HOME > Technical InformationSwitches, PBX's, and Central Offices > Chapter 13

| Introduction & Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 |
| Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 | Chapter 13 |

Survey of Telephone Switching
Chapter 13

This chapter is also available for downloading. Please choose Word 97 format
or plain text and download the images for this chapter which are contained
in the files

Chapter 13
in Microsoft Word 97 format
click here.
Chapter 13
in plain text format
click here.
Chapter 13
images contained in zip file
format :


Private Branch Exchange (PBX) service, has developed from being an insignificant part of the telephone plant to a 'point where it plays a major role in the everyday communication services of nearly all types of business.

PBX and station service is provided by means of an assembly of equipment components, some of which are relatively simple. For instance, a combined telephone set which contains all that is needed to meet most residential service requirements. However, business service may require anything from a simple wiring plan to an extensive dial PBX containing thousands of major components.

The equipment and apparatus required for these services are housed on the customer's premises. This involves an agreement between the customer and the telephone company as to space in which to place the equipment and makes it necessary to fit each installation ' to the customer's particular needs. In most cases this can be done with standard equipment assemblies but in other cases special circuits and equipment must be designed.

Information for the installation and maintenance of normal station, nonmultiple manual and small dial PBX equipment is contained in the Bell System Practices. Special station services, multiple manual and large dial PBX systems must be engineered to meet the requirements of a particular installation.

The installation of non-multiple manual and small dial PBX's and station equipment is completed by the Plant department. Multiple manual and large dial PBX installations may be made either by the Western Electric Company or the Plant department.

PBX Systems

Manual Systems - There are two general types of manual PBX switchboards, cord and cordless.

A manual cord PBX consists of one or more sections of framework containing a jack field on which are terminated station lines, central office trunks and tie lines of various types, a quantity of double ended cord circuits for connecting the jacks together, a telephone and dial circuit, which connects the, attendant to the cord circuits by means of keys, and miscellaneous common, auxiliary and alarm circuits.

A cordless manual PBX consists of a cabinet with several rows of keys which connect stations together or to central office trunks or tie lines. Either a regular station or an operator' s headset may be used by the attendant to answer calls to the switchboard.

The operation of the central office trunk, cord and station circuits is, in general, the same for all types of manual PBX'S. Talking battery for the station is fed through the cord circuit or key circuit from the central office, when the central office trunk is used in the connection. On station to station or tie trunk calls, talking battery is fed from the cord circuit or key circuit. Supervision is obtained from the talking battery current, which passes through supervisory relays in the cord or key circuits.

The cord circuits are arranged so that the front cords are used for answering central office trunk calls and the rear cords are used for answering extension or tie trunk calls. Because of the condition which is placed on the sleeve of the jack of various circuits the cord circuit relays either pass the central office battery through the cord circuit or provide talking battery to both ends of the cord circuit as may be required.

A key is associated with each cord so that the attendant may set up a night connection or permit a station user to dial over a central office trunk.

In general non-multiple manual switchboards operate on 16 to 24 volts while multiple manual and both non-multiple and multiple switchboards used with dial systems operate on 48 volts. There are some exceptions to this rule where the customer's space and other requirements dictate special equipment.

Non-Multiple Manual PBX - A non-multiple manual cord PBX consists of one or two individual attendant's positions in which each line or trunk has only one jack appearance. There is a desk type and a shelf type switchboard. The desk type has a capacity of 60 or 120 station lines, 13 central office trunks and 15 cords, arranged as shown in Figure 13-1. The shelf type has a maximum capacity of 320 lines, 15 trunks and 15 cords.

A cordless PBX is a metal cabinet arranged with a sloping top. These have a capacity of either 3 trunks and 7 stations or 5 trunks and 12 stations. The larger type is shown in Figure 13-2. Each cabinet is a self contained unit and is connected to the station and central office over cable pairs. It is designed to be placed on an ordinary desk.

Multiple Manual PBX - The small size multiple manual PBX consists of three or more switchboard positions similar to the non-multiple cord type except that the jack field is arranged so that there is more than one appearance of each station line and trunk. The different appearances of jacks for the same line are cabled together and placed in the switchboard as a unit rather than each position having its own jacks placed at the factory. These jacks are arranged on a 3 panel multiple basis.

The large size multiple manual switchboards are of two types, one with a nominal capacity of 1760 stations and 240 trunks and the other with a capacity of 3500 stations and 240 trunks. The smaller of these two is known as the 605 type and the larger as the 607 type.

The 605 type shown in Figure 13-3 consists of several single position, two panel sections with the multiple jacks arranged on a 4 panel basis. Relays for the cord circuit and attendant's circuits are placed in each section. Manual central office trunks, tie trunks and station circuits are usually placed on relay mountings in the head and foot sections. The head and foot sections are equipped with a single jack panel located adjacent to the operating position to provide a complete multiple appearance for the attendants in the first and last positions. A distributing frame may be placed at the head of the line-up, enclosed in a casing to match the remainder of the 'switchboard so that a complete installation may be in one unit.

The 607 type of switchboard is made up on the same framework as a modern toll switchboard which has single position and three position upper units but only single position lower units. The multiple is designed so that the stations are on a 7 panel basis and the trunks on a 6 panel basis. Where capacity is not a factor this arrangement may be reduced to fewer panels.

Relays for the cord circuits as well as for central office trunks and station circuits are installed on relay racks outside the switchboards. Since this type of switchboard uses considerable current for its operation an adequate 48 volt power plant is required. A separate distributing frame is also included in these installations so that considerably more space is necessary than that occupied by the switchboard sections. The cord circuits of this switchboard operate differently than those of the usual type PBX. The rear cord is used for answering all types of calls and the front cord is used for calling all types of circuits. An additional 6 volt battery is used with marginal relays to provide the features incorporated in this circuit.

Dial Systems - A step-by-step dial PBX consists of an assembly of step-by-step switches which perform various functions to connect one station to another or to trunks or to tie lines. Line relays connect the station to the equipment. Trunk units connect trunks of various kinds to the equipment. A manual switchboard, power equipment and miscellaneous alarm and register circuits are part of the complete system.

A crossbar dial PBX consists of one or more crossbar switches and associated link, line and miscellaneous circuits all mounted in one cabinet with its power plant.

701A Dial PBX - The general cabling arrangement of the 701A step-by-step dial PBX is shown in Figure 13-4. Only the major items are indicated but miscellaneous, auxiliary and alarm units are used in addition to those shown.

In a 701A dial PBX the line finder group has a capacity of 200 lines. The line terminals are multipled before a number of line finders, the number being dependent on the requirements of the traffic from the stations. All line finders serving the same 200 lines constitute a line finder group.

A first selector is connected to each line finder so that as soon as a line has been seized by the line finder the first selector returns dial tone.

First selectors are usually arranged so that their location on the shelves correspond to the location of the line finder on its shelf.

The selector multiple bank is made up of ten different levels comprising ten separate trunk groups, each level having ten sets of terminals. The levels of first selectors are ordinarily used as follows:

Level 0 - Trunks to the attendant.

Level 9 - Trunks to the central office.

Level 8 - Trunks to a foreign exchange central office, tie trunks or station groups.

Level 2-7 - Station connectors or second selectors.

Level 1 - Special services or not used because of preliminary pulses.

If the number of calls to be handled by any group of equipment exceeds the capacity of ten trunks, graded multiple is used. This provides individual trunks for some terminals of a bank of selectors while the remainder of the terminals are connected to common trunks, as indicated in Figure 13-5. This figure shows a group of 16 trunks connected to 40 selectors. In this case 8 trunks are individual to a particular shelf while the other 8 trunks are common to all shelves.

Second selectors are connected between the first selectors and connectors to provide more trunk groups or connector hundreds than would be possible with first selectors alone. Third selectors are connected between second selectors and connectors for the same purpose.

Connectors are arranged on shelves of ten and ordinarily not more than ten are needed for each hundred stations connected to its banks. Where more than ten are required, a unit with a capacity of twenty connectors may be used.

Each station line requires a line (L) and a cutoff (CO) relay. These are mounted on a relay rack. Local wiring connects the relays to terminal strips at the top of the relay rack. These terminal strips are used to connect the various parts of the line circuit together by means of jumpers or cables. The relay rack also contains control circuits for starting and controlling the associated line finder switches.

The switchboard is usually located in a separate room adjacent to the room containing the dial equipment. This is done to prevent the noise of the switches and relays from interfering with the operator and also to keep dust and other foreign objects from interfering with the proper operation of the dial equipment.

Switchboards used with the 701A dial PBX are of three general types. The smallest is similar to the non-multiple manual switchboard and is used when only a single appearance of the jacks is required. The maximum number of lines is about 400.

When more than 400 lines are required a larger switchboard is used. This is the same as the smaller of the multiple manual switchboards described previously. With this switchboard (Figure 13-6), a capacity of about 4000 station lines may be reached.

For capacities over 4000 lines, or where special features are desired, a new switchboard similar to the larger of the multiple manual switchboards has been developed with a lower key shelf, smaller jacks, automatic ringing and several other operating refinements. This type switchboard is used only on installations involving a large number of positions. In this type of switchboard the attendant may dial the stations over attendant's switches and in this way increase the capacity considerably.

740E Dial PBX - In the 740E step-by-step dial PBX, the line relays, line finders and selector-connectors together with miscellaneous trunk units are placed on one frame called a line frame. The line finders have banks with a capacity of 100 lines as contrasted with those used in the 701A dial PBX which are used with groups of 200 lines. One line frame has a capacity of 78 lines when two levels are used for trunks as is the usual case.

The selectors, when required, are arranged on a separate frame. When more than 78 lines are required, a selector frame is added and the original line frame is built up to a capacity of 98 lines. This is accomplished by disconnecting the wiring between the line finders and the selector-connectors and running cabling from the line finders to the selector frame and from the banks of the selectors back to the selector-connectors on the line frame. The selector-connectors thus become connectors for the lines on that frame. Additional line frames are similarly treated up to a normal capacity of 300 lines.

The switchboards, jacks and trunk units used with 740E dial PBX are the same as those used with the 701A dial PBX. However, there is a small capacity desk type switchboard that may be used where only one position is required and less than 100 lines are contemplated.

755A Dial PBX - At the present time crossbar PBX's are limited to a capacity of 20 station lines. The various units in the cabinet are connected together and to terminal strips for extension to the stations and trunks by means of a local form. Station sets used with the small crossbar PBX are equipped with six buttons, but differ from those used for any other type of service. Only one talking path exists between the instrument and the equipment cabinet and either a trunk or @ local station may be connected to it by depressing the proper key in the station set or in a separately mounted key box. Incoming trunk calls are indicated by different toned bells or by lamp indicators equipped with different colored lamp caps. Incoming local calls are announced by the bell in the telephone set.

The equipment in the cabinet consists of a maximum of two crossbar switches, three link circuits, four central office trunks, twenty station circuits numbered from 20 to 39, power and miscellaneous equipment. A smaller number of these components may be ordered if desired.

Since all the required equipment is contained in one cabinet, the installation consists only of placing it in a suitable location and connecting cable to it. This cable extends the circuits to stations and the central office.

Key stations, or those having. access to trunks, require more than one pair of wires for their operation. The number varies with the type of service. The distance from the cabinet to the key station must be comparatively short due to resistance limitations. Keyless stations, or those used for intercommunication only, are not so limited. These stations may be connected to central office trunks, under control of keys, located at a key station.

Station Systems

Regular Stations and Extensions - A regular station consists of a transmitter, receiver, induction coil, switchhook, condensers, dial and ringer. These components are assembled in one housing in the usual modern set and is known as a combined telephone set. Older stations and those used for special purposes may be divided into more than one unit, such as a desk stand and subscriber set, or hand set and subscriber set. The ordinary subscriber set usually contains the induction coil, condensers, ringer and terminals for connecting the wiring. The desk stand or hand set contains the transmitter, receiver and switchhook.

Coin Stations, Booths and Attended Stations - Prepay coin stations contain a coin collecting and refunding device and a coin storage space in addition to the usual components of a non-coin station. Post-pay coin stations do not contain the collecting and refunding device. Booths are usually furnished to provide privacy for the person using the instrument. These booths are of various types ranging from a shelf with partitions between instruments, to fully enclosed booths for indoor or outdoor use.

Where conditions indicate a need for assistance in placing calls from such locations as railroad terminals, armed services communication centers, and other places of public travel, attended pay stations may be placed. These consist of a number of booths, directory racks, and an associated switchboard. The switchboard may have a counter where special attendants accept the calls and collect the charges. The switchboard is similar to a PBX switchboard, with the booths as station, and trunks to one or more toll centers as well as those to, the local central office. In some cases booth lighting is controlled by the station jack circuit or by keys at the counter.

Coin stations are also used for semi-public service. These stations are usually wall or shelf mounted.

Amplified Receiving and Transmitting Telephones - Station instruments are manufactured which contain 'an amplifier in the receiver circuit for use by deaf persons. This same equipment is arranged so that the amplifier may be placed in the transmitter circuit for persons with impaired speech.

Recently a new telephones set has been introduced, called a "Speakerphone" which contains a separate loud-speaker and a microphone transmitter so that the user does not need to hold the hand set. However, a key is provided so that the hand set can be connected for use when privacy is desired.

Key Equipment

Key equipment covers a large variety of station apparatus used to augment the telephone service where more than one line is connected to a station or other features are desired. These varieties are assigned code numbers which are outlined below:

1A and 1A1 Key Telephone Systems - IA and 1A1 key equipment provides such features as pick-up of one or more lines of various types, holding, line lamps, busy lamps, intercommunication, signaling, exclusion and cutoff of extensions or ringers. The telephone sets used for these services have keys in the base of the set, or may have separately mounted keys and lamp signals. The key buttons in the base of the set may be illuminated or not. Telephone sets with different numbers of buttons are provided as needed.

The relay units are arranged on special mounting plates, with all relay terminals wired to a terminal strip attached to the mounting plate, so that the cable to each unit may be connected under screw terminals. These units are mounted in relay cabinets but on large installations, relay racks may be used. Each relay unit and each key provides one feature.

2B Key Telephone Systems - The 2B key telephone system is an intercommunicating system which provides loud-speaker and distant talking facilities at a master station and a regular telephone station at a secondary station. Switching arrangements are included for signaling in both directions, for switching the intercommunicating line to a regular telephone at the master station where desired, and for switching the secondary station instrument to other services.

This system is not designed to be used on central office or PBX lines.

The loud-speaker, microphone, amplifier and some of the control switches are located in a special cabinet. Other switching keys and telephone sets are located nearby.

4A Key Equipment - The 4A key equipment enables the customer to use an operator's telephone set in lieu of a station for two-way service to a central office or PBX line. The key assembly for this service contains jacks to which the telephone set is connected by means of a plug. The key is used in place of the switchhook, and may be either with or without holding.

5A and 5B Key Equipment - Where loud-speaking telephones are required for conference service 5A and 5B key equipments are provided. Each of these has a loud-speaker associated with it. The 5A key equipment is arranged to permit optional amplification, through the loud-speaker, of only the incoming portion of the conversation. The 5B type is arranged for optional amplification of the incoming only or both sides of the conversation.

6A and 6B Key Equipment - The 6A key equipment is used for service observing on central office lines, PBX lines, PBX trunks or PBX attendants, using keys to select the circuit. The 6B key equipment is used for the same purpose except that the circuits are dial selected. This is done by using a step-by-step switch which has access to 100 lines.

Loud-speakers may be used with either of these systems.

100 Key Equipment - Key equipment known as the 100 type is used where one or more persons, each having his own telephone, may answer, originate, and hold telephone calls on a common group of central office, PBX or automatic private lines.

The equipment consists of a key box for each station, and a relay cabinet containing equipment for terminating and controlling the lines and miscellaneous signaling and auxiliary equipment. The key boxes are arranged with three or six lines and may be single-sided or double-sided. A line lamp and a busy lamp is provided for each line.

This equipment has individual holding keys which must be restored after the other station user has picked up the call.

101A and 101B Key Equipment - The 101A and 101B key equipments were developed to provide for the following types of lines in addition to those mentioned for the 100 type; lines to IA and 1A1 key telephone systems, other 101A or 101B key equipments and ringdown private lines. The key boxes are arranged differently in that each section contains keys and lamps for ten lines, and a maximum of four sections may be assembled as a unit. The first section of the unit contains a combined "common hold" and "flash and ring" key, and buzzer and battery cutoff keys. The 101A key box has the keys on a vertical plate facing the attendant while the 101B key box has the keys in a horizontal plate so that it can be mounted flush in a table top.

102A Key Equipment - To meet the requirements of the Civil Aeronautics Administration for use at Air Traffic Control Centers and Airport Control Towers, the 1OZA key equipment has been developed. This equipment permits one or more attendants to answer, originate, hold, and record calls on a group of lines which may include central office or PBX lines, ringdown or automatic private lines and 2-wire or 4-wire private or toll lines. These lines are terminated on keys. Three kinds of key cabinets are used. The most widely used one is similar to the 101 type key unit. It does not provide for "conference" lines. The second type key cabinet is wired for conference lines and used at some but not all control towers. The third type is designed for monitoring on positions at Airway Traffic Control Centers. A visible filament lamp signal may be used with this equipment so that the illumination will be visible from a wide angle of view even when in direct sunlight. The lamp bulbs project above the faceplate and are protected by thin plastic lamp caps which may be white or colored.

109A Key Equipment - The 109A key equipment is used at small airports or Civil Aeronautics Communication Centers where a combination of five 2-wire or 4-wire lines are to be used. A key with six buttons is used to select the desired line, through the relay equipment. A loud-speaking telephone with its own amplifier may be connected to the equipment.

110A Key Equipment - The 110A key equipment is designed to terminate telephoto channels. It provides for the alternate use of transmitting and receiving pictures and talking, over a 4-wire toll line. A relay unit controlled by a key box is the principal part of the system. Extension stations, loudspeakers and signaling devices may be added.

111A Key Equipment - The 111A key equipment is also used by the Civil Aeronautics Administration for installations at small airports. It consists of a key panel designed to fit into the CAA operating console, and apparatus console, and apparatus cabinet for housing the various types of line equipment, and miscellaneous units such as loud-speakers, amplifiers and signaling equipment. This system can connect to almost any type of 2-wore or 4-wire circuit, including those requiring dial control. Voice-operated busy signals may be provided to light a busy lamp when speech is present on a line.

Secretarial Service

Secretarial service (telephone answering) exists in different forms, such as the 1A Answering Set, key equipment, or just an ordinary extension telephone. However, this section pertains only to that form of secretarial service rendered by various companies who lease switchboards from the telephone company for telephone answering purposes.

These switchboards are located in an area containing good prospects for this service such as doctors and small business firms. The customer's line is half-tapped at the central office and extended to the secretarial switchboard where it is terminated on a trunk type jack. These jacks are located so they can be easily distinguished from central office trunks. The calls are then answered at the secretarial exchange in a manner agreeable to the customer.

557A and 557B PBX Secretarial Switchboard - The 557A switchboard is particularly suitable for installations where in addition to performing secretarial services (single ended cord operation), the attendant carries on other PBX duties which necessitate double-ended cord operation. The 557B switchboard provides for terminating secretarial lines only.

Both the 557A and B switchboards have a 30 inch high keyshelf, "packaged" cord units, removable casing panels, large writing shelf and other features similar to the 555 PBX switchboard. For the majority of installations the line and trunk equipment is self-contained. Each secretarial line unit is self contained and can be installed or replaced from the rear of the section with a minimum of effort, since plug and socket type connectors are employed. Other features, briefly described, are as follows:

a. An auxiliary telephone circuit is provided to permit double attendant operation.

b. A panel pilot lamp is provided in the piling rail at the bottom of each panel.

c. A position grouping key, when required, is provided to permit the cords of one position to be grouped with the telephone circuit of an adjacent position.

d. A privacy feature prevents the attendant from connecting to. the called party's line except during the ringing interval of an incoming call.

The capacity of the 557A PBX is:

Concentrator-Identifier Equipment - A recent development has been an adaptation of existing switching and relay apparatus, called the concentrator identifier.

The concentrator-identifier will probably have a number of applications in the telephone plant among which is a means for providing more economical facilities for extending the services for secretarial answering bureaus. For this latter application the new arrangement provides a means for receiving calls for doctors' and other similar lines which terminate in a central office, routing them over a few common pairs for considerable distances to an answering bureau, and indicating to the bureau attendant the identity of the distant line on which the call is being received. Thus this new instrumentality can substantially reduce the number of pairs normally required between "attended" lines and the answering bureaus as well as reducing the number of small outlying bureaus by making centralization of the bureaus more economical. Obviously the ability to handle such outlying business at a single central point has important operational advantages from the standpoint of the bureau personnel, provision of quarters, and other similar factors.

At the central bureau each subscriber's line is terminated (whether brought directly into the bureau from the nearby office on bridged pairs or through the concentrator-identifier) in a jack and lamp. The line lamp lights only during the ringing interval. Since this feature is usually included in answering boards, the operating procedure by the bureau attendant remains unchanged.

The equipment required for the concentrator-identifier arrangement consists of two parts (1) concentrating equipment at the originating central office and (2) identifying equipment at the secretarial answering bureau. These are inter-connected by 2, 3 or 4 pairs of conductors as may be suggested by the traffic requirements as to the number of calls to be simultaneously held or answered.

The initial basic unit will handle 50 to 100 lines. The concentrator unit consists of a relay rack unit of about 225 relays and two crossbar switches. The identifier unit is mounted in a 7 foot apparatus cabinet and consists of about 100 relays and 3 crossbar switches.

Order Turrets

Order Turrets are station equipments for answering a group of central office lines. They take the place of a large number of telephones and make the same group of station lines available to a number of order takers. They provide for some type of distribution of the calls to the order takers. A call answered at an order turret cannot be switched to any other place but some are arranged so that the call may be transferred to a PBX and there switched to other locations.

2A Order Turret - The 2A Order Turret consists of a cabinet containing equipment for four order takers, jacks for 40 or 60 central office lines and relay equipment for 10 central office lines. These turrets are usually used in groups of more than one with the jacks multipled. A sufficient number of turrets are ordinarily used to provide relay circuits for all the central office lines. In addition, 5 cord-ended stations may be provided at each turret for supervisory purposes.

The distribution of calls is left to the order takers since all lines appear before each attendant.

4A Order Turret - The 4A Order Turret consists of a key box with four keys, to which are connected an incoming line, and outgoing PBX line, an overflow line, and control equipment. The central office lines are terminated on a PBX and distributed manually to the various turrets. Since the line to the turret is not used for making calls, it has no line lamp but does have an availability lamp so that the attendant may readily select a turret for any call. The overflow line is usually associated with a group of turrets. When all turrets in a group are busy and the overflow line is used, a lamp signal is lighted on each turret in the group.

Supervisory lamps and registers are associated with the system.

No. 6A Order Turrets - The No. 6A Order Turret system consists of a key box similar to the No. 4 Order Turret except the calls are distributed mechanically. The calls are extended to the turret only in the approximate sequence in which they arrive. A gate circuit is used so that all the calls inside the gate must be answered before those arriving later will be admitted.

This system requires a considerable amount of mechanical equipment for its operation and is only economical for large volumes of traffic.

Special Equipment For Dial PBX

Special equipment associated with dial PBX's consist of those items which are not required as part of the system but which are connected through the dial equipment and assist in providing complete telephone service.

Any units required are mounted on the racks or frames associated with the dial PBX.

Dial Tie Trunks - Dial tie trunks are used to connect two dial PBX's and enable a station or the attendant on one to dial an extension on the other PBX without assistance. The tie trunk units consist of an assembly of relays, coils, condensers and other components. These units usually repeat the dial pulses and furnish transmitter battery to the local station. They also operate from end to end without auxiliary equipment, between, except when voice repeaters are necessary.

The units are manufactured in an assortment of ranges and applications. A different unit is required for each type of switchboard because of the cord and jack conditions that must be met. Different units are also required for varying ranges and types of facilities.

Incoming selectors are associated with each two-way dial tie trunk. This switch takes the place of the first selector for an ordinary dial call and is interconnected with the tie trunk unit so that when "0" is dialed the switchboard attendant will be signaled on a jack associated with the particular tie trunk.

Each dial tie trunk has its own jack appearance. Two jacks are usually required since one is for dialing and the other is for the talking path. A second cord pair is used for dialing as the front cord of the pair used to complete the call may be required for a central office trunk. The No. 607 type switchboard does not require the second jack since the calling cord has the dial associated with it.

Dial Code Call - A code call system is available for use with a dial PBX and consists of a relay and switch unit which registers the code dialed into it and then transmits the code to a group of signals. The signals may be chimes, bells or horns. In the modern system three digits from one to five are used so that a maximum of 125 codes are possible.

The party who is being called may go to any telephone on the system and dial a predetermined connector terminal and reach the person who dialed the code.

Dial Conference - The dial conference equipment consists of a relay and coil unit which permits any ten stations on a dial PBX to dial a selector level and be connected together for a telephone conference.

Special fixed conference circuits have been developed for use of the Armed Forces. These circuits assigned stations to the master station on a conference connection when a button is operated at the master station. There are two types of this service. The Crash Alarm Circuit disconnects the stations from the dial equipment whether or not it is in use and places continuous ringing on the line. The Command Conference Circuit does not disconnect a station that is in use but places a tone on the line to signal that the station is wanted on the conference circuit. If the station is not in use, continuous ringing indicates that it is a conference call.

Dial Night Service - An arrangement may be added to a dial PBX which will enable a particular station to receive calls when the associated manual switchboard is not attended and extend these calls to any station on the PBX. This is accomplished by using a relay unit with a repeating coil to connect a dial station terminal to the trunk. The repeating coil is necessary so that the battery fed from the PBX will be isolated from the battery fed from the central office.

The station equipment used is of two types: the key telephone which has a capacity of four trunks and the 101 type key equipment which may have any number of trunks associated with it. However, there is a practical limit to the number of trunks that can be connected, since the employee using this equipment cannot handle calls as expeditiously as the switchboard attendant.

Although outgoing calls may be placed by the attendant, the system is designed primarily to receive incoming calls. After the attendant has answered an incoming call, he holds it and dials the extension of the person desired over a dial station line connected to the night station. If the desired person is reached, he is instructed to hang up and dial a connector terminal associated with the particular trunk. As soon as the connector terminal is reached, the dial station is connected to the trunk and at the same time the circuit to the night station is cut off.

Telephone Dictation Service - Telephone dictation service may be provided from any station on a dial PBX by installing Telephone Dictation Trunks. These trunk units are arranged to be connected to selector levels, and are wired over house cable pairs to an eleven pair connecting block for each machine. A dictation machine which has control circuits compatible with the dictation trunk, is wired to this connecting block. These machines are usually placed in groups of more than one to serve a central stenographic pool.

The selector level code is dialed to reach the dictation machine. Tone is returned from the dictation trunk to indicate that the machine is available. The digit 11111 is dialed to start or stop the machine and when the machine is stopped the tone is returned. The digit "2" marks a correction. The digit 11311 places the machine in the playback condition. Each make of machine does not always provide the same length or condition for the playback. The digit 11411 signals the end of the dictation and readies the trunk for the next user. The digit "0" stops the machine and signals the machine attendant over a special telephone line which is associated with each machine. This is for the purpose of giving special instructions about the dictated message.

Special Equipment For Manual PBX

Manual Code Call - A code call system consisting of an apparatus box with a number of keys may be placed at a manual PBX for the purpose of locating persons not at their telephones. The apparatus box contains a mechanism for transmitting a code corresponding to the key that is depressed. The person called picks up the nearest telephone and receives a message from the attendant.

This system may be connected to a central office trunk through a key for night service, in which case each signal will operate once for each spurt of ringing current. The watchman answers the call at the station which is connected to the same trunk by means of a night cord.

Manual Conference - Manual conference circuits can be provided. These circuits consist of jacks connected together through an equipment unit. Five and six jack sizes are available. Stations and central office trunks can be connected to the jacks as desired by the customer. A cord pair is required to connect each jack to the station or trunk.

Loud-Speaker Paging Systems - Loud-speaker paging systems may be associated with a manual switchboard. A number of loud-speakers and amplifiers are connected to the attendant's transmitter by means of a key so that persons not at their telephones may be located by name. The person wanted calls the attendant who completes the call to him.

These systems may be divided into sections so that part of a plant or all of it may be covered at a time. Additional transmitting stations may also be provided. In this case busy signals are usually installed to prevent one transmitter from interfering with another.

It is possible to use a dial station terminal for paging but care must be exercised in locating the loud-speakers in relation to the stations dialing into the system so that feed back is avoided.

Tie Trunks - Tie trunks to a dial PBX may be terminated on a manual switchboard so that the attendant at the manual PBX may dial extensions on the dial PBX. Each of these circuits have two jacks and a special relay unit associated with them. One jack is for dialing and the other is the jack over which voice currents pass. This is done so that central office calls may be placed over such a tie trunk. A separate cord pair must be used for dialing since the. front cord is in use for the central office trunk jack. Only one jack is required for the No. 607 type switchboard since the calling cord has the dial associated with it.

Special Services

Special services are instrumentalities for furnishing customers with a more comprehensive communication service, and can be any arrangement that may be requested by the customer.

Private Lines - Private lines are arrangements of equipment by means of which a customer may communicate between two or more points without using central office facilities. These may be merely a magneto line with a telephone at each end or may be assembled into extensive tandem networks with ringdown tie trunks, dial repeating tie trunks and teletypewriter lines.

Private lines may be either two-wire or four-wire circuits, and may be arranged with a variety of controls for calling or cutting off individual stations.

Home Bound Students System - Where school pupils are confined to their home a loud-speaker system may be provided to operate between the student's room and his regular classroom, so that he may hear the instructions and answer questions that may be directed to him.

Explosion Proof Sets - Certain locations where telephone sets are necessary may be subject to explosive atmospheres which could be ignited by the spark produced where the switchhook or dial is operated. Special station instruments have been designed so that they will operate safely under these conditions. It is necessary to install a tight conduit system with these instruments so that a spark generated by the accidental opening of the wiring will not ignite the gas.

Buttons, Buzzers and Bells - Buttons, buzzers and bells are auxiliary equipment required for the more efficient functioning of telephone service, especially where more than one station is connected to a line. They are used for signaling between stations and may involve anything from a simple combination of one button and one buzzer to a large system with several buttons operating each of several buzzers.

Other Special Services - Other special services include paging systems, civilian defense warning systems, recorder connectors, lamp indicators, automatic answering systems, automatic recorder control, message registers.

| Introduction & Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5 | Chapter 6 |
| Chapter 7 | Chapter 8 | Chapter 9 | Chapter 10 | Chapter 11 | Chapter 12 | Chapter 13 |