Outline of operation.
Local calls: The sub picks up his receiver, which seizes a free connecting circuit (the allotter moves on to a free connecting circuit so that it is ready for the next call). On receipt of dial tone the sub dials the first digit of the local number (which is always a 2). The switch steps up to level two, where the vertical marking wipers operates a relay, which establishes local calling conditions. The switch now restores to normal and is ready to receive the last two digits of the local number, upon which it acts as a normal 2/10 PBX hunting final.
Outgoing level ‘0’ calls: These are calls to the parent manual switchboard. Here the local sub dials 0. The selector steps upto level 9, the 10th impulse being diverted away from the vertical magnet, going to a discriminating relay, which operates. The selector now hunts on level 9 for a spare junction to the parent exchange. The discriminating relay in the selector forwards the discriminating signal onwards to the junction relay set, which establishes the manual switchboard calling condition. Ringtone is now supplied back to the sub from the relayset. Further discrimination is extended to the relayset (and forward over the junction) when the call is established from a coin box line, which causes a ccb lamp to light at the manual board.
Outgoing level ‘9’ calls: These calls are to the parent automatic exchange. The sub dials 9, which steps the selector up to level 9 where it proceeds to hunt for a spare junction. As there is no discriminating signal (as used for level 0 calls), the relay set extends loop calling conditions to the junction, which establishes a call to the automatic equipment at the parent exchange. The impulses dialed at the Uax being repeated by the relayset over the junction.
Outgoing calls on other levels 6-8 are done in a similar way to the above, but are for calls to adjacent exchanges (auto or manual) One exchange being available on each level, with PBX hunting over the junctions.
Incoming calls: Incoming junctions are terminated on early-choice contacts of the banks of the linefinders, which reduces the hunting time. If dialing occurs before the linefinder has found the junction, busy tone (and flash) is transmitted. If the called subs line is free, ringing conditions are given. If the sub is engaged, busy tone (and flash) is transmitted. At this point the parent exchange operator may ‘trunk offer’, by operating the ringing key, by operating this key again ringing is applied to the subs line.
The common apparatus is operating at all times during the conversation.