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The IBM 3874 Modem
The IBM 3874 Modem:
• Is intended to operate at a speed of 4800 or 2400 bits
per second (bps) on:
Non-switched telephone lines in point-to-point or
centralized multipoint configurations.
Public switched network (U.S.A. and Canada only),
either
exclusively or in back-up operation to
non-switched line. Connection is made via a Data
Access Arrangement (DAA) type CBS* or type CDT*.
• Uses coherent-phase-shift keying, with digital echo
modulation.
• Incorporates a clock, which makes it unnecessary for the
DTE to provide clocking pulses for the transmission or
reception of data.
• Includes an automatic receive equalizer, thereby assuring
satisfactory operation on voice-grade non-switched lines
or on the public switched network.
• Accepts a Fan Out feature, which enables connection to
up to three DTEs.
• Allows an operator or the using system to detect faults
rapidly and to identify their origin (3874, using system,
or line).
• In switched network operation, provides the possibility
of using an Automatic Call Originate feature (with DAA
type CBS).
In non-switched line operation, an Alternate Voice
feature allows conversation directly between operators, via
handsets plugged into the interconnected modems. In
switclied network operation, however, conversation takes
place via the associated telephone
h~nrl~et.
To determine the IBM machines to which the 3874 can
be attached, consult your IBM representative.
HIGHLIGHTS
The 3874 offers the following basic advantages:
• Reduced error rate
• High
reliability
• Operation on economical non-switched lines
U.S.A. and Canada: "Cl" conditioning.
Other countries: telephone line meeting characteristics
defined under "Communications Channel Specifica-
tions" in Part 3.
• Operation on a switched network (in U.S.A. and Canada
only).
• Possibility of manual or DTE-controlled switching from
4800 bps to 2400 bps if a non-switched line becomes
degraded.
• Possibility of switching from a non-switched line to a
switched network
if non-switched line operation
becomes impractical, and to operate at 4800 bps on the
*
Manufactured by American Telephone and Telegraph Company
switched network (in U.S.A. and Canada only).
• Automatic equalization, which provides a practical
means for efficiently equalizing each connection when it
has been established.
• Turn around time can be reduced by use of four-wire
duplex facilities (instead of four-wire half-duplex).
• Use of the fan out feature to reduce the number of
modems required by allowing up to three DTEs at the
same location to be connected to the same modem.
• Ability to operate in duplex data mode.
• Built-in test facilities to enable the operator to determine
the
location
of
a
system
fault
(teleprocessing
. transmission path).
All these advantages increase the efficiency, availability,
and flexibility of teleprocessing systems.
VARIATIONS
The 3874 is adapted for a specific application (for example,
point-to-point) by the addition of special circuitry, called
features, and by the selection of suitable strapping options.
Note: Strapping options are jumpers installed by the IBM
customer engineer during the installation of the 3874.
Features available are as follows (allowable configurations
are shown in Figure 2):
1. Point-to-Point
2. Multipoint Tributary
3. Multipoint Control
4. Switched Network**
5. Switched Network Back-Up** (for use with DAA-CBS
[with auto-answer]
or with DAA-CDT
[without
auto-answer])
6. Alternate Voice
7. Fan Out
8. Automatic Call Originate**
9. Direct Line Attachment.
Required Features
Note: The "required features" are mutually exclusive, but
each of them can have one or more optional features (see
Figure 2).
Point-to-Point
The Point-to-Point feature allows data communication
between two stations via a four-wire non-switched line.
Multipoint Tributary
The Multipoint Tributary feature is similar to the
point-to-point feature, but allows connection to a 3874
with the Multipoint Control feature. A maximum of six
tributary stations may be used with one control modem.
**
Available
in
U.S.A. and Canada only
Part 1. General Information - IBM 3 87 4 Modem
3
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