MAGNETIC DRUM TRANSLATOR FOR TOLL SWITCHING OFFICES 743 



inserted piecemeal by working in each track successively. The manual 

 switching operation of connecting a single pair of writing amplifiers to 

 each of eighty magnetic heads, in turn, is accomplished partly by setting 

 the nine-position switch shown at the center of the panel, and partly by 

 sliifting the plug of the ten-conductor cable. At the left are two signal 

 lights which serve as alarms to warn the operator of possible incorrect 

 functioning of the equipment. 



The operation of the administration unit can best be traced with the 

 aid of the schematic block-diagram of Fig. 15. A ten-stage binary counter 

 is supplied with b sync pulses from the translator; the 1,024 possible 

 states of the counter are traversed in the course of exactly one revolu- 

 tion of the translator drum. The f pulse from the translator will, mid- 

 way between two b pulses, set all counter stages to zero, once per revolu- 

 tion. After the first such reset, however, if the counter is working 

 properly, it will always have returned to the zero condition just before 

 the occurrence of the f pulse, by having counted 1,024 b pulses; under 

 these conditions the f pulse, though still initiating reset action, does not 

 change the state of the counter. The basis for the alarm signals mentioned 

 above is a circuit arranged to detect if a change of state is occasioned by 

 the F pulse. 



Associated with the counter is a coincidence circuit with a keyboard 

 on which may be set up any "address" between and 1,023. When the 

 count of B pulses ecjuals the address set up on the keyboard, the coinci- 

 dence circuit delivers a pulse which persists until the next b pulse alters 

 the count; this coincidence pulse spans the time of occurrence of an a 

 pulse, and is used in the read sync selector to gate-out a "selected" a 

 pulse uniquely assigned to the address set up on the keyboard. A slot- 

 spanning pulser, triggered bj^ the selcted a pulse, gates-out the associ- 

 ated "selected" b pulse. 



These selected pulses, which occur once per revolution of the drum, are 

 passed through gates under control of bistable electron-tube pairs which 

 can be set by the manual writing keys and are re-set by the writing action 

 itself. This insures that the desired action takes place only once per key 

 operation, instead of repeating, once per drum-revolution, as long as the 

 keys are held operated. The manually-gated unique selected a or selected 

 B sync pulse is then slightly delayed in time to become a selected write- 

 sync pulse. It is passed on through further gates under direct control of 

 the writing keys, and is emplo3''ed as an input to a writing amplifier. 



A pair of writing amplifiers is provided, one to write " 1" and the other 

 to write "0"; the circuits are identical quiescent blocking-oscillators shar- 

 ing a common output transformer, and one or the other is triggered into 



