Patent GB-1264745-A (GB)
IPC: H04M11/06
Filed: 1970-04-22
Abstract: 1,264,745. Automatic exchange systems. SIEMENS A.G. 22 April, 1970 [23 April, 1969], No. 19220/70. Heading H4K.  An exchange system has subscribers with telephone sets and teleprinter apparatus, a station being able to effect transfer of the nature of the connection from speech, or from a ringing connection if so desired, to teleprinting transmission, the receipt of such a transfer instruction at the exchange resulting in signals to both stations to connect up the teleprinters. Successful connection of the teleprinters is detected at the exchange which reverses battery feed and allows transmission to proceed. A substation, Fig. 1, has a telephone subset TN and a teleprinter D. Circuits E1 to E4 are loop signal detectors. At the exchange, Fig. 2, one side of a dual-purpose transmission relay set is shown with battery feed bridge arrangements MA1, MA2. Relays SA, HA, VA and WA effect calling-side supervision. Relays SB, HB, VB and WB, of which only the last is shown, are symmetrically arranged for called-side supervision. When connected to a transmission relay set, a short period of audio tone is returned to a subscriber as notification that the set is dualpurpose. Relay SA is operated to the loop and holds up VA. When, at the instance of either end if a call has been answered, or at the instance of the calling party if the called party is still subject to ringing, transfer from a speech mode to a teletype mode is required, a special key may be operated or the handset may be placed in a special cradle to operate such a key in the form of a cradle switch. Such switch is shown at t in Fig. 1 and operates K to place R1 in the calling loop. SA falls back in the relay set, Fig. 2, and VA follows while HA responds. Relay WA now operates and W follows in consequence. With W up the feed-bridges are held by resistive shunts while the backward and forward loops are split off for the application of positive battery leg signalling. If, as described here, the calling subscriber effects transfer, relay WA puts battery on the b-leg to the calling station and relay WB puts battery on the a-leg of the called station. Had the called station effected transfer WB would be up and WA down to reverse the leg signals. At the calling station, presumed to be effecting transfer, detector E1 responds to the leg signal to operate TR and disconnect the loop while charging a capacitor for subsequent operation of UH1 and U. At the called station E2 responds to operate TR and prepare for the operation of UH2 and U. With W operated in the relay set a counter Z is started to measure a delay before operation of RF which in operating releases itself in releasing W and WA. With W fallen back the loops are restored allowing relays UH1 and U, and UH2 and U, to operate to capacitative discharge at the calling and called stations where relays TR fall back. In this condition the teleprinters D are connected to line with resistors R2 in each loop. The transfer key t at the called station is disconnected. In the relay set the relays HA, HB are operated to the loops and bring up WQ followed by FU which reverses feed current. With reversed feed at the stations, diodes G1 short resistors R2, detectors E3 responding to operate relays X which prepare release detection by circuits E4. Lamps are lighted at the stations to indicate controlling or passive status. A teleprinter may be put out of use by short circuit of resistor R2. Teleprinter transmission uses alternating send/receive modes, mode being reversed when a message end is detected. By breaking the key or cradle contacts t the teleprinter equipment is disconnected, K releasing to insert R2 into the loop so dropping SA, VA and Fn at the exchange, consequent battery reversal being detected at E4 in both stations to release the U and UH relays. So long as the calling (or controlling) station keeps the handset off the telephone set cradle the connection is by this means restored to the speech mode with the called station being rung.