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Shelf of 1st Stage Group Selectors
Each shelf contains space for 10 selectors. Typically 6 or 8 shelves per rack
The 1st Group Selector accepts the first dialled digit
A 2 motion selector consists of a ratchet mechanism operated by 2 electromagnets - the vertical magnet and the rotary magnet to make it step up and around the bank of outlets. Relays provide the control logic (below are 4 selectors with covers removed)
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Dial tone
The uniselector has connected the caller through to a free 1st stage 2 motion selector which returns dial tone and waits for the first digit to be dialled.
The 2 motion selector has a bank of 100 outlets arranged in 10 levels of 10. Group selectors accept just one digit and step up to the appropriate level of outlets accordingly. Dialling a 1 steps to the 1st level, a 0 steps to the 10th level. The selector then automatically hunts around the outlets on that level, looking for one that's free. In our exchange it has a 4 figure numbering scheme. If the 1st digit dialled is a 5, the 1st Group selector steps up 5 levels and locates a free outlet on that level to a 2nd Group selector handling numbers starting 5xxx
The 2nd group selector accepts the 2nd digit. If this was a 2, it would step up 2 levels and connect us to a Final Selector handling lines starting 52xx
The Final Selector accepts the last 2 digits, as by this stage we have narrowed the choice down to 100 lines. If we dial 14 the Final Selector steps up to level 1 then round to outlet 4. The call is now through to line 5214 and the Final Selector tests it to check if its free.
The Final Selector returns the appropriate tone to the caller (ring or engaged) and also applies ringing current to the called line.
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