Patent GB-960062-A (GB)
Title: Improvements in or relating to automatic telephone systems
IPC: H04M3/08
Filed: 1961-02-15
Abstract: 960,062. Automatic exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. Feb. 15, 1961 [Feb. 15, 1960], No. 5713/61. Heading H4K. Tone is returned to a calling subscriber from a connecting relay set if the wanted number is busy, or unobtainable, or if the digits were dialled dilatorily, a register employed for the call being released and the relay set staying in service to send such tone unless, with all such relay sets employed, another subscriber initiates a call when one of the relay sets occupied with sending such tone is forcibly seized for the new call before the connected subscriber releases it. As shown in Fig. 1, a calling one of the 1000 subscribers Al to AN is identified by IDS and with the use of marker SLM is connected over switching stages SLA, SLB, to an idle one of the 50 connecting relay sets SNR provided an idle relay set can be associated with an idle register REG by means of a selector RS and marker RSM. The register REG, of which there are five, receives digits and supervises the connection. Line circuits LU comprise a single cut-off relay which is held by the connecting relay set. In the acknowledgement of prior art it is pointed out that the line circuits would also contain a so-called blocking relay which operates with the cut-off relay and releases slowly after the cut-off relay is released by the connecting relay set. During its release time the blocking relay connects tone to the subscriber from a source SU shown in dotted outline since it is not part of the present invention. In this invention the tone SU is connected by the relay sets SNR whenever the register REG decides that a connection cannot be set up and retires. A relay set SNR occupied in sending tone SU can be seized for a call if no other relay set is free and to distinguish itself from relay sets otherwise busy it connects resistance battery markings to the marker SLM whereas the sets otherwise busy connect battery marking direct. In the marker SLM three relay chains are employed to connect an idle relay set SNR to a calling subscriber over stages SLA, SLB, which (as shown in more detail in Figs. 2 to 6, not shown) are code-bar relay selectors. The first relay chain marks all the idle inlets to SLA available to the subscriber while a second relay chain marks inlets to SLB associated with idle relay sets SNR, and the third relay chain marks the SLB selectors having at least one idle inlet. As many relays in each chain operate as there are idle markings presented. If no relay sets SNR are idle, none of the relays in the relay chains operate when a caller seizes SLM to set up a path to a register. In this condition all the relays of each of the first and second chains provide a path for the energization of congestion relays which connect the relays of all three chains to battery potential such that they will not operate to full battery marking from the connecting relay sets SNR but will operate to the battery marking extended from such relay sets as are occupied with transmitting the tone SU. By this means the relay set is forcibly appropriated and congestion is forestalled.