DIY Telephone System: Sequence Diagram Description 
 

Altogether there are 53 relays plus 4 rotary stepping switches. Most of the relays control the progress of the call. See the sequence of operations chart below. This version was redrawn in 2022 and based on the badly water damaged original from 1969. 
 

Individual relay names are given alongside their relative time of action. Time advances downward. The diagram does not show the operation of every relay. Only the main actors are listed. A relay on (engaged) action is represented by an x symbol, a relay off action is represented by a      . 

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Notice how many ops are needed before a new caller receives dial tone.