This causes scanning to progress in step with the trunk scanner address generator. During the address advance interval, the next scanner word is addressed and, during the read interval, the word is read from memory and stored in the trunk scanner read buffer. At this point, the trunk scanner 25 determines the operations to be performed by analyzing the word instruction.

As indicated above, scanning is performed sequentially. If all trunks in all groups are scanned in numerical sequence beginning with trunk- 0000, scanning would proceed in the following manner:

Step 1. Trunk 0000 located in frame 00 (lineup 0, column 0) in the top file, leftmost card position would be scanned first.

Step 2. All trunks located in frame 00 and the leftmost card position would be scanned next from the top file to the bottom.

Step 3. Scanning advances to frame 01 (lineup 0, column l) and proceeds as in Step 2.

Step 4. Scanning proceeds as in Step 3 until frame 04 has been scanned.

Step 5. The scanner returns to frame 00 and Step 2 is repeated for the next to leftmost card position.

Step 6. The sequence just described continues until the ten card positions in all five columns have been examined.

Step 7. The entire process is repeated in lineups one through five.

When a memory word instruction identifies a trunk group word, the status receivers are cleared to prepare for scanning the trunks specified in the group word. The trunk group digits stored in the trunk scanner read buffer (TSRB) are transferred into the equipment group register.

After the trunk group number is decoded, it is transformed into binary code .decimals (BCD), processed through a l-out-of-N'check circuit, and applied to the ACbus drivers (ACBD); The drivers activate the scan point circuits via the group leads and the trunk'status is'returned tothereceivers.

A group'address applied to'the drivers causes the sta-' tus of all trunks in one lineup and one card position and all columns to be returned to the receivers. The group 10s digit specifies the trunk frame lineup andv the group units digit identifies the card slot.

When a status wordis read from memory, it sets the previous count of a trunk timer(-TT) into the trunk timer.

If the trunk is equipped and the forced disconnect sequence equals 2 (FDS=2), a request to force release the trunk is transmitted to the marker 11. If FDS does not equal 2, the present condition of the ticketing contacts in the trunk is tested. If the instruction indicates that the trunk is in an updated condition (the trunks associated memory word was reprogrammed) it is tested for idle. If the trunk is idle, its instruction'is changed to denote that it is ready for new calls. If the trunk is not idle, no action is taken and the trunk scanner 25 proceeds to the next trunk.

If the trunk is not in the updated condition and FDS=3, the trunk is tested for idle. If the trunk is idle, FDS is et to and TT is reset.

lf FDS does not equal 3 and'a match exists between the present contact status and the previous contact status stored in memory (bits five and six) the FDS memory bits are inspected for a count equal to 1. If FDS=1, T1" is reset and the memory contact status is updated. If FDS does not equal 1', TT'is not reset.

During any analysis of a trunk status, a change in the contact configuration of a trunk is not considered valid until it has been examined twice.

One bit (SFT) is provided in each memory status word to indicate whether or not a change in status of the trunk was detected during the previous scan cycle.

When a change in status is detected, SFI is set to 1. If SFT=1 on the next cycle, the status is analyzed and SFI is set to 0.

If a mismatch exists between the present contact condition and that previously stored in memory, the status has changed and a detailed examination of the status is started.

If CT=l the trunk is busy and so the previous condition of the contact is inspected. If the trunk previously was idle, CM=0. Before continuing the analysis, it must be determined if this is the first indication of change in the trunk status by examining the second look bit (SFI). If SFT=0,it is set to equal 1, and the analysis of this trunk status is discontinued until the next scanner cycle. If SFT=l the memory status is updated and SFT is set to equal 0.

If CT=l, the trunk is cut through and CM is inspected to determine if the memory status was updated. If CM=l, the GT contact status must differ from GM since it was already determined that a mismatch exists. If GT=0, answer has not occurred. If GT=1, and this condition existed during the previous scan cycle, SPT=1 also. If these conditions are true and FDS does not equal 1, TT is advanced and answer timing begins. If these conditions persist for eight scanner cycles (approximately 1 second), answer is confirmed and an entry will be stored in the trunk scanner formater (TSF). If answer is aborted (possibly hookswitch fum ble) before the 1 second answer time (time is adjustable) expires, TT remains at its last count. When the answer condition returns, answer timingcontinues from the last 'IT count. Thus, answer timing is cumulative.

After ananswer entry is stored, which includes the TT count, T1" is reset, SPT is set to 0, andthe new contact status is written into memory. f I

If a mismatch exists and CT=0, the previous state of this contactis inspected by examining bit in the trunk scannerread buffer (TSRB). If CM=l, the state of the terminating end of the trunk is tested. If GT=1, then the condition of the trunk has just changed from 'answer to disconnect. If this condition existed during the previous scan cycle, SFT=1 and a disconnect entry is If a mismatch exists and the originating end of a trun is, not released, both CT and CM equals 1.. If vGT=0 after the previous scan. cycle, FDS is tested. If this change just occurred, F DS does not equall. Since FDS does not equal 1, it will be set equal to l and T1" will reset. FDS=1 indicates that forced disconnect timing is in progress.

While the conditions just described exist, i.e., mismatch, CT=l, CM=l, GT=0 and FDS=1,' TT will ad vance 1 count during each scanner cycle, if one half second has elapsed since the last scan cycle. Tlwill continue to advance until it reaches a count of 20 (approximately 10 seconds) when a'forced disconnect entry will be stored in' the TSF.

When the entry is stored, FDS is set atZ indcating that the trunk is to be force released. After tne'entry is stored, which includes the TI count, TT isreset, SFT is set to 0, and the new status is written into memory.

After the status and test sections of the memory have been accessed, the Last Program word-is read from memory and stored in the trunk scanner read buffer. This word causes read/write in the trunk scanner portion of memory to be inhibited and deactivates the scan point test. The trunk scanner address generator will continue to advance, however, until sufficient words have been addressed to account for one scan cycle. When a predetermined address, the Last Address, is reached, block read/write is removed and the address generator returns to the Start Address (First Program Word) of the scanner memory.

From the above description of the centralized automatic message accounting system, it can be seen that the trunks 10 are connected to the service circuits 15, including the receivers 16 and the senders 19, by the switching network 12 which is a correed matrix. As further indicated above, the contacts of the correed matrix are very susceptible to damage due to large currents switched when sections of cables connected to each side of the dry reed switches and having different potentials on them are connected together.

In accordance with the present invention, the switching network 12 or matrix is protected by circuitry within the service circuits 15 which senses the wetting current provided through the matrix by the trunks. This current is approximately 2-3 milliamps and is placed on the matrix 12 via 15K resistors in the trunks 10. It is electronically sensed with a transistor sensing circuit in each of the service circuits 15 and causes a potential which is close to ground to be switched on to certain ones of the leads connecting the service circuits 15 to the matrix 12, before the matrix path can be released. This potential discharges the inherent capacitance of the cabling in the matrix and thereby insures that the use of this cabling for succeeding connections will not cause the systems marker to connect two sections of cable having different store charges on them. This arrangement, as more fully set forth below, removes the requirement for the common control circuitry of the system to constantly monitor both the trunks l and the service circuits l and sense when the path is to be released. Furthermore, it also removes the requirement for additional external circuitry to access the paths and discharge them remotely.

More particularly, in FIG. 2, one of the service circuits 15, a receiver 16, is illustrated and it can be seen that it as well as each of the other service circuits is connected to the C stage of the switching. network 12 or matrix via four leads (T, R, C and H) which are extended via the matrix 12 to a trunk 10. Two other leads (not shown) are used by the marker 1 1 for circuit identification and busy/idle status of the receiver 16.

During call setup, the H relays in a trunk and a receiver 16 both are operated by a 50 volt potential extended through these relays to a ground potential, in the generally well-known fashion. The H relay in the receiver 16, upon operating, extends ground through its normally open contact H1 to the base of transistor O4, to cause transistor O4 to turn ON. The transistor O4, in turning ON, causes transistor O5 to turn OFF. Transistor O5 in turning OFF, removes the 50 volts on the base of transistor Q6, thus causing it to turn ON. This action, in turn, causes the 49 volts on the emitter of transistor O6 to be coupled to the base of the transistor Q12 and cause transistor Q12 to turn ON. The transistors Q6 and Q12 being ON, prevent the transistors Q10 and Q13 from turning ON, by placing the near ground potential (-1 volt) on the bases of these two transistors. These transistors Q10 and Q13 are held OFF, to prevent contact stagger on the correed relay HC from causing them to operate or turn ON, during call setup.

The marker 11 during the first stages of call set up applies a ground potential to the lead AST (or EST), and this ground potential keeps the transistor Q9 turned ON and the transistor Q41 turned OFF. However, during switch through of the trunks and receivers, this ground potential is removed and the transistors Q9 and 041 are caused to turn OFF and ON, respectively. When the transistor Q41 turns ON, the correed relay HC and the relay HD both are caused to operate. The correed relay HC operates via the near ground (-3 volts) potential on the emitter of the transistor 041 being extended through it to the 50 volts on the emitter of transistor Q4. The relay HD operates via the same near ground potential on the emitter of transistor Q41 being extended through it and the diode CR89 to the -50 volt potential, when the contact HC2 of the correed relay l-IC operates. When these relays HC and HD operate, the receiver 16 is connected through the matrix 12 to the trunk 10. During a call setup, as described above, the marker 11 performs a continuity test and a foreign potential check before connecting the re.-

ceiver 16 to the trunk 10, as more fully described in a copending application Ser. No. 357,310; filed by Melvin A. Jacobs on May 4, 1973 and assigned to the same assignee as is the present application. Reference may be made to this copending application for a more complete description of the systems operation during call setup, however, for the purposes of the present invention, it is only necessary to generally state that prior to the receiver 16 being connected through the matrix 12 to the trunk 10 the above described operations occur in the receiver 16.

The attached trunk 10 provides volts on the T and R leads through the 15K ohm resistors R5 and R6, which 50 volts is extended through the contacts HC4 and HC5 of relay HC and the two resistors R12 and R13. This 50 volt potential is sensed by the transistor Q14, via the resistors R12 and R13, and the transistor Q14 is turned ON by it. The transistor Q14, in turning ON, provides a 3 volt signal through either the external relay contacts 88 or the timer 89, to the base of transistor Q36, causing transistor 36 to turn ON. The external relay contacts 88 and the timer 89 are for purposes separate from the present invention and hence are not more fully discussed herein. When transistor Q36 turns ON, it couples the 49 volt potential on its emitter to the base of transistor Q12, to hold it turned ON and this allows the marker 11 to turn OFF transistor Q6. The transistor Q6 initially was operated by the marker 11, to prevent transistors Q10 and Q13 from turning ON, due to contact stagger on the correed relay HC.

Now, when the trunk 10 begins to disconnect the -50 volts on the leads T and R is removed. This lack of current flow causes transistor Q14 to turn OFF, which action, in turn, causes transistors Q36 and Q12 to turn OFF. The transistor Q12 in turning OFF, allows the -50 volts through contact HC2 of the correed relay BC to reach the transistors Q10 and Q13. This potential on the bases of these two transistors causes them to turn ON. The transistor Q10 in turning ON extends the -2 volts on its emitter to the C lead, and the transistor Q13 in turning ON extends the --2 volts on its emitter to the T and R leads. This 2 volts on these leads T, R and C discharges the connected cable capacitance to such a low level that a future connection with these cables will not find a sufficient voltage difference to cause destructive currents to flow through the correed contacts in the switching network or matrix 12.

Accordingly, from the above description, it can be seen that a potential close to ground is switched onto the leads T, R and C of the matrix 12 before the matrix path is released, to discharge the inherent capacitance of the cabling in the matrix. By doing so, the use of this cabling for succeeding connections will not cause the marker 11 to connect two sections of cable having different store charges on them which could damage the contacts of the correed relays in the matrix 12.

It will thus be seen that the objects set forth above among those made apparent from the preceding description, are efficiently attained and certain changes may be made in carrying out the above method and in the construction set forth. Accordingly, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Now that the invention has been described, what is claimed as new and desired to be secured by Letters Patent is:

1. In a common control communication switching system including a plurality of trunks connectable through a switching network to any one of a plurality of service circuits under the control of a marker, said switching network being formed with cabling and correed relays having contacts for establishing such connections, said trunks once a connection is established coupling a wetting current through said switching network to said service circuits and removing the same at the beginning'of disconnect, an arrangement within each of said service circuits for discharging the inherent capacitance of the cabling in the switching network by means of which a connection is established to protect the contacts of the correed relays from damage comprising a potential source, switching means for coupling said potential source to said cabling to discharge said inherent capacitance of said cabling, sensing means for sensing said wetting current and being operable to render said switch means inoperable as long as said wetting current is sensed and to operate said switch means to couple said potential source to said cabling upon sensing the absence of said wetting current, whereby said potential source is coupled to said cabling at the beginning of disconnect when said trunk removes said wetting current and before the-connection is released.

2. The arrangement of claim 1, wherein said sensing means comprises a transistor sensing circuit in each of said service circuits.

3. The arrangement of claim 1, wherein said switch means comprises a transistor, said potential source being coupled to the emitter of said transistor and the collector thereof being coupled to said cabling, whereby said transistor upon being rendered conductive extends said potential source to said cabling, said transistor being rendered conductive by said sensing means when the latter senses the absence of said wetting current.

4. The arrangement of claim 1, wherein said sensing means comprises a transistor sensing circuit in each of said service circuits, said transistor sensing circuits each comprising a pair of resistors connected in series with the cabling through which said wetting current flows and a transistor having a base connected to the juncture between said pair of resistors and rendered conductive when said wetting current flows through said pair of resistors, said transistor being coupledto and'rendering said switch means inoperable when it is conductive.

i l III I

Claims (4)

1. In a common control communication switching system including a plurality of trunks connectable through a switching network to any one of a plurality of service circuits under the control of a marker, said switching network being formed with cabling and correed relays having contacts for establishing such connections, said trunks once a connection is established coupling a wetting current through said switching network to said service circuits and removing the same at the beginning of disconnect, an arrangement within each of said service circuits for discharging the inherent capacitance of the cabling in the switching network by means of which a connection is established to protect the contacts of the correed relays from damage comprising a potential source, switching means for coupling said potential source to said cabling to discharge said inherent capacitance of said cabling, sensing means for sensing said wetting current and being operable to render said switch means inoperable as long as said wetting current is sensed and to operate said switch means to couple said potential source to said cabling upon sensing the absence of said wetting current, whereby said potential source is coupled to said cabling at the beginning of disconnect when said trunk removes said wetting current and before the connection is released.

2. The arrangement of claim 1, wherein said sensing means comprises a transistor sensing circuit in each of said service circuits.

3. The arrangement of claim 1, wherein said switch means comprises a transistor, said potential source being coupled to the emitter of said transistor and the collector thereof being coupled to said cabling, whereby said transistor upon being rendered conductive extends said potential source to said cabling, said transistor being rendered conductive by said sensing means when the latter senses the absence of said wetting current.

4. The arrangement of claim 1, wherein said sensing means comprises a transistor sensing circuit in each of said service circuits, said transistor sensing circuits each comprising a pair of resistors connected in series with the cabling through which said wetting current flows and a transistor having a base connected to the juncture between said pair of resistors and rendered conductive when said wetting current flows through said pair of resistors, said transistor being coupled to and rendering said switch means inoperable when it is conductive.

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