AT&T Bell System 401E Series Description And Operation page 3

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ISS 6, SECT ION 594 -014 -1 00
Fig. 2-Da ta Set 401 E4 or 401 ES
oscillator generates a Hrest " frequency provided
the customer keying contacts are closed. Rest
frequencies are also transmitted for approximately
50 milliseconds after the keying leads are opened.
This rest frequency is normally used for echo
suppression; rest frequencies may also be used as
supplemental data frequencies. The frequencies
produced by data set 401E-type are shown in Table
A.
3.03
A two-stage answer-back amplifier monitors
both incoming answer-back tones and
transmitted data. The amplifier output is usually
fed to a loudspeaker in the business machine .
3.04
Electronic circuits are powered by the central
office supply on the local telephone loop. A
polarity guard circuit within the data set maintains
the correct voltage polarity.
3.05
The leads present at the interface connector
are numbered and designated as shown in
Table B.
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