Patent GB-663365-A (GB)
Title: Main distributing frame in telecommunication exchange systems
IPC: H04Q1/14
Filed: 1947-12-19
Abstract: 663,365. Automatic exchange equipment. STANDARD TELEPHONES & CABLES, Ltd. Dec. 19, 1947 [Oct. 29, 1943], No. 33620/47. Class 40 (iv). In an exchange system the line circuit equipment is mounted on the local side of the M.D.F. and is housed in or mechanically associated with plugs which are inserted in jacks equipped thereon. The line circuits employed are of the type disclosed in Specification 641,819 and other features are disclosed in Specification 663,037. Subscriber's line circuit, (Figs. 1, 1A). A subscriber's line circuit comprises two resistors x (15,000 #) connected between ground and the a-wire, and between the b- and c-wires respectively, a resistor y (30,000 Q) between battery and the c-wire, and a resistor z connected to the d-wire. The connection of the other end of z and its value in ohms varies according to the class of line. For single lines or intermediate PBX lines, 19,000 # to ground; for first PBX lines, 4,300 # to ground; for absent service condition lines, 0 # to ground; and for changed number lines, 10,000 # to battery are the preferred values. The line circuit also includes a rectifier R1 between the point of junction of the second resistor x and resistor y and the common point DT leading to a call detector circuit common to fifty lines which may be of the type disclosed in Specification 641,819. If restricted service facilities are required a second rectifier R2 is connected between the c-wire and the resistors x and y by opening the connection 5. For changed number lines x, y, R1, and R2 are disconnected, and for dead number lines z is also disconnected. The four conductors are multipled in the line finders (a1, b1, c1, d1) and the final selectors (a2, b2, c2, d2) and fifth conductors e2 (connected to a source of A.C. for controlling the positioning of the final selector) and el (for metering) are also employed. To provide absentee service the d-conductor is extended to contact g2 (Fig. 1) of relay Gr (Fig. 1A) by stropping terminals 3, 4 (Fig. 1). A relay Gr is provided for each line to which the service is afforded as described in Specification 663,037, and contacts 1, 2 (Fig. 1A) have access to the absentee service desk (not shown). Equipment and wiring of line circuit, (Figs. 2, 2A). Terminals A represent terminals on the vertical side of the M.D.F., whereas B shows the part of a jack mounting and terminal strip serving one line on the horizontal side of the M.D.F. C, D, E and F illustrate plugs for line circuits of different classes; G the multipling of a line finder and final selector, H the subscriber's meter and J (Fig. 2A) a relay Gr for providing switched absentee service. B comprises five soldering tags of which four (a to d) project at the bottom, while the fifth (s) does not, and seven jack-in contacts (1-7) which appear in front of the terminal strip. Five of these contacts (1, 2, 3, 4 and 6) are wired to the taps (s, c, b, a, d) by metallic strips. Contacts 1, 5 and 7 are commoned to those of the 49 other line circuits served by the common call detector and are connected to conductor DT, ground, and battery respectively. The soldering tags at the top of the strips serve for making jumpering connections, e.g. outside line A jumpered to a and b tags, and terminal 8 (Fig. 2A) to tag d for absentee service; and those at the bottom for the connection of switchboard cabling to the line finder and final selector multiple. With no plug C, D, E or F inserted in the jack the line is in the dead number condition with the dwire open circuit. Calls directed to the line will therefore be routed to an operator or, alternatively, a special tone will be connected to the caller as described in Specification 663,037. Plugs C and D show unrestricted and restricted line circuits respectively, three types of each of which are provided with the z resistor values for single or intermediate P.B.X. lines, first P.B.X. and last P.B.X. lines respectively. These are distinctively marked or coloured so as to be immediately recognized. Absentee service circuits are as shown at E, and are marked in a manner different from C and D plugs so that absentee service lines may be readily identified. For switched absentee service however, the normal line plug (C or D) is retained but a relay Gr (Fig. 2A) is allotted to the line and contact 8 is jumpered to the d tag. Relay Gr is operated or released from the absentee service desk by grounding terminals 9 or 10 respectively and on operation grounds the d conductor. The plug F (also distinctively marked) is used when a subscriber's number is changed, being jacked into the old line circuit jack so as to re-route calls to an operator. Special service facilities, (Fig. 3). In order to afford subscriber line observation to a line, the resistors x and y are replaced by line and cut-off relays which signal the conversational conditions of the line to an observation desk for both originating and terminating calls. A monitoring jack OJ is equipped at the desk for each such line, which may, if desired, be connected to recording equipment to register the numbers dialled by the subscriber. Unrestricted and restricted lines have strappings between terminals 1 and 3, and 1 and 2 respectively. The arrangement shown may also be employed for supplying wired radio relay programmes to subscribers or for malicious call service and other special services. Plugging-up circuits, (Figs. 4, 4A, 4B). Fig. 4 shows a subscriber's line A jumpered to the terminal and jack strip B. C (Fig. 4A) shows a common service observation line circuit connected by switchboard cable to a terminal and jack strip D (Fig. 4), which may be connected to any desired subscriber's line by a 4-conductor patching cord or by jumpering between the tags a, b, c and s, according as the connection is for a short or long period. Each 21st position on the horizontal side of the M.D.F. may be equipped as at D but without soldering tags and all the 21st positions may be wired to a frame whereon they may be interconnected as desired. Thus by means of two short patching cords, from the subscriber's line to the nearest 21st position, and from another 21st position connected thereto to a D position any line may be connected to a special service circuit. In such a case a small plug jacking into contacts 5, 6, 7 of the subscriber's jack strip is required. Such a plug (illustrated at G) contains only the relevant resistor z. A plugging-up circuit E (Fig. 4B), shown wired to the strip F (Fig. 4), comprises only means for connection to the outside line for detecting intermittent faults. Plugs H, J or K are used in the case of single or intermediate P.B.X. - lines, first P.B.X. and last P.B.X. lines respectively to allow calls incoming to plugged-up lines to be dealt with. The 10,000 # resistor W1 of plug H causes  calls to be re-routed to the changed-number operator who advises the caller that the line is plugged-up. Plug J contains two resistors W2 and W4 which place on the d-wire busy first P.B.X. line conditions and the final selector steps on to the next line of the group. In the case of a last P.B.X. line being pluggedup, plug K produces the busy last P.B.X. line condition on the d conductor and a caller receives busy tone.