Patent GB-1028381-A (GB)
Title: Improvements in or relating to party line telecommunications systems
IPC: H04Q5/20
Filed: 1963-09-13
Abstract: 1,028,381. Automatic exchange systems; transistor frequency-selective relay circuits. SIEMENS & HALSKE A.G. Sept. 13, 1963 [Sept. 14, 1962], No. 36114/63. Headings H3T, H4K and H4R. In a party-line system, the instruments of subscribers not participating in a call are prevented from connection to the line by a disabling signal from the exchange such as a reversal of line current, but this signal is rendered ineffective at a calling or called substation by the receipt of an individual V.F. signal, which, in the case of a calling substation, results from the subscriber dialling his own line number. The V.F. signal is fed by way of a high-pass filter F1 to a pair of transistors Tr1, Tr2 in cascade associated with a two-winding relay B and a band-pass filter F2. When the frequency received is that passed by F2, its output, rectified by G3, charges capacitor C4 and thus provides a voltage driving the transistors further into the conductive region to energize relay B. The delay in the operation of B is sufficient to obtain voice immunity. When the frequency received is not that passed by F2, relay B acts as a transformer and charges capacitor C3 through rectifier G2, giving a voltage which blocks the transistors. On the initiation of a call at a substation, relay A is energized by the discharge of a capacitor C5 and holds over the line in series with rectifier G6 and relay J1 at the exchange. The low resistance loop reduces the charge on C5 at the other substations and thus prevents any operation of A thereat. A second winding of A provides a holding circuit during dialling. Outgoing call.-The digit 2 is dialled and switch DM1 is set to a position in which relay EX energizes. A line finder AS operates to find the calling line. The second dialled digit, which identifies the caller, sets switch DM2, relay Q being up, and at the end of the digit a selected frequency is applied to the party line over coil LT II and is maintained by relay R. At the calling substation relay B responds and establishes a loop independent of G6. The third and subsequent digits are repeated forward over the finder AS, and after the third digit relay X pulls up and reverses the connections of relay J1 to block the rectifiers G6 of the non-participating substations. Switch DM2 selects the meter Z to be actuated. Revertive call.-The first digit 3 sets switch DM1 to operate relay IN, the second digit as before identifies the caller and the third digit in this case sets switch DM3. Relay X pulls up and reverses the line connections to block all the rectifiers G6. Since there is now no low resistance loop, relay J falls back and the frequency selected by the third digit is applied via LT II to operate relay B at the wanted substation. At the same time relay K controlled by a timer applies ringing current. When the called party replies, his relay A energizes followed by Jl, ringing is cut off and the frequency identifying the calling substation is applied to operate relay B thereat, this operation being timed by relay M. Incoming call.-Relay C2 is energized from the final selector LW, and switch DM3 is stepped by self-interrupting relay J2 to a position corresponding to that of LW, there being a separate position for each party line substation. Relay P pulls up and the frequency of wanted party is transmitted, while relay K operates to effect ringing. Emergency calls.-When the party line is in use another subscriber lifting his receiver causes a lamp SZ to light. He may, however, break in by actuating push-button T which connects the microphone over capacitor C2 to the line while short-circuiting the receiver F to prevent listening in.