TM 11-5805-609-14
which, in turn, enables the 425 Hz oscillator at a two
second on, four second off rate. The 425 Hz ringback
9.1 General
signal is coupled (via the 425 Hz tone signal output
Reference the block diagram of Figure 4 for this
on the four-wire send pair) to the Converter and to
description. The electronic processing of a call from
the automatic switching center. The operator of the
the automatic switching center begins with a seize
Converter recognizes the request-for-service and
signal of 2250 Hz originating at the automatic
inserts the answer cord, from the operators cord
switching center and appearing on the four-wire
circuit on the SB-22 or the SB-86 switchboard, into
receive port of the Converter. The seize signal is
the appropriate channel jack. The operator then
inductively coupled to the input amplifier, amplified
depresses the access switch which clears the service
and coupled from the output stage of this amplifier
request indicator and talks to the four-wire sub-
to the input to the squaring amplifier. This amplifier
s c r i b e r to determine with whom the four-wire
accepts the sinusoidal signals from the input am-
subscriber wishes to converse. The operator may
plifier and produces a square wave output. In this
extend the call from the SB-22, or the SB-86, switch-
case, the 2250 Hz seize signal is transformed into a
board to the two-wire subscriber in the normal
square wave and coupled to the seize detector circuit.
manner. After connection to the two-wire subscriber
The signal is detected and coupled from the detector
is established, the operator plugs the two-wire
output to the turn-on delay circuit. This circuit
subscribers answer cord into the appropriate channel
changes the 2250 Hz signal into a DC potential or
and the voice path from the four-wire subscriber to
logic output. The turn-on delay circuit provides
the two-wire subscriber is complete. The switch-
protection against false hits on the four-wire port
b o a r d operator can either monitor the call or
from being recognized as legitimate signals. The
remove the operators answer cord from the channel
turn on delay circuit is basically a transistor switch-
jack and wait for a release indication. When the call
i n g circuit controlled from the common Zener
is terminated and a release signal is received from
the automatic switching center (fig. 6) the sequence
the collector of the first stage. The output of the
of events in the Converter (in response to the release
turn-on delay circuit is coupled to the turn-off delay
signal) is identical to the sequence of events when
circuit. T h i s is a transistor switching circuit
receiving the 2250 Hz seize signal as explained
designed to respond to the logic levels provided to it
above. The Converter transmits a 570 Hz
by the previous stages and is utilized to provide a
acknowledge signal as explained above and the
turn-off delay which insures that a hit or momentary
automatic switching center recognizes the signal as a
interruption of the seize signal on the four-wire
release acknowledge signal and stops sending release
receive port will not interrupt the call sequence. A
to the Converter. The operator of the Converter
logic output is obtained from the turn-off delay
recognizes the service request indicator as a service
circuit and is coupled to the detector output stage.
r e q u e s t and inserts the answer cord from the
The detector output circuitry is a transistor switch-
operators cord circuit into the free jack associated
ing circuit designed to function as a control for the
with the particular channel and challenges the call.
The switchboard operator now depresses the AC-
The logic 0 output obtained from the detector output
CESS pushbutton and removes the cords from the
circuit is coupled to the latch circuit as an enable
Converter. The channel is now idle.
condition and to the 425 Hz tone gate as an inhibit.
The latch circuit output is coupled to the 570 Hz
oscillator and enables the oscillator at this time. The
latch circuit also activates the service request in.
The electronic processing of a call from the
dicator for that particular channel indicating a
automatic switchboard begins with a seize signal of
request-for-service. T h e o u t p u t o f t h e 5 7 0 H z
2250 Hz originating at the automatic switchboard
oscillator is coupled to the 570 Hz tone gate and
and appearing on the four-wire receive port of
inductively coupled out on the four-wire send pair of
the Converter to the automatic switching center. The
automatic switching center then recognizes the 570
the series network of resistor R3, resistor R4 and
Hz signal from the Converter as a seize acknowledge
capacitor C1. C2 and R5 makeup the hybrid balance
signal and stops transmitting seize tone. The ab-
compensating network. Capacitor C1 connects to a
sence of seize tone at the Converter allows the
positive reference and is utilized as an RF bypass.
detector circuitry to return to normal. This removes
Diode CR7 is utilized to protect transistor Q1 during
the inhibit signal from the 425 Hz tone gate. The
receipt of a 90 volt rms, 20 Hz ring signal. The ring
latch circuit remains activated and a logic 1 output
signal is coupled through resistor R2 to the base of
from the latch circuit enables the 425 Hz interrupter
transistor Q1 which forms the input terminal to the