Advanced Telephone Intercom

  
In order to achieve a status between two levels of switching for two calls at the same time, you must first establish privacy on each call. Otherwise, you will end up with all four phones talking to each other, and that just won't do. So the Link 'P' is an intermediate stage between the basic Link design, and the fully blown one (yet to be realized, but not that far away,) with all the 'bells and whistles' features. In the diagram above, you can see in block format, how this is achieved. The RTC (Ring Trip Circuit,) Reg. U (Register, which acts like an old style Uniselector,) and the STG (Service Tone Generator,) are all part of the basic Link design.
Advanced Telephone Intercom - schematic

Advanced Telephone Intercom - img1

However, I've added in two new 'blocks' of circuitry, and these are LLO (Line Lock Out, which provides busy tone back to callers who can't access the Link to make a call,) and Vertical Control (which governs who can and can't make a call - simple logic circuitry to determine who's first in and best dressed!) The relay switching matrix is akin to a simplified 'crossbar (XBAR) system. At the same time, the AND gate daisy chain 'sees' the chain broken by one of the four Q bar outputs going low, as the flip flop changes state, and this 'breaks the chain' thus making the output of the last AND gate in the chain change to a logic low. This output at logic low, allows none of the other three phones to pick up 'off hook' and enable their LA relays. They cannot now gain access to the dial up level of the Link, unless they are the 'called party' whose number the caller will now dial into the REG U Register device (IC2.) Dial pulses are received via OC5 into IC2 pin 14, and the Link then operates as per normal. Pin 3 of IC2 goes low, allowing C1 to charge up and produce Ring Pulses out of pin 9. This also enables IC1a to produce an interrupted Ring Tone into the 8R winding of TX via C3 and R6, coupling this signal to the calling party's phone. When both parties have hung up back to the 'on hook' status, the link P will reset itself due to capacitor C5 charging up, after the dialing loop has been broken, the two LEDS inside OC1 and...



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