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has been pulsed to the DAA, the ACO feature drops
(turns off) 'present next digit'. The DTE must immedi-
ately drop (turn off) the 'digit signal' and' digit present'
lines.
Note: The use of EON is not supported by IBM programs
at this time. If EON (end of number) is not used, 'present
next digit' is raised by the modem and remains up after
'the DTE has transferred the
~ast
digit of the telephone
number.
It
drops when the answer tone is received, or
when the modem times out.
• 'Distant station connected' - The ACO feature raises
(turns on) this line when the distant modem's 3.5-second,
2 lOOHz answer tone has stopped. 'Data set ready' is up
and the calling modem has the use of 'data tip' and 'data
ring' when 'distant station connected' is up. 'Distant
station connected' stays up until the DTE initiates a
disconnect.
• 'Abandon call' - The ACO feature raises (turns on) this
line momentarily when dialing fails to result in a switched
network connection to an answering station. Only
another dial digit within 20 seconds (with EON) or an
answered call inhibits this line. Optional time allowances
for an answer tone or another dial digit (without EON),
or an answered call, are: 20, 40, or 60 seconds. If an
invalid digit is transferred from the DTE, or if a call rings
in, 'abandon call' rise immediately. 'Call request' should
be dropped by the DTE when 'abandon call' drops.
Operation
An automatically originated switched network connection
operates like any other switched network connection (see
"Switched Network Feature", preceding), and there is no
difference at the automatically answering station. The
unique operation with the Automatic Call Origination (ACO)
feature is the actual making of the call.
To automatically place a data call on the switched net-
work, the DTE uses the interface to the ACO feature. When
the called station answers, the modem uses the channel for
serial data communications. At that point, the function of
automatic call origination is complete.
The process of automatic call origination (Figure 3-12)
modifies the meanings of 'data modem ready' and 'off hook'
to the DAA. 'Data modem ready' not on (down) identifies
a dialing interval, during which the dropping and raising of
'off hook' is a dialing pulse rather than a disconnect signal.
When the distant station answers, the modem becomes
ready to the DTE. The call is connected when the ACO
feature raises 'distant station connected' to the DTE and
'data modem ready' to the DAA. 'Call request' can then be
dropped and 'request to send' can be raised by the DTE.
DIRECT LINE ATTACHMENT FEATURE
(WORLD TRADE COUNTRIES)
This feature provides a transformer for each channel con-
nected to the modem. Two-wire modems require one trans-
former; four-wire modems require two transformers. The
Direct Line Attachment feature must be used in certain
World Trade countries (especially the United Kingdom) for
iselation of the modem from the communications channel.
ALTERNATE VOICE FEATURE
The Alternate Voice feature provides facilities for voice
communication and audible line monitoring between 3872
Modems equipped for dedicated communications channel
operation. This feature, in most cases, eliminates any need
for separate toll calls. Voice communication interrupts data
communications.
Both modems must have the Alternate Voice feature.
This feature is available for single modems and for either or
both modems of a dual modem. See Part 2 of this manual
for calling and answering procedures.
A telephone handset (user-provided) plugs into the 3872
VOICE jack on the front panel; the CALL switch
(momentary) signals the remote end. The telephone har.d-
set used with the 3872 Alternate Voice feature may be of
any common-carrier or PTT-approved type with the follow-
ing characteristics: (a) carbon microphone, (b) conventional
electromagnetic receiver, and ( c) three-wire connection
(microphone, receiver, and common).
The following handsets (or equivalents) are suitable:
Automatic Electric, Type 811
Stromberg-Carlson, Type S-C G-3
Note: The transmitter does not discriminate between
room noise and voice, nor between talking and listening;
locally transmitted room noise can interfere with voice
reception on two-wire lines. If it is necessary (because
of the room noise level at a station on a two-wire line)
to avoid this kind of interference, use a "push-to-talk"
handset. The following "push-to-talk" handsets (or
equivalent) are suitable:
Automatic Electric, Type 841
Stromberg-Carlson, Type S-C G-5
These handsets come without plugs; the correct plug is
provided with the Alternate Voice feature. Figure 3-13
illustrates the correct connections of the handset cord to
the plug.
FAN-OUT FEATURE
The DTE interface with the Fan-Out feature conforms to
the description given earlier. Operation of the two or three
attached DTEs as multipoint tributary stations provides the
line control discipline necessary for efficient operation;
that is:·
• All tributary DTEs (maximum of three) are connected to
all interface lines, and all must monitor 'received data'.
• Only one tributary DTE may have 'request to sned' active.
(The 'request to send' lines have no priority order and
are not interlocked.)
Special Feature Descriptions
3-19
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