Patent GB-788872-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: DOLAN HENRY
IPC: H04W84/14
Filed: 1955-07-22
Abstract: 788,872. Exchange systems. A. T. & E. (BRIDGNORTH), Ltd. June 29, 1956 [July 22, 1955], No. 21225/55. Class 40 (4). [Also in Group XL (c)] Two links 1LK, 2LK are available for connecting radio sub-stations A to F (not shown) to corresponding line circuits SLCA to SLCF in a telephone exchange. The substations are equipped for duplex working on individual frequencies. A free link searches for a calling signal by tuning the link receiver for 3 seconds to each of the six substation transmitting frequencies in a cycle of 20 seconds. When one link is seized the other takes over the search. If both links become busy, a warning signal is given to the parties using the link first seized and after a delay of 3 minutes that link is forcibly released. Channel switching equipment CSE common to the two links comprises two groups of relays 1A to 1F, 2A to 2F. When link 1LK is searching, relay 2R is up and relays 1A to 1F pull up for 3 seconds each in a cycle of 20 seconds as the contacts CCA to CCF are closed by cams driven by a motor 1SM. Relays 1A to 1F connect the line circuits SLCA to SLCF in turn to the link 1LK. Relays 1RA to 1RC and 1TA to 1TC tune the receiver 1REC and the transmitter 1TRANS to the frequencies of the corresponding substations by switching the crystals 1RXE, 1TXE, &c. When only link 2LK is in use, relay 1R is up and 2R back. Relays 2A to 2F pull up in turn and tune the receiver 2REC and the transmitter 2TRANS of the second link. When link 1LK detects a call from a substation A, a signal on lead 1F makes tube VB conductive and relay RS pulls up and completes the loop over line circuit SLCA, relay 1A being up. Hold relay BA pulls up and is followed by RG and RH. Relay RH starts the link transmitter. Relay 1A locks to contact rh4, relay 1R pulls up and 2R falls back. Link 2LK continues the search as relays 2A to 2F pull up in turn. Relay RS repeats dialled impulses over the line circuit SLCA to the exchange. Relay CD short-circuits both the hybrid H and a retard relay AA during impulsing out and relay CA prevents spurious operation of the exchange equipment when CD releases. Both links busy. Relay PB applies guarding earth to the P-wires of all the line circuits. Both relays 1R, 2R are up and relay 1S is locked up if link 1LK is the first to be seized, relay 1R then being up alone. With both 1R, 2R up, PG follows in a circuit over normally closed contacts CC2 and locks. A synchronous motor 2SM starts and closes cam contacts CC1 until the cams operating contacts CC1 to CC4 complete a 3-minute cycle. Towards the end of the cycle contact CC4 is twice closed to connect warning tone from WT to windings of retard relay AA in link 1LK. If the connection is not voluntarily released, relay TA pulls up over contact CC3 before the end of the cycle and releases link 1LK. If link 2LK had been the first in use, relay TS would not have been up and link 2LK would have been warned and then released by relay 1B. Call from exchange. When the line circuit is connected during the searching operation, relay RG responds to ringing current from the exchange, relay RH follows and the link is seized on closure of contact rh4 as in a call to the exchange. The transmitter is started at contact rh1 and on reply RS pulls up to complete the loop. When RG pulls up, condenser C1 delays its release for at least 40 seconds so that the substations may listen out intermittently for 3 seconds in every 30.